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Publications (341)
Five out of six La Niña events since 1998 have lasted two to three years. Why so many long-lasting multiyear La Niña events have emerged recently and whether they will become more common remains unknown. Here we show that ten multiyear La Niña events over the past century had an accelerated trend, with eight of these occurring after 1970. The two t...
The tropical Atlantic climate is characterized by prominent and correlated multidecadal variability in Atlantic sea surface temperatures (SSTs), Sahel rainfall and hurricane activity1–4. Owing to uncertainties in both the models and the observations, the origin of the physical relationships among these systems has remained controversial3–7. Here we...
Most current climate models predict that the equatorial Pacific will evolve under greenhouse gas–induced warming to a more El Niño-like state over the next several decades, with a reduced zonal sea surface temperature gradient and weakened atmospheric Walker circulation. Yet, observations over the last 50 y show the opposite trend, toward a more La...
Prediction of the seasonal monsoon rainfall over India relies largely on the well-known relationship with El Niño and Southern Oscillation (ENSO) and is possible because reasonably reliable seasonal predictions of ENSO are now available. Usually, the cold phase of ENSO is associated with above-normal monsoon rainfall and the warm phase of ENSO with...
Tropical rainfall variations are of direct societal relevance and drive climate variations worldwide via teleconnections. The convective rainfall tends to occur when sea surface temperature (SST) exceeds a threshold, SST thr , usually taken to be constant in time and space. We analyze 40-year monthly observations and find that SST thr varies by up...
The Atlantic multidecadal variability (AMV) – a basin-scale sea surface temperature (SST) fluctuation in the Atlantic – has dramatic influences on climate 1–8 . Understanding its causes has important social-ecological implications. However, the driver of AMV and its impacts remains debated because of limitations of current climate models 9–13 . Her...
Plain Language Summary
The global sea surface temperature (SST) shows a positive trend of ∼0.55 K century⁻¹ during the last century (1920–2005), due primarily to the rise in atmospheric CO2 values. In contrast, the SST in the subpolar North Atlantic has cooled at a rate of ∼0.4 K century⁻¹, known as the “warming hole.” Many previous studies using c...
The trends over recent decades in tropical Pacific sea surface and upper ocean temperature are examined in observations-based products, an ocean reanalysis and the latest models from the Coupled Model Intercomparison Project phase six and the Multimodel Large Ensembles Archive. Comparison is made using three metrics of sea surface temperature (SST)...
Atlantic multidecadal variability (AMV) impacts temperature, precipitation, and extreme events on both sides of the Atlantic Ocean basin. Previous studies with climate models have suggested that when external radiative forcing is held constant, the large-scale ocean and atmosphere circulation are associated with sea surface temperature (SST) anomal...
Hitting time, or first passage time, is the first time that a process reaches (from above or below) a pre-specified area. Estimating expected hitting time is an important practical problem in many areas of science. In this paper, we discuss a novel and practical approach to estimating expected hitting time of [b, ∞), for a pre-specified boundary le...
The North Atlantic Oscillation (NAO) is predictable in climate models at near-decadal timescales. Predictive skill derives from ocean initialization, which can capture variability internal to the climate system, and from external radiative forcing. Herein, we show that predictive skill for the NAO in a very large uninitialized multi-model ensemble...
Persistent multiyear cold states of the tropical Pacific Ocean drive hydroclimate anomalies worldwide, including persistent droughts in the extratropical Americas. Here, the atmosphere and ocean dynamics and thermodynamics of multiyear cold states of the tropical Pacific Ocean are investigated using European Centre for Medium-Range Weather Forecast...
This paper attempts to enhance our understanding of the causes of Atlantic Multidecadal Variability, the AMV. Following the literature, we define the AMV as the SST averaged over the North Atlantic basin, linearly detrended and low-pass filtered. There is an ongoing debate about the drivers of the AMV, which include internal variability generated f...
The European Great Famine of 1315–1317 triggered one of the worst population collapses in European history and ranks as the single worst European famine in mortality as a proportion of population. Historical records point to torrential rainfall, land saturation, crop failure, and prolonged flooding as important causes of the famine. Here we use the...
El Niño’s intensity change under anthropogenic warming is of great importance to society, yet current climate models’ projections remain largely uncertain. The current classification of El Niño does not distinguish the strong from the moderate El Niño events, making it difficult to project future change of El Niño’s intensity. Here we classify 33 E...
As exemplified by El Niño, the tropical Pacific Ocean strongly influences regional climates and their variability worldwide1–3. It also regulates the rate of global temperature rise in response to rising GHGs⁴. The tropical Pacific Ocean response to rising GHGs impacts all of the world’s population. State-of-the-art climate models predict that risi...
