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Abstract and Figures

El Niño events differ substantially in their spatial pattern and intensity. Canonical Eastern Pacific El Niño events have sea surface temperature anomalies that are strongest in the far eastern equatorial Pacific, whereas peak ocean warming occurs further west during Central Pacific El Niño events. The event types differ in their impacts on the location and intensity of temperature and precipitation anomalies globally. Evidence is emerging that Central Pacific El Niño events have become more common, a trend that is projected by some studies to continue with ongoing climate change. Here we identify spatial and temporal patterns in observed sea surface temperatures that distinguish the evolution of Eastern and Central Pacific El Niño events in the tropical Pacific. We show that these patterns are recorded by a network of 27 seasonally resolved coral records, which we then use to reconstruct Central and Eastern Pacific El Niño activity for the past four centuries. We find a simultaneous increase in Central Pacific events and a decrease in Eastern Pacific events since the late twentieth century that leads to a ratio of Central to Eastern Pacific events that is unusual in a multicentury context. Compared to the past four centuries, the most recent 30 year period includes fewer, but more intense, Eastern Pacific El Niño events.
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https://doi.org/10.1038/s41561-019-0353-3
1School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia. 2ARC Centre of Excellence for Climate System Science, University of
Melbourne, Parkville, Victoria, Australia. 3Climate and Energy College, University of Melbourne, Parkville, Victoria, Australia. 4ARC Centre of Excellence
for Climate Extremes, University of Melbourne, Parkville, Victoria, Australia. 5School of Earth, Atmosphere and Environment, Monash University, Clayton,
Victoria, Australia. 6NESP Earth Systems and Climate Change Hub, CSIRO, Aspendale, Victoria, Australia. 7School of Earth, Atmospheric and Life Sciences,
University of Wollongong, Wollongong, New South Wales, Australia. 8Research School of Earth Sciences, Australian National University, Canberra,
Australian Capital Territory, Australia. 9ARC Centre of Excellence for Climate Extremes, Australian National University, Canberra, Australian Capital
Territory, Australia. 10ARC Centre of Excellence for Climate Extremes, Monash University, Clayton, Victoria, Australia. *e-mail: freundm@unimelb.edu.au
Canonical Eastern Pacific (EP) El Niño events exhibit their
largest sea surface temperature anomalies (SSTA) in the far
eastern tropical Pacific near the Peruvian coast1. Over recent
decades, peak warming during several El Niño events has been
displaced by approximately 11,000 km, or 100° longitude, west-
wards into the central equatorial Pacific. These El Niño events are
described as Central Pacific (CP) El Niño, warm-pool El Niño2, El
Niño Modoki3 or Dateline El Niño4. The displacement of maximum
SSTA towards the central Pacific drives substantial shifts in atmo-
spheric convection and circulation57, which alter the location and
intensity of temperature and precipitation impacts associated with
El Niño around the globe3,4,811.
Evidence is emerging that changes in the El Niño Southern
Oscillation (ENSO) behaviour occurred during the instrumental
period1215. After the climate regime shift in 1976/1977, zonal SSTA
propagation during El Niño changed from westward to eastward16.
Coincident with the shift to a positive phase of the Interdecadal
Pacific Oscillation1618 in 1999/2000, Pacific trade winds strength-
ened19,20. Observations indicate an increasing El Niño event
amplitude21, decadal variations in event frequency22, changes in
maximum SSTA propagation direction12,23 and delays in the onset
of El Niño events24.
Since the late 1990s there has been a higher number of CP events
relative to EP events, unprecedented in instrumental records2,21,22.
It is unclear whether this recent increase is part of natural climate
variability25 or a consequence of anthropogenic climate change22. A
precise picture of El Niño diversity is a challenge due to model defi-
ciencies in simulating El Niño and the short and sparse coverage of
instrumental observations across the equatorial Pacific25,26. In this
study, we extend the record of El Niño diversity into the past using
a network of coral data that spans the tropical Indo-Pacific ocean.
Spatial and temporal patterns of El Niño types
We used a network of 27 seasonally resolved coral records to recon-
struct past EP and CP El Niño events (Methods and Supplementary
Information). The network includes four Sr/Ca records (a proxy
for sea surface temperature (SST)) and 23 oxygen isotope (δ18O)
records. The δ18O signal preserved in coral banding is determined
by the source isotopic composition of the surrounding seawater
(δ18OSW) and the equilibrium isotopic fractionation between the
seawater and carbonate, which is inversely related to temperature27.
