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ENSO and greenhouse warming
Abstract
The El Nino/Southern Oscillation (ENSO) is the dominant climate phenomenon affecting extreme weather conditions worldwide. Its response to greenhouse warming has challenged scientists for decades, despite model agreement on projected changes in mean state. Recent studies have provided new insights into the elusive links between changes in ENSO and in the mean state of the Pacific climate. The projected slow-down in Walker circulation is expected to weaken equatorial Pacific Ocean currents, boosting the occurrences of eastward-propagating warm surface anomalies that characterize observed extreme El Nino events. Accelerated equatorial Pacific warming, particularly in the east, is expected to induce extreme rainfall in the eastern equatorial Pacific and extreme equatorward swings of the Pacific convergence zones, both of which are features of extreme El Nino. The frequency of extreme La Nina is also expected to increase in response to more extreme El Ninos, an accelerated maritime continent warming and surface-intensified ocean warming. ENSO-related catastrophic weather events are thus likely to occur more frequently with unabated greenhouse-gas emissions. But model biases and recent observed strengthening of the Walker circulation highlight the need for further testing as new models, observations and insights become available.
... index (Supplementary Fig. 1), grid-point SST variability ( Supplementary Fig. 2) or using 50-year instead of 60-year periods to calculate variability ( Supplementary Fig. 3). Thus, models under observed climate change forcing reproduce a post-1960 increase in E-index variability with an increased frequency of strong El Niño and increased C-index variability with an increased frequency of strong La Niña, consistent with the projected change for the future climate 20,38,50 . The post-1960 increase in variability is simulated even though each model has unique, independent internal variability, as well as contrasting model physics. ...
... Instead, the mechanism behind such an increase is similar to that responsible for the projected ENSO shifts, namely, changes in ocean stratification. In response to increasing greenhouse gas emissions, the upper equatorial Pacific exhibits enhanced mean vertical stratification 28,38,50 : the near-surface ocean warms faster than the ocean below. The faster near-surface warming is a result of greenhouse gas-induced radiative forcing and increased precipitation-related freshening. ...
... Moreover, 25,868 years of pre-industrial virtual climate further highlight that observed ENSO variability is unusually high in the post-1960 period. These simulated findings agree with palaeoclimatic evidence that ENSO variability in the twentieth century and early-twenty-first century is higher than in the distant past 21,23,26 and is consistent with projections suggesting continued and increasing ENSO SST variability in the future 38,50 . These changes in ENSO variability -both in the present and the future -are underpinned by an intensified upper ocean stratification of the equatorial Pacific. ...
... First, several studies indicate a possible increase in Central Pacific El Niño events relative to East Pacific El Niño events in the future, based on both the CMIP3 and CMIP5 models (Kim & Yu, 2012;Yeh et al., 2009). In addition, climate models tend to agree on future changes in the equatorial Pacific zonal SST gradient, with preferential warming in the eastern equatorial Pacific, representing a weakening of the equatorial Pacific zonal SST gradient (review paper by Cai, Santoso, et al. (2015), Fredriksen et al. (2020)). Finally, some studies find an increase in the frequency of ENSO events. ...
... Finally, some studies find an increase in the frequency of ENSO events. Cai, Wang, et al. (2015) found almost a doubling in the frequency of La Niña events in the future, based on the Niño 4 index in the CMIP5 models, and Cai et al. (2014) found a doubling of extreme El Niño event frequency, based on rainfall in the eastern equatorial Pacific in the 3 of 18 CMIP3 and CMIP5 models. Similarly, a future increase in East Pacific SST variability was found in the CMIP5 models that capture the two types of ENSO , and among a subset of 11 CMIP6 models, 10 models project an increase in SST variance in the Niño 3.4 region (Fredriksen et al., 2020). ...
The El Niño—Southern Oscillation (ENSO) is an important mode of tropical Pacific atmosphere‐ocean variability that drives teleconnections with weather and climate globally. However, prior studies using state‐of‐the‐art climate models lack consensus regarding future ENSO projections and are often impacted by tropical Pacific sea‐surface temperature (SST) biases. We used 173 simulations from 29 climate models participating in the Coupled Model Intercomparison Project, version 6 (CMIP6) to analyze model biases and future ENSO projections. We analyzed two ENSO indices, namely the ENSO Longitude Index (ELI), which measures zonal shifts in tropical Pacific deep convection and accounts for changes in background SST, and the Niño 3.4 index, which measures SST anomalies in the central‐eastern equatorial Pacific. We found that the warm eastern tropical‐subtropical Pacific SST bias typical of previous generations of climate models persists into many of the CMIP6 models. Future projections of ENSO shift toward more El Niño‐like conditions based on ELI in 48% of simulations and 55% of models, in association with a future weakening of the zonal equatorial Pacific SST gradient. On the other hand, none of the models project a significant shift toward La Niña‐like conditions. The standard deviation of the Niño 3.4 index indicates a lack of consensus on whether an increase or decrease in ENSO variability is expected in the future. Finally, we found a possible relationship between historical SST and low‐level cloud cover biases in the ENSO region and future changes in ELI; however, this result may be impacted by limitations in data availability.
... ENSO is one of the strongest ocean-atmosphere coupling signals on Earth, and its strength has a significant impact on climate in most parts of the world (Cai et al., 2015). MEI is an important index with regard to characterizing the strength of ENSO, which is considered an important large-scale atmosphere circulation pattern with regard to climate change in the Northern Hemisphere, Frontiers in Earth Science frontiersin.org ...
... especially for the regions in or around the Pacific Ocean (Rogers and Coleman, 2003;Jin et al., 2016;Thirumalai et al., 2017). Previous studies have pointed out that MEI is closely related to the temperature extremes at different regional scales around the world (Cai et al., 2015;Fasullo et al., 2018). From a previous analysis, it could be clearly seen that although the relationship between MEI and different temperature extreme indices is different in CRC, almost all the temperature extreme indices have obvious 2-4-year periodic oscillations with MEI. ...
The cold regions of China (CRC) are important and vulnerable freshwater recharge areas on land, and any changes in them are related to the survival of millions of people in East Asia. However, for nearly half a century, in cold regions, the extreme temperature response to global warming is still poorly understood. In this study, we systematically studied the temperature extreme changes in cold regions of China since 1961 and discussed the possible circulation factors in detail. The results showed that 1) the warming magnitudes in cold nights and warm nights are greater than those in cold days and warm days, and decreases in cold nights and cold days and increases in warm days and warm nights appeared in almost all of cold regions of China. Most of the temperature indices displayed the largest magnitudes of warming in winter. 2) Spatially, for most of the temperature extremes, the stations located at Qinghai–Tibet Plateau (TPC) and Northwest China (NWC) showed a larger warming trend than that shown by the station at Northeast China (NEC). 3) The responses of temperature extremes at different cold regions to each circulation index are variable. Atlantic Multidecadal Oscillation (AMO) has a significant relationship with almost all the indices in cold regions of China. Almost all the temperature extremes of TPC and NWC showed closely relationship with the North Atlantic Oscillation (NAO), especially for diurnal temperature range (DTR), daily maximum temperature, and the cold extremes. Multivariate ENSO Index (MEI) is significantly related to most the temperature indices of Northwest China and Northeast China. However, MEI has a significant impact on only TPC’s diurnal temperature range and warmest night (TNx). 4) Atlantic Multidecadal Oscillation displayed significant relationships with most the temperature extremes in every season in cold regions of China. However, the summer and winter MEI and the summer and winter North Atlantic Oscillation showed significant impacts on only diurnal temperature range, daily minimum temperatures (TNm), and TNx.
... Although either α_large or α_small have similar regression properties, diminished Walker circulation, and enhanced local Hadley circulation, the surface temperature rise in the equatorial Pacific was further increased in α_small as the GMST increased. This was in line with the ENSO determined in several studies, which has shown that its amplitude increases when the east-west SST contrast as a background field was reduced (Cai et al., 2015;England et al., 2014;Guilyardi et al., 2009;Kim et al., 2014). The changes in the net downward radiation flux values became increasingly negative in the subtropics for α_small. ...
Internally generated radiative feedback at interannual timescales can influence climate sensitivity estimates. This study has clarified the mechanisms of variability in radiative feedback associated with the El Niño and Southern Oscillation based on preindustrial control simulations for Coupled Model Intercomparison Project Phase 6. Radiative feedback showed large modulation with a comparable magnitude to the intermodel uncertainty associated with internal changes in the zonal gradient of the ocean thermocline and sea surface temperature (SST) in the equatorial Pacific on interdecadal timescales. When the interdecadal SST anomalies in the eastern and western equatorial Pacific were simulated as positive and negative, respectively, the statistical properties of the interannual variations were also modified with diminished Walker circulation changes per unit of global mean surface temperature rise. Meanwhile, the enhanced local Hadley circulation led to a middle‐to upper‐level cloud decrease and a low‐level cloud increase in the subtropics, causing enhanced negative radiative feedback as a global average.
... Without climate change, there would not be a strong link to La Niñas (Figures 8a and 8b). It is worth noting that "strong gradient La Niñas" have been identified in observations (Johnson, 2013), and are expected by climate change model projections later in the 21st century (Cai et al., 2022;Cai et al., 2015a;Cai et al., 2015b). ...
During and after recent La Niña events, the decline of the eastern East African (EA) March‐April‐May (MAM) rains has set the stage for life‐threatening sequential October‐November‐December (OND) and MAM droughts. The MAM 2022 drought was the driest on record, preceded by three poor rainy seasons, and followed by widespread starvation. Connecting these dry seasons is an interaction between La Niña and climate change. This interaction provides important opportunities for long‐lead prediction and proactive disaster risk management, but needs exploration. Here, for the first time, we use observations, reanalyses, and climate change simulations to show that post‐1997 OND La Niña events are robust precursors of: (a) strong MAM “Western V sea surface temperature Gradients” in the Pacific, which (b) help produce large increases in moisture convergence and atmospheric heating near Indonesia, which in turn produce (c) regional shifts in moisture transports and vertical velocities, which (d) help explain the increased frequency of dry EA MAM rainy seasons. We also show that, at 20‐year time scales, increases in atmospheric heating in the Indo‐Pacific Warm Pool region are attributable to warming Western V SST, which is in turn largely attributable to climate change. As energy builds up in the oceans and atmosphere, during and after La Niña events, we see stronger heating and heat convergence over warm tropical waters near Indonesia. The result of this causal chain is that increased Warm Pool atmospheric heating and moisture convergence sets the stage for dangerous sequential droughts in EA. These factors link EA drying to a stronger Walker Circulation and explain the predictable risks associated with recent La Niña events.
... Climate variability can control mangrove productivity, reproduction success and landscape cover via changes in rainfall, humidity, sea level and porewater salinity (Duke et al., 2022;López-Medellín et al., 2011;Lovelock et al., 2017). Climate variability is seldom considered a driver of mangrove trajectories in planning performance measures associated with rehabilitation projects, despite the fact that a higher frequency of El Niño Southern Oscillation (ENSO) cold and warm extreme events and alterations of rainfall patterns are expected in future (Cai et al., 2015;Chen et al., 2017;Ward et al., 2016). In addition, there are few long-term studies of mangrove cover, structure and tree recruitment that can track changes in processes and mechanisms driven by climate variability associated with ENSO events in South American mangroves (Chan-Keb et al., 2018). ...
Hydrological rehabilitation is widely used to improve soil conditions and promote passive recovery. However, climate variability that is seldom considered in restoration planning can affect hydrological rehabilitation goals.
We used long‐term observations to assess the effects of El Niño Southern Oscillation (ENSO) climate variability and hydrologic rehabilitation on tree recruitment and landscape cover changes in a South American mangrove complex.
ENSO climate variability and rehabilitation hydrology measure types collectively explained 88%, 90% and 70% of mangrove cover change rates in the whole mangrove system and porewater salinity in basin and riverine sites, respectively. Therefore, major and detrimental rehabilitation measures counterbalanced or reinforced the effects of ENSO phase intensity on the system. In addition, climate variability explained the rates of change of salinity in the basin site (58%).
Porewater salinity, its rates of change and ENSO explained 28%–75% of propagule density and seedling/sapling growth rates of Rhizophora mangle , Laguncularia racemosa and Avicennia germinans in representative basin and riverine sites.
Tree colonization in the basin site, under high irradiance, was triggered by high seedling/saplings diameter growth rates (0.5–1.6 cm year ⁻¹ ) of L. racemosa and R. mangle generated by strong porewater salinity drop rates during consecutive La Niña episodes, while widespread tree mortality occurred during a strong El Niño. Tree size categories were more stable to ENSO climate variability in the riverine site.
Synthesis and implications . This study shows that hydrologic rehabilitation effectiveness depended on its influence on freshwater flows and on the effects of ENSO climate variability. ENSO, porewater salinity and its rates of change regulated tree recruitment and landscape cover changes. Thus, besides porewater salinity, rates of change of porewater salinity, light availability and climate variability need to be included in monitoring and mangrove restoration planning to better understand and predict mangrove trajectories. Climate‐smart restoration in mangroves should implement the types of hydrological rehabilitation measures that offset or avoid reinforcing ENSO strong phases.