Large-scale modes of climate variability can force widespread crop yield anomalies and are therefore often presented as a risk to food security. We quantify how modes of climate variability contribute to crop production variance. We find that the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), tropical Atlantic variability (TAV)...
North Atlantic sea surface temperatures (SST) exhibit a lagged response to the North Atlantic Oscillation (NAO) in both models and observations, which has previously been attributed to changes in ocean heat transport. Here we examine the lagged relationship between the NAO and Atlantic multidecadal variability (AMV) in the context of the two other...
Author summary
Mathematical models are now used to forecast infectious disease incidence at the population scale. By better understanding how errors in prediction systems are introduced, grow and impact the predictability of infectious disease, forecast accuracy could be improved. Here we explore the growth pattern of errors introduced from two maj...
The Matlab code for the perturbed EAKF and ILI+ data for 95 cities in seasons from 2003 to 2013.
(ZIP)
Forecast results from the baseline and perturbed EAKF for 1- to 4-week predictions, in the format of the CDC influenza forecast challenge.
(ZIP)
Tropical Pacific decadal variability (TPDV), though not the totality of Pacific decadal variability, has wideranging climatic impacts. It is currently unclear whether this phenomenon is predictable. In this study, we reconstruct the attractor of the tropical Pacific system in long, unforced simulations from an intermediatecomplexity model, two gene...
From 1875 to 1878, concurrent multiyear droughts in Asia, Brazil, and Africa, referred to as the Great Drought, caused widespread crop failures, catalyzing the so-called Global Famine, which had fatalities exceeding 50 million people and long-lasting societal consequences. Observations, paleoclimate reconstructions, and climatemodel simulations are...
Previous studies suggest that internal variability, in particular the Atlantic Meridional Overturning Circulation (AMOC), drives the Atlantic Multidecadal Oscillation (AMV), while external radiative forcing only creates a steady increase in sea surface temperature (SST). This view has been recently challenged and new evidence has emerged that aeros...
The Tropical Pacific Ocean displays persistently cool sea surface temperature (SST) anomalies that last several years to a decade, with either no El Niño events or a few weak El Niño events. These cause large-scale droughts in the extratropics, including major North American droughts such as the 1930s Dust Bowl, and also modulate the global mean su...
The recent California drought was associated with a persistent ridge at the west coast of North America that has been associated with, in part, forcing from warm SST anomalies in the tropical west Pacific. Here it is considered whether there is a role for human-induced climate change in favoring such a west coast ridge. The models from phase 5 of t...
Abstarct
In this model study the authors explore the possibility that the internal component of the Atlantic multidecadal oscillation (AMO) sea surface temperature (SST) signal is indistinguishable from the response to white noise forcing from the atmosphere and ocean. Here, complex models are compared without externally varying forcing with a one-...
We analyze the Atlantic multidecadal oscillation (AMO) in the Pre-industrial (PI) and Historical (HIST) simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to assess the drivers of the observed AMO from 1865 to 2005. We draw 141-year samples from the 41 CMIP5 model's PI runs and compare the correlation and variance between th...
Focusing on ENSO seasonal phase locking, diversity in peak location, and propagation direction, as well as the El Niño–La Niña asymmetry in amplitude, duration, and transition, a set of empirical probabilistic diagnostics (EPD) is introduced to investigate how the ENSO behaviors reflected in SST may change in a warming climate.
EPD is first applie...
Over the last two centuries, the impact of the Human System has grown dramatically, becoming strongly dominant within the Earth System in many different ways. Consumption, inequality, and population have increased extremely fast, especially since about 1950, threatening to overwhelm the many critical functions and ecosystems of the Earth System. Ch...
The Atlantic Multidecadal Oscillation (AMO) affects climate variability in the North Atlantic basin and adjacent continents with potential societal impacts. Previous studies based on model simulations and short-term satellite retrievals hypothesized an important role for cloud radiative forcing in modulating the persistence of the AMO in the tropic...
A suite of empirical model experiments under the empirical model reduction framework are conducted to advance the understanding of ENSO diversity, nonlinearity, seasonality, and the memory effect in the simulation and prediction of tropical Pacific sea surface temperature (SST) anomalies. The model training and evaluation are carried out using 4000...
Ocean circulation changes not needed
What causes the pattern of sea surface temperature change that is seen in the North Atlantic Ocean? This naturally occurring quasi-cyclical variation, known as the Atlantic Multidecadal Oscillation (AMO), affects weather and climate. Some have suggested that the AMO is a consequence of variable large-scale ocean...