The oxygen isotopes are fractionated throughout the annual cycle
(and thus preserve the SST variations across the calendar year) and
the δ18OSW is affected by the advection of water masses with dif-
ferent isotope signatures and precipitation–evaporation changes.
Precipitation–evaporation changes also affect the sea surface salin-
ity (SSS), so δ18O is often used as an SST, SSS or SST–SSS proxy. The
relationship between salinity and δ18OSW is complex and can depend
on local conditions28. In general, if the SSS is relatively constant,
the δ18O in corals is mainly determined by SST variability and vice
versa. High variability of SSS and SST can make the interpretation
of δ18O in corals more complex. We carried out extensive testing of
our methods and explored possible sources of error, which included
testing the δ18O signal in the individual coral records and our net-
work as a whole (Supplementary Information). We found that all of
the coral δ18O records in our network have a strong link to ENSO,
with correlations to SST and SSS, parameters which in turn vary
with the spatial and temporal patterns of CP and EP El Niño events.
Higher frequency of Central Pacific El Niño events
in recent decades relative to past centuries
Mandy B. Freund 1,2,3*, Benjamin J. Henley 1,2,4,5, David J. Karoly1,2,6, Helen V. McGregor 7,
Nerilie J. Abram 8,9 and Dietmar Dommenget5,10
El Niño events differ substantially in their spatial pattern and intensity. Canonical Eastern Pacific El Niño events have sea sur-
face temperature anomalies that are strongest in the far eastern equatorial Pacific, whereas peak ocean warming occurs further
west during Central Pacific El Niño events. The event types differ in their impacts on the location and intensity of temperature
and precipitation anomalies globally. Evidence is emerging that Central Pacific El Niño events have become more common, a
trend that is projected by some studies to continue with ongoing climate change. Here we identify spatial and temporal patterns
in observed sea surface temperatures that distinguish the evolution of Eastern and Central Pacific El Niño events in the tropical
Pacific. We show that these patterns are recorded by a network of 27 seasonally resolved coral records, which we then use to
reconstruct Central and Eastern Pacific El Niño activity for the past four centuries. We find a simultaneous increase in Central
Pacific events and a decrease in Eastern Pacific events since the late twentieth century that leads to a ratio of Central to Eastern
Pacific events that is unusual in a multicentury context. Compared to the past four centuries, the most recent 30 year period
includes fewer, but more intense, Eastern Pacific El Niño events.
NATURE GEOSCIENCE | VOL 12 | JUNE 2019 | 450–455 | www.nature.com/naturegeoscience
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... To investigate these matters, we considered a series of pertinent questions based on the impacts and processes observed in mangroves during mass dieback events and other abrupt declines in canopy condition across northern Australia [11,14,15] We also planned to evaluate these observations in the context of global climate change, especially where the frequency of extreme environmental drivers, like Taimasa and severe ENSO, were expected to increase [37]. But, while we consider suggestions for better management to minimise harm to mangroves, our primary goal with this article has been to raise awareness of the implications of the newly recognised impacts of Taimasa and other manifestations of extreme low sea level oscillations (coincident with severe El Niño) on shoreline mangroves. ...
... In this way, the unusually low 12-month SOI anomaly levels in 1982 ( Fig 4C) were both consistent with the earlier Taimasa, and definitive in identifying the specific months of occurrence of that dieback event in the absence of local sea level data. In summary, the two unusually extreme low sea level (Taimasa) periods in the western Pacific region in 1982/83 [16][17][18] and 2015/16 [9,27,37], were consistent with each event causing widespread mass dieback of mangroves [11]. ...
... In recent decades, these notably abrupt extreme lows in sea level have on two occasions, at least, resulted in severe desiccation and catastrophic mass dieback of shoreline mangroves in northern Australia [11]. The underlying drivers responsible have been the unusually severe El Niño events in 1982 and 2015, the two most severe such events in recent centuries, depicted in the 400 year coral record [37]. Of further concern, such occurrences have been predicted to re-occur as greenhouse warming escalates [58]. ...