... Our hypotheses will evaluate the effects of natural and anthropogenic stressors separately, and synergistically to assess whether the intensity of coupled influences increases negative effects on the abundance of marine species or if they cancel each other out. Considering that the higher intensity and frequency of ENSO events experienced in the last decades are related to global warming (Cai et al., 2015;Freund et al., 2019), this work will inform how anthropogenic and natural effects related to climate change affected species composition and abundance in coastal systems, and what species were most affected and should be prioritized in conservational efforts. ...
Full text available: https://authors.elsevier.com/a/1i3xLB8ccyiHO
Climate and anthropogenic stressors are frequent in coastal systems, affecting biological communities in different intensities and directions. When acting synergistically, their effects may be intensified. ENSO strongly affects the climate globally, being responsible for increased rainfall in the Atlantic Southwestern during El Niño and droughts during La Niña phases. Contrasting, human-made breakwaters have static influence in decreasing estuarine salinity. Using a 23-year of fish abundance dataset, we identified that intense El Niño events and breakwater extension decreased the marine fish abundance, with potential additive synergistic effects, whereas La Niña showed no influence. Species composition changes were observed after the breakwater extension, probably related to opportunistic habits of euryhaline species. Anthropogenic and natural climatic disturbances affect habitat use, and their synergic effects must be considered to evaluate ecosystem responses in the current climate change scenario, and constant human modification of coastal zones.
... Capturing the influence of ENSO on temperature extremes would require realistic projections of future ENSO events into the 21st century, which is currently beyond the state of the art of most (if not all) GCMs (see, e.g., refs. [56][57][58], rendering the assessment of how future ENSO variability affects the statistics of future extreme temperatures beyond the scope of this study. ...
Global warming in the 21st century will alter the frequency of extreme climatic events, such as high-temperature anomalies and “heat waves”. Observations of extreme high temperatures during recent decades have detected upward trends in their frequency of occurrence, and recent state-of-the-art Global Climate Models (GCMs), e.g., Climate Model Intercomparison Projects (CMIPs), notably CMIP5 and CMIP6, have predicted acceleration of temperature trends and high-temperature events by 2100 under projected greenhouse-gas emission scenarios. Hence, the 21st century is expected to experience substantial shifts in the occurrence of extreme events, where present-day, extreme-but-rare high-temperature events will become common during the summer months. The increasing frequency of extreme heat may affect the health and resiliency of social, biological, and infrastructure systems in many regions worldwide, underscoring the need for accurate and reliable long-term assessments of climatic change across global and regional scales. So far, many investigations of high-temperature extremes have been carried out under end-point scenarios, e.g., by comparing GCM-projected changes in the frequency of high-temperature extremes expected in the late 21st century to the late 20th century. In this study, we use extreme value theory and decades of observations of high-temperature extremes at thousands of meteorological stations across North America to investigate continuous shifts in the frequency of extreme high-temperature events due to projected local warming trends. We find that the odds of exceedance of 50-year extreme high-temperature events increases exponentially with increases in mean local temperature. At a majority of the stations studied here, a local mean temperature increase of 0.5–1 ∘\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{\circ }$$\end{document}C can double the odds of exceedance of 50-year extreme high-temperature events. Based on time-dependent temperature projections, the odds of exceedance of 50-year extreme high-temperature events doubles approximately every 20 years (or sooner) for ∼\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim $$\end{document} 96% of the stations. Moreover, we find that, for ∼\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim $$\end{document} 80% of the stations in North America, investigated here, the 50-year extreme high-temperature events will be exceeded annually before 2100.
... Meanwhile, fires in SEAS were not exclusively natural phenomena, the linkage between fire and climate drivers and weather conditions was also confounded by anthropogenic factors. Nevertheless, the evaluations of how ENSO-induced variations in fire weather risk over SEAS and EQAS regions could still benefit in improving appropriate fire management strategies locally, especially under the serious threat of increasingly frequent extreme El Niño events related to climate warming (Cai et al., 2014(Cai et al., , 2015. ...
Interannual variability of tropical fire activity has been linked to the El Niño‐Southern Oscillation (ENSO). Here we analyze changes in fire and simultaneous fire weather related to ENSO over Southeast (SEAS) and Equatorial Asia (EQAS) and quantitively evaluate the contribution of key meteorological variables to the fire weather variations for the past decades. During the fire seasons, large increases in fires emerge in El Niño years, which greatly coincides with the more fire‐susceptible weather conditions compared to La Niña years. Moreover, the fires exhibit a more significant correlation with the ENSO index in El Niño years, mainly due to the influence of asymmetry response of the fire weather to ENSO. Composite analysis shows that it is the positive anomalous low‐level geopotential height and less water vapor transportation over SEAS (March–May) and EQAS (August–October) during El Niño that forms the fire‐favoring weather. By conducting tests in calculating fire weather index (FWI) under different ideal conditions, we identified the daily precipitation as the dominant driver of FWI variation, with the contribution reaching 65.5%–77.5% and 60.9%–90.5% for SEAS and EQAS respectively, while decreased relative humidity induces nearly a quarter of increases in FWI in El Niño years. Changes in temperature and wind speed, however, rarely perturb the fire weather for both SEAS and EQAS regions. The quantitative evaluations of individual meteorological variables in shaping fire weather as well as their asymmetrical response in ENSO years could support fire management during fire seasons in Tropical Asia.
... ENSO cycles are known to reduce certain crop yields in some regions 2 . El Niño generally brings drier than average weather to the region during key growing months 3 , impacts that are likely to become more severe as global temperatures rise 4 . ...
Controlling for factors such as criminal violence and poverty, we tested if drier than usual growing season weather was a predictor of emigration from El Salvador, Guatemala, and Honduras to the US between 2012 and 2018. We focus on growing season weather because agriculture is a primary transmission pathway from the effects of climate change upon migration. We secured the migration apprehensions data for our analysis through a FOIA request to US Customs and Border Protection. Border Patrol intake interviews recorded the original home location of families that arrived at the southern US border. We used this geographic information to measure recent weather patterns and social circumstances in the area that each family departed. We found 70.7% more emigration to the US when local growing seasons in Central America were recently drier than the historical average since 1901.
... The people of the South Pacific islands are highly vulnerable to the effects of both climate variability and climate change. The region is affected by climate extremes, leading to floods and droughts, with acute effects on agriculture (Held and Soden, 2006;Xie et al., 2010;Barnett, 2011;Cai et al., 2015). Climate variability in the South Pacific is influenced by seasonal shifts of the South Pacific Convergence Zone (SPCZ): the most significant rain belt in the Southern Hemisphere and a key driver of rainfall variability in the region (Higgins et al., 2020). ...
This study utilizes speleothem trace elements as climate proxies to reconstruct hydroclimate variability over approximately 350 years in the Southern Cook Islands. Stalagmites Pu17 and Pu4 from Pouatea cave were analyzed using high-resolution LA-ICP-MS for trace elements (Mg, Na, Sr, P, U, Y). By monitoring cave dripwater and conducting regression analysis, we found that Mg, Sr, and Na in Pouatea dripwater mostly originated from marine aerosols, while Sr and Ba were primarily from bedrock, with additional Ba coming from marine aerosols and weathered oceanic basalt leaching. Mg was identified as the most reliable element for hydroclimate reconstruction due to its predominantly marine aerosol origin. Infiltration, via dilution of marine aerosols and bedrock inputs, was identified as the main driver of trace element variations in Pouatea at a seasonal scale. Transfer functions were established between each trace element and effective infiltration was calculated, with Mg showing the strongest correlation. The reconstructed infiltration data were compared with climate indices, showing an overarching role of the SPCZ and ENSO in controlling rainfall in the South Pacific. This research demonstrates the potential of speleothem trace elements for paleohydroclimate reconstructions, improving understanding of rainfall variability in the climatically vulnerable South Pacific Islands over the past millennia.
... From territorial zoning, strategic health facility placement in the Regions, and versatile information and communication strategies to governance structures and financial mechanisms. Moreover, since the frequency of ENSO events will likely increase and intensify with unabated greenhouse gas emissions [12,16], there is a need to improve territorial and environmental management and preparedness. This can be addressed through local-level climate change risk assessments (i.e., city level, community level) with a framework of auditable indicators and ensure the assignment of a focal point person at the local level that manages adaptation and mitigation plans, response and preparedness, communication, strategies, and effective budget allocation. ...
Climate-related phenomena in Peru have been slowly but continuously changing in recent years beyond historical variability. These include sea surface temperature increases, irregular precipitation patterns and reduction of glacier-covered areas. In addition, climate scenarios show amplification in rainfall variability related to the warmer conditions associated with El Niño events. Extreme weather can affect human health, increase shocks and stresses to the health systems, and cause large economic losses. In this article, we study the characteristics of El Niño events in Peru, its health and economic impacts and we discuss government preparedness for this kind of event, identify gaps in response, and provide evidence to inform adequate planning for future events and mitigating impacts on highly vulnerable regions and populations. This is the first case study to review the impact of a Coastal El Niño event on Peru’s economy, public health, and governance. The 2017 event was the third strongest El Niño event according to literature, in terms of precipitation and river flooding and caused important economic losses and health impacts. At a national level, these findings expose a need for careful consideration of the potential limitations of policies linked to disaster prevention and preparedness when dealing with El Niño events. El Niño-related policies should be based on local-level risk analysis and efficient preparedness measures in the face of emergencies.
... Global warming poses a pressing challenge to society at large, prompting countries worldwide to adopt measures such as controlling greenhouse gas emissions and implementing a 'low-carbon development' strategy (Kosaka and Xie 2013, Cai et al 2015, Sarkodie and Strezov 2019. Concurrently, the continuous release of greenhouse gases heightens human vulnerability to various diseases, constituting a significant potential threat to public health (Georgescu et al 2014, Mora et al 2022, Fuhrman et al 2023. ...
China is faced with significant challenges in simultaneously promoting rural development and reducing carbon emissions. However, the issue of quantifying and addressing carbon emissions in rural areas has not been adequately addressed. Accurately quantifying these emissions is crucial for developing effective strategies to reduce carbon output. In this study, the historical evolution and spatial distribution of rural carbon emissions in northwestern China from 2006 to 2019 were evaluated across five key sectors: residential energy consumption, agricultural machinery, solid waste management, planting practices, and breeding industry activities. During this period, total carbon emissions in rural areas of northwest China steadily increased from 60.15Mt to 83.49Mt at an annual growth rate of 2.55%. Given the complex interplay between economic and social factors driving these changes, the future trajectory of rural carbon emissions remains uncertain. To analyze the underlying drivers behind regional variations in carbon emissions over time, we constructed an LMDI model which revealed that economic growth primarily contributed to regional increases in carbon output. Furthermore, due to a remarkable annual growth rate of 35.17% in renewable energy generation (such as photovoltaic and wind power), it can be inferred that if renewable electricity were included within our calculations for carbon emission statistics, northwest China's rural areas achieved a state of being effectively "carbon-neutral" by 2019 solely from a production-based perspective.
... Under GHG-induced warming, an increased equatorial Pacific warming and a weakening of the Walker circulation (Vecchi et al., 2006) are projected to lead to a stronger ENSO magnitude and frequency (Cai et al., 2015); this has been inferred through ENSO proxies (Grothe et al., 2020), reanalyzes and multi-model projections (Cai et al., 2021). Given the need for long simulations in order to properly sample the underlying processes, provided the high variability and a comparatively long period of an average ENSO cycle, few results are available for SRM simulations (Gabriel & Robock, 2015). ...
The specifics of the simulated injection choices in the case of stratospheric aerosol injections (SAI) are part of the crucial context necessary for meaningfully discussing the impacts that a deployment of SAI would have on the planet. One of the main choices is the desired amount of cooling that the injections are aiming to achieve. Previous SAI simulations have usually either simulated a fixed amount of injection, resulting in a fixed amount of warming being offset, or have specified one target temperature, so that the amount of cooling is only dependent on the underlying trajectory of greenhouse gases. Here, we use three sets of SAI simulations achieving different amounts of global mean surface cooling while following a middle‐of‐the‐road greenhouse gas emission trajectory: one SAI scenario maintains temperatures at 1.5°C above preindustrial levels (PI), and two other scenarios which achieve additional cooling to 1.0°C and 0.5°C above PI. We demonstrate that various surface impacts scale proportionally with respect to the amount of cooling, such as global mean precipitation changes, changes to the Atlantic Meridional Overturning Circulation and to the Walker Cell. We also highlight the importance of the choice of the baseline period when comparing the SAI responses to one another and to the greenhouse gas emission pathway. This analysis leads to policy‐relevant discussions around the concept of a reference period altogether, and to what constitutes a relevant, or significant, change produced by SAI.
... However, many important phenomena are influenced by changing external factors, resulting in time-dependent governing rules. Examples include collective motion of particles and organisms in response to changes in their environment 4,5 , neuronal dynamics under stimuli 6,7 , mixing and coherent structure formation under a time-dependent fluid flow 8,9 , and the variability of the Earth's climate under natural and anthropogenic forcings 10,11 . In response, at the beginning of the previous decade, extensions of operator-theoretic techniques to non-autonomous dynamics were developed 12,13 , enabling the analysis of a much wider class of systems. ...