We use a multilevel vector autoregressive model (VAR-L), to forecast sea surface temperature anomalies (SSTAs) in the Atlantic hurricane Main Development Region (MDR). VAR-L is a linear regression model using global SSTA data from L prior months as predictors. In hindcasts for the recent 30 years, the multilevel VAR-L outperforms a state-of-the-art...
The authors investigate a sea surface temperature anomaly (SSTA)-only vector autoregressive (VAR) model for prediction of El Niño-Southern Oscillation (ENSO). VAR generalizes the linear inverse method (LIM) framework to incorporate an extended state vector including many months of recent prior SSTA in addition to the present state. An SSTA-only VAR...
Despite the tremendous progress in the theory, observation and prediction of El Niño over the past three decades, the classification of El Niño diversity and the genesis of such diversity are still debated. This uncertainty renders El Niño prediction a continuously challenging task, as manifested by the absence of the large warm event in 2014 that...
Workshop on Connecting the Tropics to the Polar Regions; New York, New York, 2–3 June 2014
East African precipitation is characterized by a dry annual mean climatology compared to other deep tropical land areas and a bimodal annual cycle with the major rainy season during March–May (MAM; often called the ''long rains'') and the second during October–December (OND; often called the ''short rains''). To explore these distinctive features,...
Significance
There is evidence that the 2007−2010 drought contributed to the conflict in Syria. It was the worst drought in the instrumental record, causing widespread crop failure and a mass migration of farming families to urban centers. Century-long observed trends in precipitation, temperature, and sea-level pressure, supported by climate model...
East Africa has two rainy seasons: the long rains [March–May (MAM)] and the short rains [October–December (OND)]. Most CMIP3/5 coupled models overestimate the short rains while underestimating the long rains. In this study, the East African rainfall bias is investigated by comparing the coupled historical simulations from CMIP5 to the corresponding...
Decadal variability of the East African precipitation during the season of March–May (long rains) is examined and the performance of a series of models in simulating the observed features is assessed. Observational results show that the drying trend of the long rains is associated with decadal natural variability associated with sea surface tempera...
A multiscale-modeling framework for daily rainfall is considered and diagnostic results are presented for an application to the winter season in Northwest India. The daily rainfall process is considered to follow a Hidden Markov Model (HMM), with the hidden states assumed to be an unknown random function of slowly varying climatic modulation of the...
I must first pay tribute to Klaus Wyrtki, a hero of mine who passed away earlier this year. At a time when ideas about El Niño pointed all over the place, he told me that Bjerknes's hypothesis was the way forward. He was right, of course, but Bjerknes stopped short of explaining the oscillatory nature of ENSO, and it took Klaus's tide gauge data to...
The presence of rich ENSO variability in the long unforced simulation of GFDL’s CM2.1 motivates the use of tools from dynamical systems theory to study variability in ENSO predictability, and its connections to ENSO magnitude, frequency, and physical evolution. Local Lyapunov exponents (LLEs) estimated from the monthly NINO3 SSTa model output are u...
The presence of rich ENSO variability in the long unforced simulation of GFDL’s CM2.1 motivates the use of tools from dynamical systems theory to study variability in ENSO predictability, and its connections to ENSO magnitude, frequency, and physical evolution. Local Lyapunov exponents (LLEs) estimated from the monthly NINO3 SSTa model output are u...
From 2005 to 2010, Syria experienced the most severe and persistent drought in the instrumental record, devastating the agriculture and causing widespread crop failure. A mass migration of farming families to urban peripheries followed the resulting food shortages, unemployment, and disruption of rural social structure. The addition of nearly 1.5 m...
Snowmelt-dominated streamflow of the Western Himalayan rivers is an important water resource during the dry pre-monsoon spring months to meet the irrigation and hydropower needs in northern India. Here we study the seasonal prediction of melt-dominated total inflow into the Bhakra Dam in northern India based on statistical relationships with meteor...
During the last glacial period, the North Atlantic regionexperienced
pronounced, millennial-scale alternations between cold, stadial
conditions and milder interstadial conditions--commonly referred to as
Dansgaard-Oeschger oscillations--as well as periods of massive iceberg
discharge known as Heinrich events. Changes in Northern Hemisphere
temperat...
In this paper, zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ECMWF Interim Re- Analysis (ERA-Interim). It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure g...
As a result of global warming, precipitation is likely to increase in high latitudes and the tropics and to decrease in already dry subtropical regions. The absolute magnitude and regional details of such changes, however, remain intensely debated. As is well known from El Niño studies, sea-surface-temperature gradients across the tropical Pacific...