Article
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Recent ENSO-related, extreme low oscillations in mean sea level, referred to as 'Taimasa' in Samoa, have destabilised shoreline mangroves of tropical northern Australia, and possibly elsewhere. In 1982 and 2015, two catastrophic Taimasa each resulted in widespread mass dieback of~76 km 2 of shoreline mangroves along 2,000 km of Australia's Gulf of Car-pentaria. For the 2015 event, we determined that a temporary drop in sea level of~0.4 metres for up to six months duration caused upper zone shoreline mangroves across the region to die from severe moisture deficit and desiccation. The two dramatic collapse events revealed a previously unrecognised vulnerability of semi-arid tidal wetland habitats to more extreme ENSO influences on sea level. In addition, we also observed a relationship between annual sea level oscillations and mangrove forest productivity where seasonal oscillations in mean sea level were co-incident with regular annual mangrove leaf growth during months of higher sea levels (March-May), and leaf shedding during lower sea levels (September-November). The combination of these periodic fluctuations in sea level defined a mangrove 'Goldilocks' zone of seasonal productivity during median-scale oscillations, bracketed by critical threshold events when sea levels became unusually low, or high. On the two occasions reported here when sea levels were extremely low, upper zone mangrove vegetation died en masse in synchrony across northern Australia. Such extreme pulse impacts combined with localised stressors profoundly threaten the longer-term survival of mangrove ecosystems and their benefits, like minimisation of shoreline erosion with rising sea levels. These new insights into such critical influences of climate and sea level on mangrove forests offer further affirmation of the urgency for implementing well-considered miti-gation efforts for the protection of shoreline mangroves at risk, especially given predictions of future re-occurrences of extreme events affecting sea levels, combined with ongoing pressure of rapidly rising sea levels.
... During EP "canonical" El Niño events, new production is depressed in the eastern equatorial Pacific and elevated west of 180°W when compared with neutral conditions (Strutton & Chavez, 2000;Turk et al., 2001Turk et al., , 2011. Central Pacific (CP) "Modoki" El Niño events have become more frequent relative to EP events since the 1990s (Ashok et al., 2007;Freund et al., 2019;Marathe et al., 2015;T. Lee & McPhaden, 2010), and have been shown to reduce new production and CO 2 flux in the western Pacific, but are similar to neutral conditions in the EP (Gierach et al., 2012;Liao et al., 2020;Messié & Chavez, 2013;Racault et al., 2017;Turk et al., 2011;Valsala et al., 2014). ...
... The shift from cool to warm Pacific Decadal Variability around 2015 may have further influenced the CP like trends presented (Figure 3 right column and Figure 4, center column). Therefore, as CP and La Niña events are becoming more frequent Freund et al., 2019), they could be causing or amplifying the decadal trends in the carbon budget, as explained in detail below. ...
... CO 2 outgassing and new production are strongly influenced by ENSO conditions (Table 1; Figures 2, 4 and 5). ENSO is changing character over time, toward less frequent but more intense EP events and more common CP and La Niña events Freund et al., 2019;T. Lee & McPhaden, 2010;Yeh et al., 2009). ...
Article
Full-text available
The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal climate variability. New production, the amount of phytoplankton net primary production driven by upwelled nitrate, plays a significant role in modulating air‐sea CO2 fluxes as the biological carbon pump removes carbon from the surface ocean. We aim to understand how the physical drivers of sea surface temperature and wind speed influence interannual and decadal variability of the equatorial Pacific carbon cycle. In the equatorial Pacific, there are three biogeochemical regimes: the upwelling cold tongue east of 140°W and south of the equator (3°N–15°S); the eastern Pacific warm pool north of the equator (3°–15°N); and the 28.5°C western Pacific warm pool, west of 140°W. We find that between 2000 and 2020, air‐sea CO2 flux and ΔpCO2 increased in the cold tongue (45 mmolC m⁻² yr⁻², 1.5 μatm yr⁻¹, respectively) but decreased elsewhere, while new production decreased everywhere. The western Pacific occasionally became a weak carbon sink, depending on ENSO and this sink was strongest at 165°E during central Pacific “Modoki” El Niño events. We find that changes in wind speed, temperature and ENSO frequency have altered the surface carbon budget. The mean basin‐wide (150°E−90°W and 15°N–15°S) new production for 2000–2020 was 1.2 ± 0.1 PgC yr⁻¹ and air‐sea CO2 flux was 0.5 ± 0.1 PgC yr⁻¹. New production decreased at −7.7 ± 1.6 TgC yr⁻², compared to the CO2 flux trend of −1.7 ± 1.4 TgC yr⁻².