An important problem in modern applied science is characterizing the behavior of systems with complex internal dynamics subjected to external forcings from their environment. While a great variety of techniques has been developed to analyze such non-autonomous systems, many approaches rely on the availability of ensembles of experiments or simulations in order to generate sufficient information to encapsulate the external forcings. This makes them unsuitable to study important classes of natural systems such as climate dynamics where only a single realization is observed. Here, we show that operator-theoretic techniques previously developed to identify slowly decaying observables of autonomous dynamical systems provide a powerful means for identifying trends and persistent cycles of non-autonomous systems using data from a \emph{single} trajectory of the system. Using systematic mathematical analysis and prototype examples, we demonstrate that eigenfunctions of Koopman and transfer operators provide coordinates that simultaneously capture nonlinear trends and coherent modes of internal variability. In addition, we apply our framework to two real-world examples from present and past climate dynamics: Variability of sea surface temperature (SST) over the industrial era and the mid-Pleistocene transition (MPT) of Quaternary glaciation cycles. Our results provide a nonparametric representation of SST and surface air temperature (SAT) trends over the industrial era, while also capturing the response of the seasonal precipitation cycle to these trends. In addition, our paleo-climate analysis reveals the dominant glaciation cycles over the past 3 million years and the MPT with a high level of granularity.
... The onset of an El Niño-Southern Oscillation (ENSO) event is induced by the coupling of SST to the Walker Circulation, namely, the Bjerknes feedback (Bjerknes, 1969). The ENSO phenomenon and its rainfall response are suggested to be significantly changed under anthropogenic warming (Santoso et al., 2013;Cai et al., 2015;Cai et al., 2021). The Indian Ocean Dipole (IOD) and corresponding rainfall pattern are also related to the coupling of SST to the atmospheric circulation (Saji et al., 1999). ...
Sea surface temperature (SST) is an important element in studying the global ocean-atmospheric system, as well as its simulation and projection in climate models. In this study, we evaluate the simulation skill of the Coupled Model Intercomparison Project Phase 6 (CMIP6) models in simulating the climatological SST in the Asian Marginal Seas (AMS), known as the most rapidly warming region over the global ocean. The results show that the spatial patterns and seasonal variability of Asian Marginal Seas (AMS) climatological SST simulated by the CMIP6 models are generally in good agreement with the observations, but there are simulation biases in the values. In boreal winter, the simulated climatological SST tends to be overestimated in the Japan/East Sea and the East China Seas (ECSs) by up to 2°C, while being underestimated in the Sea of Okhotsk by up to 2°C. In boreal summer, the simulated climatological SSTs are overestimated in the Indonesian seas and western Arabian Sea, while being underestimated in the Sea of Okhotsk and the northern ECSs by 1.2–1.5 and 2°C, respectively. Furthermore, we calculate the projected sea surface warming trends in the AMS under different future scenarios in the CMIP6 models. The results show warming trends of 0.8–1.8, 1.7–3.4, and 3.8–6.5°C/century for the Shared Socio-Economic Pathway (SSP) of low- (global radiative forcing of 2.6 W/m² by the year 2100), medium- (global radiative forcing of 4.5 W/m² by 2100) and high-end (8.5 W/m² by 2100) pathways, respectively. In addition, the middle and high latitudes of the AMS are found to have faster warming trends than the low latitudes, with the most rapidly warming occurring in the Sea of Okhotsk, which is around 2 times larger than the global mean SST warming trend. The SST warming trends are relatively slow in the South China Sea and the Indonesian seas, roughly equal to the global mean SST warming trend.
... By performing experiments with an atmosphere model, it was found that SST warming patterns are the main cause of the weakened Walker Circulation over 1950-2009 3 . Finally, the majority of current climate models predict that the tropical Pacific will exhibit an El Niño-like meanstate response to strongly elevated atmospheric CO 2 -concentrations, i.e. a reduced SST gradient across the equator, with more warming in the east than in the west, and slower PWC [4][5][6][7] . ...
The globally averaged sea-surface temperature (SST) has steadily increased in the last four decades, consistent with the rising atmospheric greenhouse gas concentrations. Parts of the tropical Pacific exhibited less warming than the global average or even cooling, which is not captured by state-of-the-art climate models and the reasons are poorly understood. Here we show that the last four decades featured a strengthening atmospheric circulation and stronger trade winds over the tropical Pacific, which counteracted externally-forced SST warming. Climate models do not simulate the trends in the atmospheric circulation irrespective of whether an external forcing is applied or not and model bias is the likely reason. This study raises questions about model-based tropical Pacific climate change projections and emphasizes the need to enhance understanding of tropical Pacific climate dynamics and response to external forcing in order to project with confidence future climate changes in the tropical Pacific sector and beyond.
... It suggests more pIODs in a warmer climate. On the other hand, more El Niños can be expected based on an El Niño-like warming pattern in the Pacific in a warmer climate 71 . Considering both El Niño and pIODs may occur more often, a warmer climate may exhibit cooccurrences of El Niños and pIOD more often. ...
The El Niño-Southern Oscillation (ENSO) and the Indian summer monsoon (ISM, or monsoon) are two giants of tropical climate. Here we assess the future evolution of the ENSO-monsoon teleconnection in climate simulations with idealized forcing of CO 2 increment at a rate of 1% year ⁻¹ starting from a present-day condition (367 p.p.m.) until quadrupling. We find a monotonous weakening of the ENSO-monsoon teleconnection with the increase in CO 2 . Increased co-occurrences of El Niño and positive Indian Ocean Dipoles (pIODs) in a warmer climate weaken the teleconnection. Co-occurrences of El Niño and pIOD are attributable to mean sea surface temperature (SST) warming that resembles a pIOD-type warming pattern in the Indian Ocean and an El Niño-type warming in the Pacific. Since ENSO is a critical precursor of the strength of the Indian monsoon, a weakening of this relation may mean a less predictable Indian monsoon in a warmer climate.
... Alteration of the hydrological regime in regions where the snow accumulation season is out of phase with the melt season [96][97][98][99][100][101] Precipitation variability and increase in extreme events ENSO effects, which increase precipitation by releasing water stored in the atmosphere due to higher temperatures ENSO reduces electric power generation due to floods [102][103][104][105][106][107][108][109] High evaporation and need for water ...
This paper discusses the increasing demand for energy due to population growth and civilization's rapid development and the negative impacts of using fossil fuels to generate energy, including greenhouse gas emissions and climate change. It highlights the vulnerability of the energy industry to climate change and its negative consequences on people's health. Developing countries have implemented plans to generate secure electric energy through renewable energy resources (RER) and have pledged to achieve net-zero emissions. The paper presents hydropower as a reliable alternative to fossil fuels and discusses its potential in reducing greenhouse gas emissions and decarbonizing the energy system. It also highlights the impact of climate change on hydropower and the need for global modeling of social, economic, and environmental aspects to provide sustainable RER. The paper provides evidence to support the claims made and highlights the significance of reducing greenhouse gas emissions to mitigate the effects of climate change on the energy industry and the environment.
... In addition, most CMIP6 models project an increase in year-toyear variability of seasonal SST in the north Atlantic, tropical central Pacific, and near the Maritime Continent region ( Supplementary Fig. 12). Potential consequences of increases in both the mean state and variability of tropical SSTs include an increased frequency of extreme El Niño events due to promoting atmospheric convection and an eastward shift of ENSO precipitation teleconnections 60 . For precipitation, projected changes in the seasonal precipitation amounts differ by region and season ( Supplementary Fig. 13a-d). ...
Climate-driven changes in precipitation amounts and their seasonal variability are expected in many continental-scale regions during the remainder of the 21st century. However, much less is known about future changes in the predictability of seasonal precipitation, an important earth system property relevant for climate adaptation. Here, on the basis of CMIP6 models that capture the present-day teleconnections between seasonal precipitation and previous-season sea surface temperature (SST), we show that climate change is expected to alter the SST-precipitation relationships and thus our ability to predict seasonal precipitation by 2100. Specifically, in the tropics, seasonal precipitation predictability from SSTs is projected to increase throughout the year, except the northern Amazonia during boreal winter. Concurrently, in the extra-tropics predictability is likely to increase in central Asia during boreal spring and winter. The altered predictability, together with enhanced interannual variability of seasonal precipitation, poses new opportunities and challenges for regional water management.
... Despite the model consensus on the Walker circulation weakening by the late twenty-first century and the established links between the mean state and ENSO characteristics, there has been no consensus among climate models regarding the response of ENSO to global warming. A number of studies noted a stronger ENSO in warming climates in CMIP5 models, particularly when utilizing precipitation metrics (Huang and Xie 2015;Cai et al. 2015) or metrics that isolate Eastern Pacific (EP) or Central Pacific (CP) El Niño events . The tendency for a stronger ENSO in future warming scenarios has also been reported for subsets of CMIP6 models (Fredriksen et al. 2020;Cai et al. 2021;Brown et al. 2020). ...
The El Niño Southern Oscillation (ENSO) has profound impacts on weather patterns across the globe, yet there is no consensus on its response to global warming. Several modelling studies find a stronger ENSO in global warming scenarios, while other studies suggest ENSO weakening. Using a broad range of models from the Coupled Model Intercomparison Project phase 6 (CMIP6) and four types of warming experiments, here we show that the majority of the models predict a stronger ENSO by century-end in Shared Social Pathway (SSP) experiments, and in idealized 1pctCO2 and abrupt 4xCO2 experiments. Several models, however, do predict no change or ENSO weakening, especially in the idealized experiments. Critically, the strongest forcing (abrupt-4xCO2) does not induce the strongest ENSO response, while differences between the models are much greater than those between warming scenarios. For the long-term response (over 1000 years) the models disagree even on the sign of change. Furthermore, changes in ENSO sea surface temperature (SST) variability are only modestly correlated with the tropical Pacific mean state change. The highest correlation for ENSO SST amplitude is found with the mean zonal SST gradient in the SSP5-8.5 experiment (R = − 0.58). In contrast, changes in ENSO rainfall variability correlate well with changes in the mean state, as well as with changes in ENSO SST variability. When evaluating the Bjerknes Stability Index for a subset of models, we find that it is not a reliable predictor of ENSO strengthening, as this index tends to predict greater stability with warming. We argue that the enhanced ENSO stability is offset by increases in atmospheric noise or/and potential nonlinear effects. However, a robust inter-model mechanism that can explain a stronger ENSO simulated with global warming is still lacking. Therefore, caution should be exercised when considering ENSO changes based on a single model or warming scenario.
... Altogether, our results suggest that the already marked seasonality in the Chamela-Cuixmala region has T A B L E 4 Tropical cyclones climatology in the NEP # (% of total TCs) Average ± SD become more contrasting, a pattern likely to intensify in the future with drier dry seasons and wetter wet seasons (Murray-Tortarolo et al., 2016). This trend will be importantly influenced by the expected increase in both La Niña and El Niño extreme events (Cai et al., 2014(Cai et al., , 2015a(Cai et al., , 2015b, and other ocean-atmosphere oscillations could also influence seasonal rainfall patterns. Interestingly, we found a positive and significant correlation between AMO and precipitation during the wet season, and between PDO and precipitation during the dry season (Table S1). ...
Based on 40 years of meteorological data, we characterized and analysed long‐term climatic trends in the Chamela‐Cuixmala Biosphere Reserve on the Pacific coast of central Mexico. The region is still covered by a large proportion of well‐preserved tropical dry forest, characterized by a short period of rain and a dry season of around 8 months. We found a sustained temperature increase which is likely driven by global climate change. An increasing trend in rainfall was also found, but the trend was detected only during the wet season. Correspondingly, an increase in runoff during the rainfall season was also detected; this seems to be linked with an increasing number of intense rainfall events, rather than an increase in the total number of rainy days. El Niño years are likely to present below average precipitation during the wet season and above average precipitation during the dry season. The opposite is expected during La Niña years, when tropical cyclones are likely to come closer to the coast and cause intense rainfall events during the hurricane season. As many of the studied variables are likely to change under climate change scenarios, our results highlight the need to understand the expected impacts of global climate change on tropical dry forests. In particular, it is necessary to monitor changes in water availability to anticipate its consequences for the forest and the human communities that depend on it.
... The method of empirical orthogonal function (EOF) analysis can deconvolve the spatiotemporal variability of a signal into orthogonal modes, each indicated by a principal spatial pattern and the corresponding principal component time series. It is widely used to study spatial patterns of climate variability and how they change with time 60,61 . We perform the EOF analysis on simulated tropical soil moisture from CMIP6 models. ...