Here we analyze the variability of MAM (March–April–May) and JJAS (June–July–August–September) seasonal Satluj River flow into the Bhakra dam in India through Pearson anomaly correlation and composite analyses with antecedent and concurrent seasonal climatic and atmospheric circulation patterns. The MAM seasonal inflow of Bhakra dam is significantl...
During the last glacial period, the North Atlantic region experienced pronounced, millennial-scale alternations between cold, stadial conditions and milder interstadial conditions-commonly referred to as Dansgaard-Oeschger oscillations-as well as periods of massive iceberg discharge known as Heinrich events. Changes in Northern Hemisphere temperatu...
onal momentum balances of the tropical atmospheric circulation during the global monsoon mature months ( January and July ) are analyzed in three dimensions based on the ERA-Interim reanalysis. It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis forc...
Growing scientific evidence from modern climate science is loaded with implications for the environmental history of the Roman Empire and its successor societies. The written and archaeological evidence, although richer than commonly realized, is unevenly distributed over time and space. A first synthesis of what the written records and multiple na...
Snowmelt dominated streamflow of the Western Himalayan Rivers is an important water resource during the dry pre-monsoon spring months to meet the irrigation and hydropower needs in Northern India. Here we study the seasonal prediction of melt-
5 dominated total inflow into the Bhakra Dam in Northern India based on statistical relationships with meteo...
Snowmelt dominated streamflow of the Western Himalayan Rivers is an important wa-ter resource during the dry pre-monsoon spring months to meet the irrigation and hy-dropower needs in Northern India. Here we study the seasonal prediction of melt-dominated total inflow into the Bhakra Dam in Northern India based on statistical re-5 lationships with m...
The global monsoon circulation and precipitation in July and January are
simulated in a simple tropical beta-plane model with two baroclinic free
troposphere layers above plus one atmospheric boundary layer at the
bottom, forced by the sea surface temperature (SST) and prescribed water
vapor profile. The moist convective heating in the model is
par...
Observed Pacific decadal climate variability (PDV) has been described as
ENSO-like due to the similarity of the sea surface temperature and
atmospheric circulation anomalies with ENSO. From this analogy it is
also expected that PDV is predictable. However, climate models have a
much lower skill predicting PDV compared with ENSO. Why is this? Here w...
Interannual rainfall variations in equatorial East Africa are tightly
linked to the El Niño Southern Oscillation (ENSO), with more rain
and flooding during El Niño and droughts in La Niña years,
both having severe impacts on water stress and food security. Here we
report evidence from an annually laminated lake-sediment record from
southeastern Ken...
By analyzing a set of the Coupled Model Intercomparison Project phase 3 (CMIP3) climate model projections of the twenty-first century, it is found that the shallow meridional overturning of the Pacific subtropical cells (STCs) show contrasting trends between two hemispheres in a warming climate. The strength of STCs and equivalently the STC surface...
It has been proposed that changes in global climate have been responsible for episodes of widespread violence and even the collapse of civilizations. Yet previous studies have not shown that violence can be attributed to the global climate, only that random weather events might be correlated with conflict in some cases. Here we directly associate p...
Interannual rainfall variations in equatorial East Africa are tightly linked to the El Niño Southern Oscillation (ENSO), with more rain and flooding during El Niño and droughts in La Niña years, both having severe impacts on human habitation and food security. Here we report evidence from an annually laminated lake sediment record from southeastern...
Decadal variability of ENSO is present in historical and paleo records,
and has been simulated by a hierarchy of dynamical and statistical
models. The ENSO variability in the IPCC AR4 Coupled GCMs ranges from
constant periodicity or amplitude to significant inter-decadal
variability in both period and amplitude. While long runs of
intermediate dyna...
Snowmelt dominated streamflow of the Western Himalayan rivers is an
important water resource during the dry pre-monsoon months to meet the
irrigation and hydropower needs in northern India. On the other hand,
winter precipitation as the form of snow over Himalayas helps in
maintaining the glaciers, which serve as a storehouse of freshwater
througho...
El Niño–Southern Oscillation (ENSO) related precipitation anomalies in North America are related to changes in the paths of storm systems across the Pacific Ocean, with a more southern route into southwestern North America during El Niños and a more northern route into the Pacific Northwest during La Niñas. Daily reanalysis data are analyzed to con...
The midlatitude response to tropical Pacific SST anomalies involves changes in transient eddy propagation, but the processes leading to the transient eddy changes are still not clear. In a recent study, we used a series of controlled general circulation model (GCM) experiments in which an imposed tropical Pacific sea-surface temperature (SST) anoma...