... Relevant for society is the total damage resulting from consecutive or cascading hazards during several months. Modelling studies and measurements still do not provide a clear image about the effect of climate change on the El Niño Southern Oscillation, but the general expectation is that more extreme states will occur more frequently (Cai et al., 2015(Cai et al., , 2018Freund et al., 2019). For this reason, a stronger shift from the "emergency response"-mindset towards integrated disaster risk management would be advantageous for Latin America. ...
Thesis
Der technologische Fortschritt erlaubt es, zunehmend komplexe Vorhersagemodelle auf Basis immer größerer Datensätze zu produzieren. Für das Risikomanagement von Naturgefahren sind eine Vielzahl von Modellen als Entscheidungsgrundlage notwendig, z.B. in der Auswertung von Beobachtungsdaten, für die Vorhersage von Gefahrenszenarien, oder zur statistischen Abschätzung der zu erwartenden Schäden. Es stellt sich also die Frage, inwiefern moderne Modellierungsansätze wie das maschinelle Lernen oder Data-Mining in diesem Themenbereich sinnvoll eingesetzt werden können. Zusätzlich ist im Hinblick auf die Datenverfügbarkeit und -zugänglichkeit ein Trend zur Öffnung (open data) zu beobachten. Thema dieser Arbeit ist daher, die Möglichkeiten und Grenzen des maschinellen Lernens und frei verfügbarer Geodaten auf dem Gebiet der Hochwasserrisikomodellierung im weiteren Sinne zu untersuchen. Da dieses übergeordnete Thema sehr breit ist, werden einzelne relevante Aspekte herausgearbeitet und detailliert betrachtet. Eine prominente Datenquelle im Bereich Hochwasser ist die satellitenbasierte Kartierung von Überflutungsflächen, die z.B. über den Copernicus Service der Europäischen Union frei zur Verfügung gestellt werden. Große Hoffnungen werden in der wissenschaftlichen Literatur in diese Produkte gesetzt, sowohl für die akute Unterstützung der Einsatzkräfte im Katastrophenfall, als auch in der Modellierung mittels hydrodynamischer Modelle oder zur Schadensabschätzung. Daher wurde ein Fokus in dieser Arbeit auf die Untersuchung dieser Flutmasken gelegt. Aus der Beobachtung, dass die Qualität dieser Produkte in bewaldeten und urbanen Gebieten unzureichend ist, wurde ein Verfahren zur nachträglichenVerbesserung mittels maschinellem Lernen entwickelt. Das Verfahren basiert auf einem Klassifikationsalgorithmus der nur Trainingsdaten von einer vorherzusagenden Klasse benötigt, im konkreten Fall also Daten von Überflutungsflächen, nicht jedoch von der negativen Klasse (trockene Gebiete). Die Anwendung für Hurricane Harvey in Houston zeigt großes Potenzial der Methode, abhängig von der Qualität der ursprünglichen Flutmaske. Anschließend wird anhand einer prozessbasierten Modellkette untersucht, welchen Einfluss implementierte physikalische Prozessdetails auf das vorhergesagte statistische Risiko haben. Es wird anschaulich gezeigt, was eine Risikostudie basierend auf etablierten Modellen leisten kann. Solche Modellketten sind allerdings bereits für Flusshochwasser sehr komplex, und für zusammengesetzte oder kaskadierende Ereignisse mit Starkregen, Sturzfluten, und weiteren Prozessen, kaum vorhanden. Im vierten Kapitel dieser Arbeit wird daher getestet, ob maschinelles Lernen auf Basis von vollständigen Schadensdaten einen direkteren Weg zur Schadensmodellierung ermöglicht, der die explizite Konzeption einer solchen Modellkette umgeht. Dazu wird ein staatlich erhobener Datensatz der geschädigten Gebäude während des schweren El Niño Ereignisses 2017 in Peru verwendet. In diesem Kontext werden auch die Möglichkeiten des Data-Mining zur Extraktion von Prozessverständnis ausgelotet. Es kann gezeigt werden, dass diverse frei verfügbare Geodaten nützliche Informationen für die Gefahren- und Schadensmodellierung von komplexen Flutereignissen liefern, z.B. satellitenbasierte Regenmessungen, topographische und hydrographische Information, kartierte Siedlungsflächen, sowie Indikatoren aus Spektraldaten. Zudem zeigen sich Erkenntnisse zu den Schädigungsprozessen, die im Wesentlichen mit den vorherigen Erwartungen in Einklang stehen. Die maximale Regenintensität wirkt beispielsweise in Städten und steilen Schluchten stärker schädigend, während die Niederschlagssumme