Terrestrial ecosystems have taken up about 32% of the total anthropogenic CO2 emissions in the past six decades¹. Large uncertainties in terrestrial carbon–climate feedbacks, however, make it difficult to predict how the land carbon sink will respond to future climate change². Interannual variations in the atmospheric CO2 growth rate (CGR) are dominated by land–atmosphere carbon fluxes in the tropics, providing an opportunity to explore land carbon–climate interactions3–6. It is thought that variations in CGR are largely controlled by temperature7–10 but there is also evidence for a tight coupling between water availability and CGR¹¹. Here, we use a record of global atmospheric CO2, terrestrial water storage and precipitation data to investigate changes in the interannual relationship between tropical land climate conditions and CGR under a changing climate. We find that the interannual relationship between tropical water availability and CGR became increasingly negative during 1989–2018 compared to 1960–1989. This could be related to spatiotemporal changes in tropical water availability anomalies driven by shifts in El Niño/Southern Oscillation teleconnections, including declining spatial compensatory water effects⁹. We also demonstrate that most state-of-the-art coupled Earth System and Land Surface models do not reproduce the intensifying water–carbon coupling. Our results indicate that tropical water availability is increasingly controlling the interannual variability of the terrestrial carbon cycle and modulating tropical terrestrial carbon–climate feedbacks.
... Furthermore, tropical cyclones, extreme sea level events including storm surges and flooding and precipitation over the ocean are predicted to increase in intensity and frequency through the first half of this century due to ocean circulation changes (discussed below) Hartmann et al. 2013;IPCC 2014IPCC , 2019Kirtman et al. 2013;Kopp et al. 2014;Ren et al. 2013). In addition, recent models and observational data indicate that recurring climate patterns such as the El Niño-Southern Oscillation are likely to increase in frequency and intensity as the ocean warms Cai et al. 2014Cai et al. , 2015IPCC 2019;Wang et al. 2017), with potentially important impacts on fishing, aquaculture and tourism operations. River flows and flooding may also change with increased snowmelt and more variable landbased precipitation, reducing salinity, increasing sedimentation and impacting productivity in nearshore waters (IPCC 2019;Jha et al. 2006;Pervez and Henebry 2015;Siderius et al. 2013;Loo et al. 2015). ...
The ocean is critically important to our global economy. Collectively, it is estimated that ocean-based industries and activities contribute hundreds of millions of jobs and approximately US$2.5 trillion to the global economy each year, making it the world’s seventh-largest economy when compared with national gross domestic products (GDPs) (Hoegh-Guldberg 2015; IPCC 2019). In addition, the nonmarket services and benefits provided by the ocean are significant and may in fact far exceed the value added by market-based goods and services (Costanza et al. 2014).
... In recent years, extreme weather phenomena have occurred frequently, seriously affecting human survival and development. El Niño-Southern Oscillation (ENSO) is the strongest signal of interannual climate change in the climate system and is the dominant climate phenomenon affecting extreme weather worldwide [1]. A significant impact caused by ENSO on global crop production [2], economic [3], and social development [4]. ...
... Beating the Heat: Investing in Pro-Poor Solutions for Urban Resilience Zhang, Held, and Fueglistaler 2021) as well as on the future (uncertain) behavior and interactions between important heat wave drivers such sea surface temperatures in the Pacific Ocean and Indian Ocean (Cai et al. 2015;Le and Bae 2019;McKenna et al. 2020;Yeh et al. 2018;Zheng et al. 2013). ...
As countries in Asia and the Pacific step up their climate commitments in the context of urban development, they open many opportunities for pursuing pro-poor urban resilience initiatives to reduce the increasing impacts of heat stress that urban areas face—in particular, the urban poor population. With this publication, we aim to increase awareness of such opportunities in countries. The report provides eight key recommendations that also offer a good basis for the Asian Development Bank (ADB) to scale up support for countries in dealing with issues relating to extreme heat. These recommended actions will succeed if cities adopt people-centered and integrated solutions in urban planning, delivery of basic services, health, social protection, livelihoods, gender equality, and environmental management.
... To the best of our knowledge, the impacts of projected AMOC slowdown have not been explicitly assessed in the context of anthropogenic warming via fully coupled climate model experiments. Also, it is well recognized that climate model projections of ENSO intensity have large uncertainties (Cai et al., 2022(Cai et al., , 2015Collins et al., 2010;Stevenson, 2012) because ENSO is controlled by a delicate balance of amplifying and damping feedback, which is modified by climate change simultaneously (Collins et al., 2010;Fedorov et al., 2020;Fedorov & Philander, 2000Hu & Fedorov, 2018;Zheng et al., 2016). So far, efforts to understand ENSO projections have mainly focused on local processes in the tropic Pacific, whilst remote effects such as those from the future AMOC slowdown have received less attention. ...
This study quantifies the impacts of the Atlantic Meridional Overturning Circulation (AMOC) on the El Niño–Southern Oscillation (ENSO) under anthropogenic warming by comparing climate change model simulations with declining and fixed strengths of the overturning. After the 1980s, a weakened AMOC is shown to reduce the strength of the annual cycle of sea surface temperature (SST) in the eastern equatorial Pacific and induce anomalous cross‐equatorial northerly winds there, which strengthens ENSO variability by about 11%. An analysis of the Bjerknes stability index reveals that this intensification of ENSO results mainly from enhanced Ekman upwelling feedback due to amplified atmospheric wind response to SST anomalies and oceanic upwelling response to equatorial wind stress anomalies. The weakened AMOC also promotes the occurrence of Central Pacific El Niño events and reduces ENSO skewness. These AMOC impacts on ENSO magnitude and complexity throughout the twenty‐first century are however smaller than ENSO variations expected from internal climate variability.
... We find that this rainfall RPC1 and the SST PC2 have a correlation of 0.87, and the spatial correlation between their corresponding rainfall patterns is 0.97. This suggests that like its counterpart in SST, this rainfall mode also reflects part of the nonlinearities associated with ENSO, including asymmetries between El Niño and La Niña and the differences between eastern Pacific (EP) El Niño and the central Pacific (CP) El Niño (Ashok et al. 2007;Kao and Yu 2009;Compo and Sardeshmukh 2010;Takahashi et al. 2011;Dommenget et al. 2013;Cai et al. 2015). ...
This study, based on an analysis with observational and reanalysis data, highlights seasonal tropical–extratropical atmospheric teleconnections originating from tropical rainfall modes unrelated to the Niño-3.4 index for northern winters. The mode decomposition for tropical rainfall is done by first removing the Niño-3.4 index–related variability from the tropical rainfall and then applying rotated empirical orthogonal function (REOF) analysis to the residual. The corresponding teleconnection patterns are obtained by regressing global atmospheric fields against the time series of the rainfall modes. Analyses of the tropical heating–atmospheric circulation relationship indicate that the circulation anomalies corresponding to the rainfall modes are forced responses to the corresponding rainfall mode. The teleconnection patterns reveal some new features and show that some intrinsic midlatitude patterns can be triggered by tropical forcing with different rainfall patterns. Results from this study are relevant to seasonal climate attribution and prediction analyses and climate model evaluation. As an illustration, the teleconnections from the rainfall modes, together with that related to the Niño-3.4 index and linear trend, are applied to the attribution analyses for the global circulation anomalies of 2019/20 winter and the California dry condition during the strong El Niño winter of 2015/16. The overall impact of these modes in the period of 1980–2021 is also discussed.
Significance Statement
This study highlights the seasonal tropical–extratropical atmospheric teleconnections independent of the Niño-3.4 index using tropical rainfall modes for northern winters. The reason for using rainfall rather than SST in the mode decomposition is that rainfall represents vertically integrated latent heat, which is the direct forcing of the tropical atmosphere, while SST may have no definite relationship with rainfall in the Indo-Pacific warm pool region. The results of this study are applicable to the analysis of climate attribution and prediction and climate model evaluation, and further, may also have the potential to help improve seasonal climate forecasts.
... ). EP and CP El Niño events are de ned here according to an adapted Trans-Niño-Index (TNI, Trenberth and Stepaniak 2001), measuring the normalized SSTA difference between eastern Paci c (120°W-80°W, 5°S-5°N) and the western Paci c (Niño4 region). Strong EP El Niño events are de ned as EP El Niño months, in which the SSTa in Niño3 is larger than 1.5 K. Extreme EP El Niño events are strong EP El Niño events, that additionally have more than 5 mm/day PR in the Niño3 region(Cai et al. 2015). According to this de nition e.g. the El Niño of 2015/2016 is a strong EP El Niño and the El Niños of 1982/1983 and 1997/1998 are extreme EP El Niños. ...
The El Niño/Southern Oscillation (ENSO) varied considerably over the last 140 years and the reason of that is under debate. The warm phase of ENSO is termed El Niño, the cold phase La Niña. Here we show that the difference between periods of high and low ENSO variability results mainly from the amplitude of Eastern Pacific (EP) El Niños, while the amplitudes of Central Pacific (CP) El Niños and La Niñas are comparable. Further, ENSO asymmetry and amplitude covary, suggesting that the number of strong EP El Niños dominates both. We find similar relations in the 40 historical runs of the Large Ensemble from the CESM1-CAM5-BGC model. Finally, the fraction of strong EP El Niños explains the spread in ENSO amplitude and asymmetry in preindustrial control simulations from the CMIP6 database. This study emphasizes the need of a better understanding of the deterministic and stochastic factors that are important for the development of strong EP El Niño events in observations and climate models.
Emissions of biogenic volatile organic compounds (BVOCs) from the terrestrial biosphere play a significant role in major atmospheric processes. BVOCs are highly reactive compounds that influence the atmosphere's oxidation capacity and also serve as precursors for the formation of aerosols that influence global radiation budgets. Emissions depend on the response of vegetation to atmospheric conditions (primarily temperature and light), as well as other stresses, e.g. from droughts and herbivory. The El Niño–Southern Oscillation (ENSO) is a naturally occurring cycle arising from anomalies in the sea surface temperature (SST) in the tropical Pacific. ENSO perturbs the natural seasonality of weather systems on both global and regional scales and is considered the most significant driver of climate variability. Several studies have evaluated the sensitivity of BVOC fluxes during ENSO events using historical transient simulations. While this approach employs realistic scenarios, it is difficult to assess the impact of ENSO alone given the multiple types of climate forcing, e.g. from anthropogenic emissions of CO2 and aerosol. In this study, a global atmospheric chemistry–climate model with enabled interactive vegetation was used to conduct two sets of simulations: (1) isolated ENSO event simulations, in which a single ENSO event is used to perturb otherwise baseline conditions, and (2) sustained ENSO simulations, in which the same ENSO conditions are reproduced for an extended period of time. From the isolated ENSO events, we present global and regional BVOC emission changes resulting from the immediate response of vegetation to atmospheric states. More focus is given to the sustained ENSO simulations, which have the benefit of reducing the internal variability for more robust statistics when linking atmospheric and vegetation variables with BVOC flux anomalies. Additionally, these simulations explore long-term changes in the biosphere with potential shifts in vegetation in this possible climate mode, accounting for the prospect of increased intensity and frequency of ENSO with climate change. Our results show that strong El Niño events increase global isoprene emission fluxes by 2.9 % and that one single ENSO event perturbs the Earth system so markedly that BVOC emission fluxes do not return to baseline emissions within several years after the event. We show that persistent ENSO conditions shift the vegetation to a new quasi-equilibrium state, leading to an amplification of BVOC emission changes with up to a 19 % increase in isoprene fluxes over the Amazon. We provide evidence that BVOC-induced changes in plant phenology, such as the leaf area index (LAI), have a significant influence on BVOC emissions in the sustained ENSO climate mode.
A notable shift in the El Niño‐Southern Oscillation (ENSO) has been observed in the early 21st century, characterized by an increased prevalence of Central Pacific (CP) events and strengthened Pacific trade winds. This shift may be attributed to the warming tropical Indian Ocean (TIO). To investigate this, we conduct perturbation experiments using the Insitut Pierre Simon Laplace climate model and nudge TIO surface temperatures to induce warming or cooling effects. Our findings reveal that TIO warming (or cooling) leads to amplified (weakened) mean trade winds and surface warming (cooling) in the Pacific region. Surprisingly, ENSO variability increases in both TIO cooling and warming scenarios. This result is linked to stronger positive feedbacks and a less stable Bjerknes index for either TIO forcing. Additionally, we find that TIO warming leads to more frequent CP events, meridional widening of wind anomalies, and broadening of the ENSO power spectrum toward lower frequencies.
Climate change is currently the world’s most serious environmental and meteorological challenge. Climate change has a negative impact on agriculture, water resources, forests, health, biodiversity, ecology, socioeconomics, and coastlines. Agriculture is the most vulnerable to climate change, and it is India’s backbone, with 70–80% of the population relying solely on rainfed crop production for food. In the unprecedented increasing population, urbanization, industrialization creating additional stress and facing pressure with limited sources of water demands. Adaptation strategies must be designed to accommodate climatic and non-climatic stress for existing anthropogenic driven beyond climate change control. Climate change is the foremost environmental challenge associated with climate variability. It is impact on the decline of agricultural production and crop areas, to fulfil increasing food demand, water resources, forest and biodiversity, health, coastal management, ecological, socioeconomic (rapid industrialization, urbanization, economic development and increase in temperature). Climate change or climate variability also brings a susceptible epidemic pests and diseases over Indian continents. Stamping out the poverty thereby rendering good living standards and basic amenities (food, water and shelter) to Indian citizens is India’s utmost task. India has a target of mitigation towards less emission of GHG.
The El Niño Southern Oscillation (ENSO), as one of the largest coupled climate modes, influences the livelihoods of millions of people and ecosystems survival. Thus, how ENSO is expected to behave under the influence of anthropogenic climate change is a substantial question to investigate. In this paper, we analyze future predictions of specific traits of ENSO, in combination with a subset of well-established precursors—the Trade Wind Charging and North Pacific Meridional Mode (TWC/NPMM). We study it across three sets of experiments from a protocol-driven ensemble from CMIP6—the High Resolution Model Intercomparison Project (HighResMIP). Namely, (1) experiments at constant 1950’s radiative forcings, and (2) experiments of present (1950–2014) and (3) future (2015–2050) climate with prescribed increasing radiative forcings. We first investigate the current and predicted spatial characteristics of ENSO events, by calculating area, amplitude and longitude of the Center of Heat Index (CHI). We see that TWC/NPMM-charged events are consistently stronger, in both the presence and absence of external forcings; however, as anthropogenic forcings increase, the area of all ENSO events increases. Since the TWC/NPMM-ENSO relationship has been shown to affect the oscillatory behavior of ENSO, we analyze ENSO frequency by calculating CHI-analogous indicators on the Continuous Wavelet Transform (CWT) of its signal. With this new methodology, we show that across the ensemble, ENSO oscillates at different frequencies, and its oscillatory behavior shows different degrees of stochasticity, over time and across models. However, we see no consistent indication of future trends in the oscillatory behavior of ENSO and the TWC/NPMM-ENSO relationship.
The essential micronutrient iron (Fe) limits phytoplankton growth when dissolved Fe (dFe) concentrations are too low to meet biological demands. However, many of the processes that remove, supply, or transform Fe are poorly constrained, which limits our ability to predict how ocean productivity responds to ongoing and future changes in climate. In recent years, isotopic signatures (δ56Fe) of Fe have increasingly been used to gain insight into the ocean Fe cycle, as distinct δ56Fe endmembers of external Fe sources and δ56Fe fractionation during processes such as Fe uptake by phytoplankton can leave a characteristic imprint on dFe signatures (δ56Fediss). However, given the relative novelty of these measurements, the temporal scale of δ56Fediss observations is limited. Thus, it is unclear how the changes in ocean physics and biogeochemistry associated with ongoing or future climate change will affect δ56Fediss on interannual to decadal timescales. To explore the response of δ56Fediss to such climate variability, we conducted a suite of experiments with a global ocean model with active δ56Fe cycling under two climate scenarios. The first scenario is based on an atmospheric reanalysis and includes recent climate variability (1958–2021), whereas the second comes from a historical and high-emissions climate change simulation to 2100. We find that under recent climatic conditions (1975–2021), interannual δ56Fediss variability is highest in the tropical Pacific due to circulation and productivity changes related to the El Niño–Southern Oscillation (ENSO), which alter both endmember and uptake fractionation effects on δ56Fediss by redistributing dFe from different external sources and shifting nutrient limitation patterns. While the tropical Pacific will remain a hotspot of δ56Fediss variability in the future, the most substantial end-of-century δ56Fediss changes will occur in the Southern Hemisphere at middle to high latitudes. These arise from uptake fractionation effects due to shifts in nutrient limitation. Based on these strong responses to climate variability, ongoing measurements of δ56Fediss may help diagnose changes in external Fe supply and ocean nutrient limitation.
In this study, we investigate the potential changes of El Niño characteristics, including intensity, frequency and CP/EP El Niño ratio, under progressive global warming based on the 140-year CMIP6 model simulation outputs with the 1pctCO2 experiment. Major air-sea feedback mechanisms attributing to the changes are also examined. The CMIP6 ensemble means project a slight enhancement of El Niño intensity by about 2% and a modest increase of El Niño frequency by about 4% from the first to the second 70-year periods. It is found that these small changes result from the opposite response to global warming between CP and EP El Niño, i.e., the intensity of EP El Niño is projected to weaken by nearly 4.6% while the intensity of CP El Niño is projected to increase by about 4.5%. Since CP El Niño occurs more frequently than EP El Niño in CMIP6 simulations, this leads to a slight enhancement of the total El Niño
intensity if these two types of El Niño were not separated. A similar situation occurs in projecting the future change of El Niño frequency, i.e., the frequency of EP El Niño is projected to decrease by about 1.4% while the frequency of CP El Niño is projected to increase by about 2%, thereby leading to a modest increase of the total El Niño frequency. By comparing the variance explained by key air-sea feedback mechanism between the two 70-year periods, we also note that the increased CP/EP ratio can be explained by the enhanced role played by the SF (seasonal footprinting) mechanism in a warmer atmosphere. Our study also points out that, as long as a climate model can correctly produce the intensity (variance) of major air-sea feedback mechanisms, the relationship between changes in El Niño characteristics and changes in feedback mechanisms can be physically robust.
This study investigates the combined effects of Arctic tropospheric warming and La Niña events on climate variations in Eurasia during boreal winter by utilizing the reanalysis data and the output of CMIP6 models. Results show that the “Warm Arctic-Cold Eurasia” pattern emerges with enhanced intensity under the combined effects compared with the individual effect of Arctic tropospheric warming or La Niña events. The significantly intensified Eurasian cold anomaly is observed in the north of Lake Baikal during the combined years. However, this cold anomaly shrinks substantially in warm Arctic years while shifting southwestward with a weaker intensity in La Niña years. Additionally, the winter atmospheric variability under the combined effects is much stronger than that under the individual effect of these two factors. Further investigation reveals that the La Niña event is probably crucial to dominating the basic distribution of winter temperature variability and the associated tropospheric circulation anomalies. Recent Arctic tropospheric warm anomalies are also essential, possibly amplifying the impact of the La Niña event on Eurasia through the weakened stratospheric polar vortex. These two factors jointly contribute to the enhanced “+-+” wave train that displays over mid-high latitudes under the combined effects. This “+-+” wave train exhibits alternative anticyclonic, cyclonic, and anticyclonic circulation anomalies over the Ural Mountains, Lake Baikal, and North Pacific, respectively. It guides wave energy from the Barents-Kara Seas (BKS) along the Ural Mountains to Lake Baikal and promotes the development of the Ural blocking anomaly that favors the intensified extreme cold temperature in Eurasia. The results of the CMIP6 models largely replicate some major features of the combined effects in the observations.
Precipitation forecasts are of high significance for different disciplines. In this study, precipitation was forecasted using a wide range of teleconnection signals across different precipitation regimes. For this purpose, four sophisticated machine learning algorithms, i.e., the Generalized Regression Neural Network (GRNN), the Multi-Layer Perceptron (MLP), the Multi-Linear Regression (MLR), and the Least Squares Support Vector Machine (LSSVM), were applied to forecast seasonal and annual precipitation in 1- to 6-months lead times. To classify precipitation regimes, precipitation was clustered using percentiles. The indices quantifying El Niño-Southern Oscillation (ENSO) phasing showed the highest association with autumn, spring, and annual precipitation over the studied areas. The MLP and LSSVM algorithms provided satisfactory forecasts for almost all cases. However, our results indicated that the performance of LSSVM decreased in testing data, implying the tendency of this algorithm towards overfitting. The MLP showed a more balanced performance for the training and testing sets. Consequently, MLP seems best suited to be used for forecasting precipitation in our study area. The modeling algorithms provided less reliable forecasts for the regions corresponding to the 10–40th percentiles, mostly located in hyper-arid and arid environments. This underscores the inherent difficulty of precipitation forecasting in the hyper-arid and arid areas, wherein precipitation is very erratic and sparsely distributed. Our findings illustrate that clustering precipitation regimes to consider microclimate seems vital for reliable precipitation forecasting. Moreover, the results seem useful to design preventive drought/flood risk management strategies and to improve food-water security in Iran.
Improvements in the seasonal prediction system Scale Interaction Experiment‐Frontier version 2 (SINTEX‐F2) achieved via atmospheric initialisation were evaluated using hindcast experiments. The atmospheric initialisation in the system was implemented using an atmospheric nudging scheme. The prediction skill of SINTEX‐F2 with and without initialisation (new and original systems) for nine climate indices that represent the interannual variability were evaluated against the observational data. It was found that the prediction skill for some climate indices in the Tropics and most of the climate indices in the midlatitude including subtropics was improved by the atmospheric initialisation. The skills for El Niño–Southern Oscillation (ENSO) and ENSO Modoki, however, remained comparable with the original system. Further investigation for the prediction skill revealed that the sea level pressure (SLP) in the midlatitude was predicted better by the new system than in the original system. The 2‐m temperature was better predicted by the new system, due to the improved SLP prediction. The precipitation prediction was improved in regions where the improved interannual variability had an influence on the variability of precipitation. The difference in air–sea coupling regimes was found to be effective for improving the prediction skill in different latitudes. The atmospheric nudging enhances correct atmospheric feedback on the ocean in midlatitude leading to better subsurface ocean properties; thus, it can improve the prediction skill there. However, by neglecting the ocean side feedback, it deteriorates the prediction skill in Tropics where the sea surface temperature (SST) feedback on the atmosphere is dominant. The finding for the prediction skill is general and can be used for other prediction systems to increase their prediction skill using an atmospheric nudging scheme.
This commentary discusses new advances in the predictability of east African rains and highlights the potential for improved early warning systems (EWS), humanitarian relief efforts, and agricultural decision‐making. Following an unprecedented sequence of five droughts, 23 million east Africans faced starvation in 2022, requiring >$2 billion in aid. Here, we update climate attribution studies showing that these droughts resulted from an interaction of climate change and La Niña. Then we describe, for the first time, how attribution‐based insights can be combined with the latest dynamical models to predict droughts at 8‐month lead‐times. We then discuss behavioral and social barriers to forecast use, and review literature examining how EWS might (or might not) enhance agro‐pastoral advisories and humanitarian interventions. Finally, in reference to the new World Meteorological Organization “Early Warning for All” Executive Action Plan, we conclude with a set of recommendations supporting actionable and authoritative climate services. Trust, urgency, and accuracy can help overcome barriers created by limited funding, uncertain tradeoffs, and inertia. Understanding how climate change is producing predictable climate extremes now, investing in African‐led EWS, and building better links between EWS and agricultural development efforts can support long‐term adaptation, reducing chronic needs for billions of dollars in reactive assistance. In Africa and beyond, climate change brings increasingly extreme sea surface temperature (SST) gradients. Using climate models, we can often see these extremes coming. Prediction, therefore, offers opportunities for proactive risk management and improved advisory services, if we can create effective societal linkages via cross‐silo collaborations.
The mean state of the tropical Pacific ocean-atmosphere climate, in particular its east-west asymmetry, has profound consequences for regional climates and for the El Niño/Southern Oscillation variability. Here we present a new high-resolution paleohydrological record using the stable-hydrogen-isotopic composition of terrestrial-lipid biomarkers (δDwax) from a 1,400-year-old lake sedimentary sequence from northern Philippines. Results show a dramatic and abrupt increase in δDwax values around 1630 AD with sustained high values until around 1900 AD. We interpret this change as a shift to significantly drier conditions in the western tropical Pacific during the second half of the Little Ice Age as a result of a change in tropical Pacific mean state tied to zonal sea surface temperature (SST) gradients. Our findings highlight the prominent role of abrupt shifts in zonal SST gradients on multidecadal to multicentennial timescales in shaping the tropical Pacific hydrology of the last millennium, and demonstrate that a marked transition in the tropical Pacific mean state can occur within a period of a few decades.
Multi-factor experiments suggest that interactions among environmental changes commonly influence biodiversity and community composition. However, most field experiments manipulate only single factors. Soil food webs are critical to ecosystem health and may be particularly sensitive to interactions among environmental changes that include soil warming, eutrophication, and altered precipitation. Here, we asked how environmental changes interacted to alter soil nematode communities in a northern Chihuahuan Desert grassland. Factorial manipulations of nitrogen, winter rainfall, and nighttime warming matched predictions for regional environmental change. Warming reduced nematode diversity by 25% and genus-level richness by 32%, but declines dissipated with additional winter rain, suggesting that warming effects occurred via drying. Interactions between precipitation and nitrogen also altered nematode community composition, but only weakly affected total nematode abundance, indicating that most change involved reordering of species abundances. Specifically, under ambient precipitation, nitrogen fertilizer reduced bacterivores by 68% and herbivores by 73%, but did not affect fungivores. In contrast, under winter rain addition, nitrogen fertilization increased bacterivores by 95%, did not affect herbivores, and doubled fungivore abundance. Rain can reduce soil nitrogen availability and increase turnover in the microbial loop, potentially promoting the recovery of nematode populations overwhelmed by nitrogen eutrophication. Nematode communities were not tightly coupled to plant community composition and may instead track microbes, including biocrusts or decomposers. Our results highlight the importance of interactions among environmental change stressors for shaping the composition and function of soil food webs in drylands.
Convective extreme El Niño (CEE) events, characterized by strong convective events in the eastern Pacific, are known to have a direct link to anomalous climate conditions worldwide, and it has been reported that CEE will occur more frequently under greenhouse warming. Here, using a set of CO2 ramp-up and ramp-down ensemble experiments, we show that frequency and maximum intensity of CEE events increase further in the ramp-down period from the ramp-up period. These changes in CEE are associated with the southward shift of the intertropical convergence zone and intensified nonlinear rainfall response to sea surface temperature change in the ramp-down period. The increasing frequency of CEE has substantial impacts on regional abnormal events and contributed considerably to regional mean climate changes to the CO2 forcings.