in tiefliegenden Flussgebieten und bewaldeten Regionen als aussagekräftiger befunden wurde. Ländliche Gebiete in Peru weisen in der präsentierten Studie eine höhere Vulnerabilität als die Stadtgebiete auf. Jedoch werden auch die grundsätzlichen Grenzen der Methodik und die Abhängigkeit von spezifischen Datensätzen and Algorithmen offenkundig. In der übergreifenden Diskussion werden schließlich die verschiedenen Methoden – prozessbasierte Modellierung, prädiktives maschinelles Lernen, und Data-Mining – mit Blick auf die Gesamtfragestellungen evaluiert. Im Bereich der Gefahrenbeobachtung scheint eine Fokussierung auf neue Algorithmen sinnvoll. Im Bereich der Gefahrenmodellierung, insbesondere für Flusshochwasser, wird eher die Verbesserung von physikalischen Modellen, oder die Integration von prozessbasierten und statistischen Verfahren angeraten. In der Schadensmodellierung fehlen nach wie vor die großen repräsentativen Datensätze, die für eine breite Anwendung von maschinellem Lernen Voraussetzung ist. Daher ist die Verbesserung der Datengrundlage im Bereich der Schäden derzeit als wichtiger einzustufen als die Auswahl der Algorithmen.
... Statistically significant relationships exist between the Laguna Pallcacocha flood history and records from both the central and western Pacific (Dang et al., 2020) and eastern Pacific (Koutavas and Joanides, 2012) (Fig. 6), indicates there is some degree of coherence between the many flavors of the ENSO over multi-millennial timescales. A newly developed network of coral records (Freund et al., 2019) shows that the ratio of CP to EP events has increased in recent decades, while EP events have become more intense. The development of a continuous Holocene chronology of EP El Niño events will be crucial to testing hypotheses regarding the relationship between different flavors of the ENSO under different background climate conditions. ...
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The laminated sedimentary sequence of Ecuador's Laguna Pallcacocha is one of the most widely cited proxy records of Holocene El Niño Southern Oscillation (ENSO) variability. Previous efforts to reconstruct flood-driven laminae from Laguna Pallcacocha relied solely on sediment color, a useful but non-specific metric of flood events. We improved the chronology with ²¹⁰Pb and additional ¹⁴C dates over the past millennium, which allows for comparison of the sedimentary record with historically documented El Niño events. Additionally, we use elemental composition derived from X-ray Fluorescence (XRF) to reconstruct flood history at Pallcacocha. A principal component analysis (PCA) of the XRF dataset identifies minerogenic flood-driven clastic laminae. The first principal component (PC1) of the XRF data and red color intensity are positively correlated over the past 7.5 kyr, but the color record fails to capture high frequency variability that is preserved in the XRF dataset during the early Holocene (approximately 7.5-11 kyr BP). The new XRF dataset indicates moderate El Niño activity during the early Holocene, suppressed El Niño activity in the middle Holocene, and enhanced El Niño activity during the late Holocene. This pattern is relatively common among other ENSO records, and has been attributed to long-term changes in tropical insolation. Some intervals-most notably between 3-2 kyr BP and during the last millennium-deviate from expected trends if insolation was the sole forcing mechanism. Previously proposed mechanisms linking ENSO to latitudinal displacement of the ITCZ and ocean-atmospheric variabilities in other ocean basins appear to play an additional role in modulating Holocene ENSO development, as demonstrated by statistically significant correlations between the revised Laguna Pallcacocha flood history and proxy records from the Atlantic.
... On the other hand, it has been shown that the EP and CP events were alternatively prevalent every 10 to 20 years over the past century 61,62 . For example, the EP events were dominant in the 1980s while the CP El Niños were more frequently identified after 2000 63,64 . These findings imply that the decadal variability plays a crucial role in driving the transitions between the CP-and EP-dominant regimes. ...