It has been demonstrated that the increased CO2 concentration would influence El Niño and its connected precipitation anomaly over East Asia (EA). Based on the model simulations from CMIP5 and CMIP6, this study investigates projected change of the boreal winter precipitation anomaly in EA during strong Eastern-Pacific type El Niño (EP-El Niño) responding to different emission scenarios and further examines the possible mechanisms. Features of the EA precipitation anomaly associated with EP-El Niño can be reasonably captured by most of the CMIP5 models, but not substantially improved by the CMIP6 models. As emissions increase, the positive precipitation anomalies over the northern EA (NEA) during strong EP-El Niños tend to be more intense, while the precipitation anomalies decrease over southern EA (SEA). Such a change pattern is generally consistent between CMIP5 and CMIP6 models, which can be intimately related to the changes of circulation and moisture transport. That is, the changed cyclonic (anticyclonic) anomaly pattern over NEA (SEA) is favorable (unfavorable) for the formation of precipitation pattern with the associated enhanced (weakened) moisture supply anomaly. Further analysis shows that the strong EP-El Niño itself acts to increase precipitation anomaly over most of NEA compared with historical simulations, while its induced combination mode contributes to the relatively large inconsistency over SEA between CMIP5 and CMIP6.
El Niño Southern Oscillation (ENSO) has a profound influence on the occurrence of extreme precipitation events at local and regional scales in the present-day climate, and thus it is important to understand how that influence may change under future global warming. We consider this question using the large ensemble simulations of CESM2, which simulates ENSO well historically. CESM2 projects that the influence of ENSO on extreme precipitation will strengthen further under the SSP3-7.0 scenario in most regions whose extreme precipitation regimes are strongly affected by ENSO in the boreal cold season. Extreme precipitation in the boreal cold season that exceeds historical thresholds is projected to become more common throughout the ENSO cycle. The difference in the intensity of extreme precipitation events that occur under El Niño and La Niña conditions will increase, resulting in “more extreme and more variable hydroclimate extremes”. We also consider the processes that affect the future intensity of extreme precipitation and how it varies with the ENSO cycle by partitioning changes into thermodynamic and dynamic components. The thermodynamic component, which reflects increases in atmospheric moisture content, results in a relatively uniform intensification of ENSO-driven extreme precipitation variation. In contrast, the dynamic component, which reflects changes in vertical motion, produces strong regional difference in the response to forcing. In some regions, this component amplifies the thermodynamic-induced changes, while in others, it offsets them or even results in reduction in extreme precipitation variation.
The Mississippi River represents a major commercial waterway, and periods of anomalously low river levels disrupt riverine transport. These low-flow events occur periodically, with a recent event in the fall of 2022 slowing barge traffic and generating sharp increases in riverine transportation costs. Here we combine instrumental river gage observations from the lower Mississippi River with output from the Community Earth System Model v2 (CESM2) Large Ensemble (LENS2) to evaluate historical trends and future projections of Mississippi River low streamflow extremes, place the 2022 low-flow event in a broader temporal context, and assess the hydroclimatic mechanisms that mediate the occurrence of low-flows. We show that the severity and duration of low-flow events gradually decreased between 1950–1980 coincident with the establishment of artificial reservoirs. In the context of the last ~70 years, the 2022 low-flow event was less severe in terms of stage or discharge minima than other low-flow events of the mid- and late-20th century. Model simulations from the LENS2 dataset show that, under a moderate-high emissions scenario (SSP3-7.0), the severity and duration of low-flow events is projected to decrease through to the end of the 21st century. Finally, we use the large sample size afforded by the LENS2 dataset to show that low-flow events on the Mississippi River are associated with cold tropical Pacific forcing (i.e., La Niña conditions), providing support for the hypothesis that the El Niño-Southern Oscillation (ENSO) plays a critical role in mediating Mississippi River discharge extremes. We anticipate that our findings describing the trends in and hydroclimatic mechanisms of Mississippi River low-flow occurrence will aid water resource managers to reduce the negative impacts of low water levels on riverine transport.
The global emission of CO2 through fossil fuel combustion is still increasing, which is a major challenge for the international community. An integrated carbon capture and utilisation (ICCU) process with a CaO-based sorbent is a promising alternative to effectively reduce emissions. In this work, a comparative thermodynamic analysis of two CaO-based sorbents (commercial and sol-gel CaO) was performed for one cycle of ICCU. In addition, the influence of temperature was investigated from 600 to 750 °C in terms of the degree of CO2 conversion. Thermodynamic calculations were based on the actual gas composition and developed model, where heat consumption and entropy generation were calculated. The results indicate that the degree of CO2 conversion decreased from 84.6 to 41.2% and from 84.1 to 62.4% for the sol-gel and commercial material, respectively, as the temperatures increased. Furthermore, the total heat consumption during one cycle decreased with higher temperatures. The total amount of consumed heat decreased from 19.1 to 5.9 kJ/g and from 24.7 to 5.4 kJ/g for sol-gel and commercial CaO, respectively. Although commercial CaO always requires more heat during one cycle. Moreover, for both materials, the lowest generation of entropy was calculated at 650 °C with values of 9.5 and 10.1 J/g·K for the sol-gel and the commercial CaO, respectively. At all temperatures, the commercial CaO generated a greater entropy.
Improving prediction of ecological responses to climate variability requires understanding how local fish population dynamics are impacted by climate events. The present study was conducted in the Beibu Gulf of the northwestern South China Sea where the fisheries are characterized by high ecological and commercial value. We evaluated the relationship between major commercial fish population dynamics (abundance and distribution) and climate periods, using survey data from 2006–2020. The analysis using random forest and GAM models show that climate events are not the best predictors for the variations of fish abundance, because abundance of most fish stocks decreases significantly with the year, and the increasing fishing pressure over time can better explain the overall downward trend in fishery stocks. However, environmental variables that correlate significantly with interannual variation in ONI may impact fish abundance in short terms. Our research suggests that climate events leading to higher surface seawater salinity in winter favors pelagic fishes by improving habitat availability, and higher near-surface chlorophyll-α concentration during La Niña events provides better food condition for overwintering fish. In addition, there is no clear evidence that climatic events have a significant impact on gravity center of fish distribution, whereas climate change has caused most fishes to move to cooler coastal waters in the north.
El Nino events, characterized by anomalous warming in the eastern equatorial Pacific Ocean, have global climatic teleconnections and are the most dominant feature of cyclic climate variability on subdecadal timescales. Understanding changes in the frequency or characteristics of El Nino events in a changing climate is therefore of broad scientific and socioeconomic interest. Recent studies(1-5) show that the canonical El Nino has become less frequent and that a different kind of El Nino has become more common during the late twentieth century, in which warm sea surface temperatures (SSTs) in the central Pacific are flanked on the east and west by cooler SSTs. This type of El Nino, termed the central Pacific El Nino (CP-El Nino; also termed the dateline El Nino(2), El Nino Modoki(3) or warm pool El Nino(5)), differs from the canonical eastern Pacific El Nino (EP-El Nino) in both the location of maximum SST anomalies and tropical-midlatitude teleconnections. Here we show changes in the ratio of CP-El Nino to EP-El Nino under projected global warming scenarios from the Coupled Model Intercomparison Project phase 3 multi-model data set(6). Using calculations based on historical El Nino indices, we find that projections of anthropogenic climate change are associated with an increased frequency of the CP-El Nino compared to the EP-El Nino. When restricted to the six climate models with the best representation of the twentieth-century ratio of CP-El Nino to EP-El Nino, the occurrence ratio of CP-El Nino/EP-El Nino is projected to increase as much as five times under global warming. The change is related to a flattening of the thermocline in the equatorial Pacific.
El Niño-Southern Oscillation (ENSO) induces climate anomalies around the globe. Atmospheric general circulation model simulations are used to investigate how ENSO-induced teleconnection patterns during boreal winter might change in response to global warming in the Pacific-North American sector. As models disagree on changes in the amplitude and spatial pattern of ENSO in response to global warming, for simplicity the same sea surface temperature (SST) pattern of ENSO is prescribed before and after the climate warming. In a warmer climate, precipitation anomalies intensify and move eastward over the equatorial Pacific during El Niño because the enhanced mean SST warming reduces the barrier to deep convection in the eastern basin. Associated with the eastward shift of tropical convective anomalies, the ENSO-forced Pacific-North American (PNA) teleconnection pattern moves eastward and intensifies under the climate warming. By contrast, the PNA mode of atmospheric internal variability remains largely unchanged in pattern, suggesting the importance of tropical convection in shifting atmospheric teleconnections. As the ENSO-induced PNA pattern shifts eastward, rainfall anomalies are expected to intensify on the west coast of North America, and the El Niño-induced surface warming to expand eastward and occupy all of northern North America. The spatial pattern of the mean SST warming affects changes in ENSO teleconnections. The teleconnection changes are larger with patterned mean warming than in an idealized case where the spatially uniform warming is prescribed in the mean state. The results herein suggest that the eastward-shifted PNA pattern is a robust change to be expected in the future, independent of the uncertainty in changes of ENSO itself.
El Niño-Southern Oscillation (ENSO) consists of irregular episodes of warm El Niño and cold La Niña conditions in the tropical Pacific Ocean1, with significant global socio-economic and environmental impacts1. Nevertheless, forecasting ENSO at lead times longer than a few months remains a challenge2, 3. Like the Pacific Ocean, the Indian Ocean also shows interannual climate fluctuations, which are known as the Indian Ocean Dipole4, 5. Positive phases of the Indian Ocean Dipole tend to co-occur with El Niño, and negative phases with La Niña6, 7, 8, 9. Here we show using a simple forecast model that in addition to this link, a negative phase of the Indian Ocean Dipole anomaly is an efficient predictor of El Niño 14 months before its peak, and similarly, a positive phase in the Indian Ocean Dipole often precedes La Niña. Observations and model analyses suggest that the Indian Ocean Dipole modulates the strength of the Walker circulation in autumn. The quick demise of the Indian Ocean Dipole anomaly in November–December then induces a sudden collapse of anomalous zonal winds over the Pacific Ocean, which leads to the development of El Niño/La Niña. Our study suggests that improvements in the observing system in the Indian Ocean region and better simulations of its interannual climate variability will benefit ENSO forecasts.
The global climate during 1998 was affected by opposite extremes of the ENSO cycle, with one of the strongest Pacific warm episodes (El Niño) in the historical record continuing during January-early May and Pacific cold episode (La Niña) conditions occurring from July-December. In both periods, regional temperature, rainfall, and atmospheric circulation patterns across the Pacific Ocean and the Americas were generally consistent with those observed during past warm and cold episodes. Some of the most dramatic impacts from both episodes were observed in the Tropics, where anomalous convection was evident across the entire tropical Pacific and in most major monsoon regions of the world. Over the Americas, many of the El Niño- (La Niña-) related rainfall anomalies in the subtropical and extratropical latitudes were linked to an extension (retraction) of the jet streams and their attendant circulation features typically located over the subtropical latitudes of both the North Pacific and South Pacific. The regions most affected by excessive El Niño-related rainfall included 1) the eastern half of the tropical Pacific, including western Ecuador and northwestern Peru, which experienced significant flooding and mudslides; 2) southeastern South America, where substantial flooding was also observed; and 3) California and much of the central and southern United States during January-March, and the central United States during April-June. El Niño-related rainfall deficits during 1998 included 1) Indonesia and portions of northern Australia; 2) the Amazon Basin, in association with a substantially weaker-than-normal South American monsoon circulation; 3) Mexico, which experienced extreme drought throughout the El Niño episode; and 4) the Gulf Coast states of the United States, which experienced extreme drought during April-June 1998. The El Niño also contributed to extreme warmth across North America during January-May. The primary La Niña-related precipitation anomalies included 1) increased rainfall across Indonesia, and a nearly complete disappearance of rainfall across the east-central equatorial Pacific; 2) above-normal rains across northwestern, eastern, and northern Australia; 3) increased monsoon rains across central America and Mexico during October-December; and 4) dryness across equatorial eastern Africa. The active 1998 North Atlantic hurricane season featured 14 named storms (9 of which became hurricanes) and the strongest October hurricane (Mitch) in the historical record. In Honduras and Nicaragua extreme flooding and mudslides associated with Hurricane Mitch claimed more than 11 000 lives. During the peak of activity in August-September, the vertical wind shear across the western Atlantic, along with both the structure and location of the African easterly jet, were typical of other active seasons. Other regional aspects of the short-term climate included 1) record rainfall and massive flooding in the Yangtze River Basin of central China during June-July; 2) a drier and shorter-than-normal 1997/98 rainy season in southern Africa; 3) above-normal rains across the northern section of the African Sahel during June-September 1998; and 4) a continuation of record warmth across Canada during June-November. Global annual mean surface temperatures during 1998 for land and marine areas were 0.56°C above the 1961-90 base period means. This record warmth surpasses the previous highest anomaly of +0.43°C set in 1997. Record warmth was also observed in the global Tropics and Northern Hemisphere extratropics during the year, and is partly linked to the strong El Niño conditions during January-early May.