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El Ni\~no-Southern Oscillation (ENSO) is the most prominent interannual climate variability in the tropics and exhibits diverse features in spatiotemporal patterns. In this paper, a simple multiscale intermediate coupled stochastic model is developed to capture the ENSO diversity and complexity. The model starts with a deterministic and linear coupled interannual atmosphere, ocean and sea surface temperature (SST) system. It can generate two distinct dominant linear solutions that represent the eastern Pacific (EP) and the central Pacific (CP) El Ni\~nos, respectively. In addition to adopting a stochastic model for characterizing the intraseasonal wind bursts, another simple stochastic process is developed to describe the decadal variation of the background Walker circulation. The latter links the two dominant modes in a simple nonlinear fashion and advances the modulation of the strength and occurrence frequency of the EP and the CP events. Finally, a cubic nonlinear damping is adopted to parameterize the relationship between subsurface temperature and thermocline depth. The model succeeds in reproducing the spatiotemporal dynamical evolution of different types of the ENSO events. It also accurately recovers the strongly non-Gaussian probability density function, the seasonal phase locking, the power spectrum and the temporal autocorrelation function of the SST anomalies in all the three Ni\~no regions (3, 3.4 and 4) across the equatorial Pacific. Furthermore, both the composites of the SST anomalies for various ENSO events and the strength-location bivariate distribution of equatorial Pacific SST maxima for the El Ni\~no events from the model simulation highly resemble those from the observations.
... The canonical El Niño is a pattern of SST anomalies observed in central and western tropical Pacific areas, while El Niño Modoki is characterized by positive SST anomalies in the central equatorial Pacific and negative anomalies in eastern and western zones (Capotondi et al. 2015). EP-El Niño events have become less frequent in recent decades (Lee and McPhaden, 2010), contrasting with CP-El Niño events, which show a decadal periodicity (Ashok et al. 2007;Di Lorenzo et al. 2010;Couto et al. 2013;Freund et al. 2019). Phytoplankton functional groups are particularly useful for identifying the effects of El Niño events on oceanic ecosystems. ...
Article
A long-term information baseline is necessary to identify the seasonal and interannual variability of diatom production driven by environmental dynamics. This motivated our compiling hydrographic data and concurrent diatom fluxes at the Alfonso Basin sub-zone. The diatom data time series spanned from February 2008 to September 2012. These data were analyzed along with primary productivity and chlorophyll-a concentration estimates derived from satellite imagery from 2002–2012. Planktonic diatom flux (range: 1.62 × 10⁶–3.55 × 10⁷ valves·m⁻²d⁻¹) was significantly correlated (r² = 0.37) with chlorophyll-a concentration. The bimodal production season (December–July) exhibited a winter mixing period influenced by water from the Gulf of California. The diatom flux was dominated by fast-growing surface species such as Chaetoceros spp. and Thalassiosira spp. The second peak in May occurred under the influence of southwest winds concurrent with the entry of tropical surface water and a cyclonic eddy that injected nutrients into the base of the euphotic zone, eliciting a bloom of Pseudo-nitzschia spp. The summer stratification resulted in the deepening of Subtropical Subsurface Water, and the diatom flux included large species of oceanic affinity such as Chaetoceros (Phaeoceros), Rhizosolenia acuminata, and Rh. imbricata. The environment at Alfonso Basin promotes diatom sinking; thus, material from sediment traps accurately reflects the primary production cycle associated with hydrographic processes. Interannual variability was characterized by anomalously low diatom fluxes between August 2009 and July 2010. A significant negative association between diatom flux anomalies and El Niño Modoki Index values suggests that oceanographic changes are related to this event. Additional reductions in diatom fluxes were coeval with SST positive anomalies recorded from July to February 2008–2009 and 2011–2012, which showed a significant relationship with the tropical Pacific Meridional Mode. The temporal trends like the additive signals of interannual (CP-El Niño) and decadal (Pacific Meridional Mode) time scales indicate that climatic disturbances affected the local ecosystem in the basin, reducing the vertical flux of diatoms initially dominated by Pseudo-nitzschia spp., and boosting the presence of oceanic warm-water species and small highly silicified species such as Thalassionema nitzschioides var. parva.