It has been previously proposed that two El Niño (EN) regimes, strong and moderate, exist but the historical observational record is too short to establish this conclusively. Here, 1200 years of simulations with the GFDL CM2.1 model allowed us to demonstrate their existence in this model and, by showing that the relevant dynamics are also evident in observations, we present a stronger case for their existence in nature. In CM2.1, the robust bimodal probability distribution of equatorial Pacific sea surface temperature (SST) indices during EN peaks provides evidence for the existence of the regimes, which is also supported by a cluster analysis of these same indices. The observations agree with this distribution, with the EN of 1982-1983 and 1997-1998 corresponding to the strong EN regime and all the other observed EN to the moderate regime. The temporal evolution of various indices during the observed strong EN agrees very well with the events in CM2.1, providing further validation of this model as a proxy for nature. The two regimes differ strongly in the magnitude of the eastern Pacific warming but not much in the central Pacific. Observations and model agree in the existence of a finite positive threshold in the SST anomaly above which the zonal wind response to warming is strongly enhanced. Such nonlinearity in the Bjerknes feedback, which increases the growth rate of EN events if they reach sufficiently large amplitude, is very likely the essential mechanism that gives rise to the existence of the two EN regimes. Oceanic nonlinear advection does not appear essential for the onset of strong EN. The threshold nonlinearity could make the EN regimes very sensitive to stochastic forcing. Observations and model agree that the westerly wind stress anomaly in the central equatorial Pacific in late boreal summer has a substantial role determining the EN regime in the following winter and it is suggested that a stochastic component at this time was key for the development of the strong EN towards the end of 1982.
The El Nino/Southern Oscillation is Earth's most prominent source of interannual climate variability, alternating irregularly between El Nino and La Nina, and resulting in global disruption of weather patterns, ecosystems, fisheries and agriculture(1-5). The 1998-1999 extreme La Nina event that followed the 1997-1998 extreme El Nino event(6) switched extreme El Nino-induced severe droughts to devastating floods in western Pacific countries, and vice versa in the southwestern United States(4,7). During extreme La Nina events, cold sea surface conditions develop in the central Pacific(8,9), creating an enhanced temperature gradient from the Maritime continent to the central Pacific. Recent studies have revealed robust changes in El Nino characteristics in response to simulated future greenhouse warming(10-12), but how La Nina will change remains unclear. Here we present climate modelling evidence, from simulations conducted for the Coupled Model Intercomparison Project phase 5 (ref. 13), for a near doubling in the frequency of future extreme La Nina events, from one in every 23 years to one in every 13 years. This occurs because projected faster mean warming of the Maritime continent than the central Pacific, enhanced upper ocean vertical temperature gradients, and increased frequency of extreme El Nino events are conducive to development of the extreme La Nina events. Approximately 75% of the increase occurs in years following extreme El Nino events, thus projecting more frequent swings between opposite extremes from one year to the next.
El Niño Southern Oscillation (ENSO) is a naturally occurring mode of tropical Pacific variability, with global impacts on society and natural ecosystems. While it has long been known that El Niño events display a diverse range of amplitudes, triggers, spatial patterns, and life cycles, the realization that ENSO's impacts can be highly sensitive to this event-to-event diversity is driving a renewed interest in the subject. This paper surveys our current state of knowledge of ENSO diversity, identifies key gaps in understanding, and outlines some promising future research directions.
The destructive environmental and socio-economic impacts of the El Niño/Southern Oscillation (ENSO) demand an improved understanding of how ENSO will change under future greenhouse warming. Robust projected changes in certain aspects of ENSO have been recently established. However, there is as yet no consensus on the change in the magnitude of the associated sea surface temperature (SST) variability, commonly used to represent ENSO amplitude, despite its strong eeects on marine ecosystems and rainfall worldwide. Here we show that the response of ENSO SST amplitude is time-varying, with an increasing trend in ENSO amplitude before 2040, followed by a decreasing trend thereafter. We attribute the previous lack of consensus to an expectation that the trend in ENSO amplitude over the entire twenty-first century is unidirectional, and to unrealistic model dynamics of tropical Pacific SST variability. We examine these complex processes across 22 models in the Coupled Model Intercomparison Project phase 5 (CMIP5) database, forced under historical and greenhouse warming conditions. The nine most realistic models identified show a strong consensus on the time-varying response and reveal that the non-unidirectional behaviour is linked to a longitudinal difference in the surface warming rate across the Indo-Pacific basin. Our results carry important implications for climate projections and climate adaptation pathways.
The Indian Ocean dipole is a prominent mode of coupled ocean-atmosphere variability, affecting the lives of millions of people in Indian Ocean rim countries. In its positive phase, sea surface temperatures are lower than normal off the Sumatra-Java coast, but higher in the western tropical Indian Ocean. During the extreme positive-IOD (pIOD) events of 1961, 1994 and 1997, the eastern cooling strengthened and extended westward along the equatorial Indian Ocean through strong reversal of both the mean westerly winds and the associated eastward-flowing upper ocean currents. This created anomalously dry conditions from the eastern to the central Indian Ocean along the Equator and atmospheric convergence farther west, leading to catastrophic floods in eastern tropical African countries but devastating droughts in eastern Indian Ocean rim countries. Despite these serious consequences, the response of pIOD events to greenhouse warming is unknown. Here, using an ensemble of climate models forced by a scenario of high greenhouse gas emissions (Representative Concentration Pathway 8.5), we project that the frequency of extreme pIOD events will increase by almost a factor of three, from one event every 17.3 years over the twentieth century to one event every 6.3 years over the twenty-first century. We find that a mean state change--with weakening of both equatorial westerly winds and eastward oceanic currents in association with a faster warming in the western than the eastern equatorial Indian Ocean--facilitates more frequent occurrences of wind and oceanic current reversal. This leads to more frequent extreme pIOD events, suggesting an increasing frequency of extreme climate and weather events in regions affected by the pIOD.
Errors of coupled general circulation models (CGCMs) limit their utility for climate prediction and projection. Origins of and feedback for tropical biases are investigated in the historical climate simulations of 18 CGCMs from phase 5 of the Coupled Model Intercomparison Project (CMIP5), together with the available Atmospheric Model Intercomparison Project (AMIP) simulations. Based on an intermodel empirical orthogonal function (EOF) analysis of tropical Pacific precipitation, the excessive equatorial Pacific cold tongue and double intertropical convergence zone (ITCZ) stand out as the most prominent errors of the current generation of CGCMs. The comparison of CMIP-AMIP pairs enables us to identify whether a given type of errors originates from atmospheric models. The equatorial Pacific cold tongue bias is associated with deficient precipitation and surface easterly wind biases in the western half of the basin in CGCMs, but these errors are absent in atmosphere-only models, indicating that the errors arise from the interaction with the ocean via Bjerknes feedback. For the double ITCZ problem, excessive precipitation south of the equator correlates well with excessive downward solar radiation in the Southern Hemisphere (SH) midlatitudes, an error traced back to atmospheric model simulations of cloud during austral spring and summer. This extratropical forcing of the ITCZ displacements is mediated by tropical ocean-atmosphere interaction and is consistent with recent studies of ocean-atmospheric energy transport balance.
This work investigates the teleconnection patterns over the North Pacific/North America sector and regional rainfall variability over the southwestern USA during boreal autumn, associated with two types of El Nino. These two types, called cold tongue (CT) and warm pool (WP) El Ninos, have an opposing impact on atmospheric circulation over the eastern North Pacific. When CT El Nino occurs, a strong cyclonic anomaly tends to appear over the North Pacific, and the associated southwesterly winds bring unusually moist air and thereby enhance rainfall over the southwestern USA. However, during WP El Nino autumns, a tripolar anomaly develops over the North Pacific. The associated northerly and northeasterly winds transport unusually dry air to the southwestern USA causing a reduction in rainfall. In this region, the rainfall response to WP El Nino is similar to that of La Nina, but opposite to that of CT El Nino. Since the early 1990s, the WP El Nino event has occurred more frequently, while the CT El Nino events has become less. The La Nina events remain roughly unchanged in terms of the zonal location. Autumn rainfall deficits over the southwestern USA have also been more frequent after the 1990s. The El Nino regime change thus appears to contribute to a decadal difference in the regional autumn rainfall.
Observations and climate simulations exhibit epochs of extreme El Niño/Southern Oscillation (ENSO) behavior that can persist for decades. Previous studies have revealed a wide range of ENSO responses to forcings from greenhouse gases, aerosols, and orbital variations – but they have also shown that interdecadal modulation of ENSO can arise even without such forcings. The present study examines the predictability of this intrinsically-generated component of ENSO modulation, using a 4000-year unforced control run from a global coupled GCM (GFDL-CM2.1) with a fairly realistic representation of ENSO. Extreme ENSO epochs from the unforced simulation are reforecast using the same ("perfect") model, but slightly-perturbed initial conditions. These 40-member reforecast ensembles display potential predictability of the ENSO trajectory, extending up to several years ahead. However, no decadal-scale predictability of ENSO behavior is found. This indicates that multidecadal epochs of extreme ENSO behavior can arise not only intrinsically, but delicately, and entirely at random. Previous work had shown that CM2.1 generates strong, reasonably-realistic, decadally-predictable high-latitude climate signals, as well as tropical and extratropical decadal signals that interact with ENSO. However, those slow variations appear not to lend significant decadal predictability to this model's ENSO behavior, at least in the absence of external forcings. While the potential implications of these results are sobering for decadal predictability, they also suggest an expedited approach to model evaluation and development – in which large ensembles of short runs are executed in parallel, to quickly and robustly evaluate simulations of ENSO. Further implications are discussed for decadal prediction, attribution of past and future ENSO variations, and societal vulnerability.
El Niño events, the warm phase of the El Niño/Southern
Oscillation (ENSO), are known to affect other tropical ocean basins
through teleconnections. Conversely, mounting evidence suggests that
temperature variability in the Atlantic Ocean may also influence ENSO
variability. Here we use reanalysis data and general circulation models
to show that sea surface temperature anomalies in the north tropical
Atlantic during the boreal spring can serve as a trigger for ENSO
events. We identify a subtropical teleconnection in which spring warming
in the north tropical Atlantic can induce a low-level cyclonic
atmospheric flow over the eastern Pacific Ocean that in turn produces a
low-level anticyclonic flow over the western Pacific during the
following months. This flow generates easterly winds over the western
equatorial Pacific that cool the equatorial Pacific and may trigger a La
Niña event the following winter. In addition, El Niño
events led by cold anomalies in the north tropical Atlantic tend to be
warm-pool El Niño events, with a centre of action located in the
central Pacific, rather than canonical El Niño events. We suggest
that the identification of temperature anomalies in the north tropical
Atlantic could help to forecast the development of different types of El
Niño event.
It is vital to understand how the El Niño-Southern Oscillation
(ENSO) has responded to past changes in natural and anthropogenic
forcings, in order to better understand and predict its response to
future greenhouse warming. To date, however, the instrumental record is
too brief to fully characterize natural ENSO variability, while large
discrepancies exist amongst paleo-proxy reconstructions of ENSO. These
paleo-proxy reconstructions have typically attempted to reconstruct the
full temporal variability of ENSO, rather than focusing simply on its
variance. Here a new approach is developed that synthesizes the
information on common low frequency variance changes from various proxy
datasets to obtain estimates of ENSO variance. The method is tested
using surrogate data from two coupled general circulation model (CGCM)
simulations. It is shown that in the presence of dating uncertainties,
synthesizing variance information provides a more robust estimate of
ENSO variance than synthesizing the raw data than identifying its
running variance. We also examine whether good temporal correspondence
between proxy data and instrumental ENSO records implies a good
representation of ENSO variance. A significant improvement in
reconstructing ENSO variance changes is found when combining several
proxies from diverse ENSO-teleconnected source regions, rather than by
relying on a single well-correlated location, suggesting that ENSO
variance estimates provided derived from a single site should be viewed
with caution. Finally, identifying the common variance signal in a
series of existing proxy based reconstructions of ENSO variability over
the last 600 yr we find that the common ENSO variance over the period
1600-1900 was considerably lower than during 1979-2009.
Climate models participating in the third Coupled Model Inter Comparison
Project (CMIP3) suggest a significant increase in the transport of the
New Guinea Coastal Undercurrent (NGCU) and the Equatorial Undercurrent
(EUC, in the central and western Pacific) and a decrease in the Mindanao
current and the Indonesian Throughflow. Most models also project a
reduction in the strength of the equatorial Trade winds. Typically, on
ENSO time scales, a weakening of the equatorial easterlies would lead to
a reduction in EUC strength in the central Pacific. The strengthening of
the EUC projected for longer timescales, can be explained by a robust
projected intensification of the south-easterly trade winds and an
associated off-equatorial wind-stress curl change in the Southern
Hemisphere. This drives the intensification of the NGCU and greater
water input to the EUC in the west. A 1½-layer shallow water
model, driven by projected wind stress trends from the CMIP3 models
demonstrates that the projected circulation changes are consistent with
a purely wind driven response. While the equatorial winds weaken for
both El Niño events and in the projections, the ocean response
and the mechanisms driving the projected wind changes are distinct from
those operating on interannual timescales.
Changes to the El Niño/Southern Oscillation (ENSO) and its
atmospheric teleconnections under climate change are investigated using
simulations conducted for the Coupled Model Intercomparison Project
(CMIP5). The overall response to CO2 increases is determined
using 27 models, and the ENSO amplitude change based on the multi-model
mean is indistinguishable from zero. However, changes between ensembles
run with a given model are sometimes significant: for four of the eleven
models having ensemble sizes larger than three, the 21st century change
to ENSO amplitude is statistically significant. In these four models,
changes to SST and wind stress do not differ substantially from those in
the models with no ENSO response, indicating that mean changes are not
predictive of the ENSO sensitivity to climate change. Also, ocean
vertical stratification is less (more) sensitive to CO2 in
models where ENSO strengthens (weakens), likely due to a regulation of
the subsurface temperature structure by ENSO-related poleward heat
transport. Atmospheric teleconnections also show differences between
models where ENSO amplitude does and does not respond to climate change;
in the former case El Niño/La Niña-related sea level
pressure anomalies strengthen with CO2, and in the latter
they weaken and shift polewards and eastwards. These results illustrate
the need for large ensembles to isolate significant ENSO climate change
responses, and for future work on diagnosing the dynamical causes of
inter-model teleconnection differences.
The El Niño/Southern Oscillation (ENSO) is the Earth's most prominent source of interannual climate variability, exerting profound worldwide effects. Despite decades of research, its behaviour continues to challenge scientists. In the eastern equatorial Pacific Ocean, the anomalously cool sea surface temperatures (SSTs) found during La Niña events and the warm waters of modest El Niño events both propagate westwards, as in the seasonal cycle. In contrast, SST anomalies propagate eastwards during extreme El Niño events, prominently in the post-1976 period, spurring unusual weather events worldwide with costly consequences. The cause of this propagation asymmetry is currently unknown. Here we trace the cause of the asymmetry to the variations in upper ocean currents in the equatorial Pacific, whereby the westward-flowing currents are enhanced during La Niña events but reversed during extreme El Niño events. Our results highlight that propagation asymmetry is favoured when the westward mean equatorial currents weaken, as is projected to be the case under global warming. By analysing past and future climate simulations of an ensemble of models with more realistic propagation, we find a doubling in the occurrences of El Niño events that feature prominent eastward propagation characteristics in a warmer world. Our analysis thus suggests that more frequent emergence of propagation asymmetry will be an indication of the Earth's warming climate.
Large discrepancies exist between twentieth-century tropical Indo-Pacific sea surface temperature trends determined from present reconstructions. These discrepancies prevent an unambiguous verification and validation of climate models used for projections of future climate change. Here we demonstrate that a more consistent and robust trend among all the reconstructions is found by filtering each data set to remove El Niño/Southern Oscillation (ENSO), which is represented not by a single-index time series but rather by an evolving dynamical process. That is, the discrepancies seem to be largely the result of different estimates of ENSO variability in each reconstruction. The robust ENSO-residual trend pattern represents a strengthening of the equatorial Pacific temperature gradient since 1900, owing to a systematic warming trend in the warm pool and weak cooling in the cold tongue. Similarly, the ENSO-residual trend in sea-level pressure represents no weakening of the equatorial Walker circulation over the same period. Additionally, none of the disparate estimates of post-1900 total eastern equatorial Pacific sea surface temperature trends are larger than can be generated by statistically stationary, stochastically forced empirical models that reproduce ENSO evolution in each reconstruction.
The El Niño–Southern Oscillation (ENSO) exhibits well-known asymmetries: 1) warm events are stronger than cold events, 2) strong warm events are more likely to be followed by cold events than vice versa, and 3) cold events are more persistent than warm events. Coupled GCM simulations, however, continue to underestimate many of these observed features.
To shed light on these asymmetries, the authors begin with a widely used delayed-oscillator conceptual model for ENSO and modify it so that wind stress anomalies depend more strongly on SST anomalies (SSTAs) during warm conditions, as is observed. Then the impact of this nonlinearity on ENSO is explored for three dynamical regimes: self-sustained oscillations, stochastically driven oscillations, and self-sustained oscillations disrupted by stochastic forcings. In all three regimes, the nonlinear air–sea coupling preferentially strengthens the feedbacks (both positive and delayed negative) during the ENSO warm phase—producing El Niños that grow to a larger amplitude and overshoot more rapidly and consistently into the opposite phase, than do the La Niñas. Finally, the modified oscillator is applied to observational records and to control simulations from two global coupled ocean–atmosphere–land–ice models [Geophysical Fluid Dynamics Laboratory Climate Model version 2.1 (GFDL CM2.1) and version 2.5 (GFDL CM2.5)] to elucidate the causes of their differing asymmetries.
Due to errors in complex coupled feedbacks that compensate differently
in different global climate models, as well as nonlinear nature of El
Niño-Southern Oscillation (ENSO), there remain difficulties in
detecting and evaluating the reason for the past and future changes in
the ENSO amplitude, σniño. Here we use physics
parameter ensembles, in which error compensation was eliminated by
perturbing model parameters, to explore relationships between mean
climate and variability. With four such ensembles we find a strong
relationship between σniño and the mean
precipitation over the eastern equatorial Pacific (P¯nin˜o).
This involves a two-way interaction, in which the wetter mean state with
greater P¯nin˜o acts to increase the ENSO amplitude by
strengthening positive coupled feedbacks. Such a relationship is also
identified in 11 single-model historical climate simulations in the
Coupled Model Intercomparison Project phase 5 despite mean precipitation
biases apparently masking the relationship in the multi-model ensemble
(MME). Taking changes in σniño and
P¯nin˜o between pre-industrial and recent periods eliminates
the bias, and therefore results in a robust
σniño-P¯nin˜o connection in
MME, which suggests a 10-15% increase in the ENSO amplitude since
pre-industrial era mainly due to changing mean state. However, the
σniño-P¯nin˜o connection is
less clear for their future changes, which are still greatly uncertain.
Climate model experiments are analyzed to elucidate if and how the changes in mean climate in response to doubling of atmospheric CO2 (2xCO2) influence ENSO. The processes involved the development, transition, and decay of simulated ENSO events are quantified through a multimodel heat budget analysis. The simulated changes in ENSO amplitude in response to 2xCO2 are directly related to changes in the anomalous ocean heat flux convergence during the development, transition, and decay of ENSO events. The weakening of the Walker circulation and the increased thermal stratification, both robust features of the mean climate response to 2xCO2, play opposing roles in ENSO–mean climate interactions. Weaker upwelling in response to a weaker Walker circulation drives a reduction in thermocline-driven ocean heat flux convergence (i.e., thermocline feedback) and, thus, reduces the ENSO amplitude. Conversely, a stronger zonal subsurface temperature gradient, associated with the increased thermal stratification, drives an increase in zonal-current-induced ocean heat flux convergence (i.e., zonal advection feedback) and, thus, increases the ENSO amplitude. These opposing processes explain the lack of model agreement in whether ENSO is going to weaken or strengthen in response to increasing greenhouse gases, but also why ENSO appears to be relatively insensitive to 2xCO2 in most models.
The amplitude of the El Niño-Southern Oscillation (ENSO) is known
to fluctuate in long records derived from observations and general
circulation models (GCMs), even when driven by constant external
forcings. This involves an interaction between the ENSO cycle and the
background mean state, which affects the climatological precipitation
over the eastern equatorial Pacific. The changes in climatological
rainfall may be ascribed to several factors: changes in mean sea surface
temperature (SST), changes in SST variability, and changes in the
sensitivity of precipitation to SST. We propose a method to separate
these effects in model ensembles. A case study with a single GCM
demonstrates that the method works well, and suggests that each factor
plays a role in changing mean precipitation. Applying the method to 16
pre-industrial control simulations archived in the Coupled Model
Intercomparison Project phase 5 (CMIP5) reveals that the inter-model
diversity in mean precipitation arises mostly from differences in the
mean SST and atmospheric sensitivity to SST, rather than from
differences in ENSO amplitude.
This study examines preindustrial simulations from Coupled Model Intercomparison Project, phase 3 (CMIP3), models to show that a tendency exists for El Niñ o sea surface temperature anomalies to be located farther eastward than La Niñ a anomalies during strong El Niñ o–Southern Oscillation (ENSO) events but farther westward than La Niñ a anomalies during weak ENSO events. Such reversed spatial asymmetries are shown to force a slow change in the tropical Pacific Ocean mean state that in return modulates ENSO amplitude. CMIP3 models that produce strong reversed asymmetries experience cyclic modulations of ENSO intensity, in which strong and weak events occur during opposite phases of a decadal variability mode associated with the residual effects of the reversed asymmetries. It is concluded that the reversed spatial asymmetries enable an ENSO–tropical Pacific mean state interaction mechanism that gives rise to a decadal modulation of ENSO intensity and that at least three CMIP3 models realistically simulate this interaction mechanism.
Extreme El Niño (e.g., 1983/1983 and 1997/1998) causes severe weather and climate impacts globally, but the associated dynamics is not fully understood. The present study shows that advection of mean temperature by anomalous eastward zonal current plays an important role in producing such extreme events especially during the early part of the developing period. While the climatological direction of the upper oceanic current in the equatorial Pacific is westward, at times the direction reverses. These eastward current events are well distinguished from the normal, westward conditions. The upper-layer zonal current in the equatorial Pacific is basically in geostrophic balance and forced by wind stress. However, in the case of the eastward zonal current events, persistent westerly winds are observed in the Western Pacific, and the current becomes synchronized with the westerly wind stress above. The advection of the mean temperature by the anomalous zonal current in the early developing period always precedes strong El Niño, though it does not significantly contribute to the growth of La Niña, neutral, and moderate El Niño; and is the major contributor of asymmetry in the early developing phase.
We analyse the ability of CMIP3 and CMIP5 coupled ocean–atmosphere general circulation models (CGCMs) to simulate the tropical Pacific mean state and El Niño-Southern Oscillation (ENSO). The CMIP5 multi-model ensemble displays an encouraging 30 % reduction of the pervasive cold bias in the western Pacific, but no quantum leap in ENSO performance compared to CMIP3. CMIP3 and CMIP5 can thus be considered as one large ensemble (CMIP3 + CMIP5) for multi-model ENSO analysis. The too large diversity in CMIP3 ENSO amplitude is however reduced by a factor of two in CMIP5 and the ENSO life cycle (location of surface temperature anomalies, seasonal phase locking) is modestly improved. Other fundamental ENSO characteristics such as central Pacific precipitation anomalies however remain poorly represented. The sea surface temperature (SST)-latent heat flux feedback is slightly improved in the CMIP5 ensemble but the wind-SST feedback is still underestimated by 20–50 % and the shortwave-SST feedbacks remain underestimated by a factor of two. The improvement in ENSO amplitudes might therefore result from error compensations. The ability of CMIP models to simulate the SST-shortwave feedback, a major source of erroneous ENSO in CGCMs, is further detailed. In observations, this feedback is strongly nonlinear because the real atmosphere switches from subsident (positive feedback) to convective (negative feedback) regimes under the effect of seasonal and interannual variations. Only one-third of CMIP3 + CMIP5 models reproduce this regime shift, with the other models remaining locked in one of the two regimes. The modelled shortwave feedback nonlinearity increases with ENSO amplitude and the amplitude of this feedback in the spring strongly relates with the models ability to simulate ENSO phase locking. In a final stage, a subset of metrics is proposed in order to synthesize the ability of each CMIP3 and CMIP5 models to simulate ENSO main characteristics and key atmospheric feedbacks.
Canonical El Niño has a warming center in the eastern Pacific (EP), but in recent decades, El Niño warming center tends to occur more frequently in the central Pacific (CP). The definitions and names of this new type of El Niño, however, have been notoriously diverse, which makes it difficult to understand why the warming center shifts. Here, we show that the new type of El Niño events is characterized by: 1) the maximum warming standing and persisting in the CP and 2) the warming extending to the EP only briefly during its peak phase. For this reason, we refer to it as standing CP warming (CPW). Global warming has been blamed for the westward shift of maximum warming as well as more frequent occurrence of CPW. However, we find that since the late 1990s the standing CPW becomes a dominant mode in the Pacific; meanwhile, the epochal mean trade winds have strengthened and the equatorial thermocline slope has increased, contrary to the global warming-induced weakening trades and flattening thermocline. We propose that the recent predominance of standing CPW arises from a dramatic decadal change characterized by a grand La Niña-like background pattern and strong divergence in the CP atmospheric boundary layer. After the late 1990s, the anomalous mean CP wind divergence tends to weaken the anomalous convection and shift it westward from the underlying SST warming due to the suppressed low-level convergence feedback. This leads to a westward shift of anomalous westerly response and thus a zonally in-phase SST tendency, preventing eastward propagation of the SST anomaly. We anticipate more CPW events will occur in the coming decade provided the grand La Niña-like background state persists.