... We distinguish regional WRs from global oscillations that occur at larger spatial scales [e.g., El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Interdecadal Pacific Oscillation (IPO), Atlantic Multidecadal Oscillation (AMO), Madden-Julian Oscillation (MJO)]. A significant body of literature has focused on reconstructing the latter using environmental proxies (Hernández et al. 2020), including tree-ring chronologies (D'Arrigo et al. 2001;MacDonald and Case 2005;Cook et al. 2008;D'Arrigo et al. 2015;Buckley et al. 2019), ice cores (Chylek et al. 2012;Vance et al. 2015), speleothems (Chen et al. 2016), corals (Urban et al. 2000Crueger et al. 2009;Cobb et al. 2013;Freund et al. 2019) and sediments (Dean and Kemp 2004;White et al. 2018). Reconstructions vary in length across the Common Era and Holocene depending on the chosen proxy, and they can often conflict due to nonstationarity in the teleconnections (Schmutz et al. 2000;An and Jin 2004;Raible et al. 2014;Newman et al. 2016). ...
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Extreme weather variability has long posed difficulties for food, energy, and water systems in the Western US and is only projected to become more variable in a changing climate. The key to understanding weather in the West lies in understanding variability of large-scale weather regimes that dictate regional weather patterns. In this study, we propose a novel, multi-objective optimization and regression-based framework that reconstructs the annual frequency of regional weather regimes based on a gridded, tree-ring based reconstruction of cold season precipitation. The approach optimally smooths and selects the tree-ring based information used in the weather regime reconstruction to enhance out of sample performance and minimize model complexity. Multiple objectives are considered within the optimization to balance the preservation of low and high frequency modes of variability in the multivariate weather regime dynamics. We reconstruct weather regime frequencies back to 1400 CE and show that the reconstructed weather regimes are consistent with previously identified megadroughts and pluvials. Further, the reconstructed weather regimes exhibit significant variability in the 3–15-year frequency band and extend far beyond the bounds of the instrumental period. Overall, the weather regime reconstructions developed here provide important insight into the extent of natural atmospheric variability that can influence Western US weather.
... It oscillates irregularly every 2-8 years and significantly disrupts global rainfall and SST anomalies patterns (McPhaden et al., 2006;Ropelewski and Halpert, 1987). The teleconnection regulations of ENSO in the global ocean climate system show evident spatiotemporal heterogeneity in terms of intensity, spatial scope, onset, duration, and cessation (Freund et al., 2019). The SCS is located on the northern edge of the Western Pacific Warm Pool (WPWP), and its interannual climate change is primarily regulated by ENSO and associated climate systems (Xie et al., 2016;Zhou and Chan, 2010;Wang et al., 2000). ...
Article
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Massive Porites corals are widely used for paleoclimate reconstruction, however the potential of Porites coral microatoll δ¹⁸O as high-resolution archive of paleoclimate has only been recognized recently, and the systematic chemistry-climate relationship has yet to be well characterized in the western Pacific. In this study, we examined the reproducibility of microatoll δ¹⁸O records against two adjacent Porites corals from the Xisha Islands in the northern South China Sea (SCS) and evaluated the reliability of microatoll δ¹⁸O as a proxy for reconstructing regional climate and El Niño-Southern Oscillation (ENSO) variability. The seasonal to interannual variability in microatoll δ¹⁸O was primarily controlled by local sea surface temperature (SST), while the δ¹⁸O signal might be suppressed by sea surface salinity (SSS) variations on interannual timescale. Despite the overlapping coral δ¹⁸O records exhibited similar patterns of variability, the mean values were consistently offset by ~0.2‰ and the sensitivity of δ¹⁸O proxy to climate also varied across different coral colonies. The microatoll δ¹⁸O exhibited relatively high proxy-SST sensitivities and amplitude of the seasonal variabilities. These results suggested that intercolony δ¹⁸O variability was a significant source of uncertainty in coral-based paleoclimate reconstructions. Microatoll δ¹⁸O anomaly appeared to serve as a sensitive and relatively reliable proxy for ENSO variability, although the imprints of weak-to-moderate ENSO events could not be fully captured due to the complex relationship between the East Asian Monsoon and ENSO, as well as the local seawater salinity changes. This study further strengthened the evidence for microatoll as an alternate climate archive in the SCS and highlighted its potential in helping resolve poorly understood paleoclimate before instrumental observations.
... However, no observational studies have examined the relationship between Arabian Sea aerosols and Indian monsoon rainfall and its sensitivity to ENSO. Also, recent studies have indicated that El-Nino-like conditions will increase in the future due to anthropogenic climate change 32,33 . Therefore, it is essential to understand how ENSO may modulate the relationship between AS dust aerosol and monsoon rainfall. ...
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