Warm Pool Sea Surface Temperature Research Articles

Pliocene Weakening of Gradients in Temperature but Not in Productivity in the Eastern Equatorial Pacific

AbstractThe modern eastern equatorial Pacific Ocean (EEP) exhibits strong upwelling, producing pronounced gradients in sea surface temperature (SST), nutrient concentration, and biological productivity between 80° and 140°W. During the globally warmer late Pliocene (3.0–3.6 Ma), the EEP may have experienced permanent El Niño‐like conditions, supported by a reduced SST gradient across the equatorial Pacific. However, the weakened east‐west SST gradient has been controversial, with disparate results depending on the proxy used to monitor Western Warm Pool SSTs. We present new Pliocene alkenone‐based SST and paleoproductivity records from four Ocean Drilling Program (ODP) cores spanning an east‐west transect across the EEP, which present an internally consistent picture of SST and productivity gradients in the modern cold tongue, resolved at orbital‐scale variability. Strong agreement between core top reconstructions and satellite estimates indicates that alkenone paleotemperature and paleoproductivity proxies are appropriate for reconstructing Pliocene EEP conditions. The average SST gradient between 90° and 120°W was reduced from the modern 1.8°C gradient to 0.9°C in the late Pliocene. Despite the weakened SST gradient, the surface productivity gradient was stronger during the late Pliocene compared to modern, based on calibrated X‐ray fluorescence biogenic opal and alkenone average accumulation rates. Contrary to modern El Niño SST and productivity patterns, reduced Pliocene surface productivity did not accompany the weakened SST gradient. Instead, strong Pliocene biogenic opal and alkenone concentration accumulation gradients in the eastern EEP suggest that subsurface tilting of the nutricline and thermocline persisted to supply vigorous upwelling of warm but nutrient‐rich subsurface waters in a warmer climate.

A Hyperactive End to the Atlantic Hurricane Season October–November 2020

Abstract The active 2020 Atlantic hurricane season produced 30 named storms, 14 hurricanes, and 7 major hurricanes (category 3+ on the Saffir–Simpson hurricane wind scale). Though the season was active overall, the final two months (October–November) raised 2020 into the upper echelon of Atlantic hurricane activity for integrated metrics such as accumulated cyclone energy (ACE). This study focuses on October–November 2020, when 7 named storms, 6 hurricanes, and 5 major hurricanes formed and produced ACE of 74 × 104 kt2 (1 kt ≈ 0.51 m s−1). Since 1950, October–November 2020 ranks tied for third for named storms, first for hurricanes and major hurricanes, and second for ACE. Six named storms also underwent rapid intensification (≥30 kt intensification in ≤24 h) in October–November 2020—the most on record. This manuscript includes a climatological analysis of October–November tropical cyclones (TCs) and their primary formation regions. In 2020, anomalously low wind shear in the western Caribbean and Gulf of Mexico, likely driven by a moderate-intensity La Niña event and anomalously high sea surface temperatures (SSTs) in the Caribbean, provided dynamic and thermodynamic conditions that were much more conducive than normal for late-season TC formation and rapid intensification. This study also highlights October–November 2020 landfalls, including Hurricanes Delta and Zeta in Louisiana and in Mexico and Hurricanes Eta and Iota in Nicaragua. The active late season in the Caribbean would have been anticipated by a statistical model using the July–September-averaged ENSO longitude index and Atlantic warm pool SSTs as predictors.

Synchronized spatial shifts of Hadley and Walker circulations

Abstract. The El Niño–Southern Oscillation (ENSO) influences the most extensive tropospheric circulation cells on our planet, known as Hadley and Walker circulations. Previous studies have largely focused on the effect of ENSO on the strength of these cells. However, what has remained uncertain is whether interannual sea surface temperature anomalies can also cause synchronized spatial shifts of these circulations. Here, by examining the spatiotemporal relationship between Hadley and Walker cells in observations and climate model experiments, we demonstrate that the seasonally evolving warm-pool sea surface temperature (SST) anomalies in the decay phase of an El Niño event generate a meridionally asymmetric Walker circulation response, which couples the zonal and meridional atmospheric overturning circulations. This process, which can be characterized as a phase-synchronized spatial shift in Walker and Hadley cells, is accompanied by cross-equatorial northwesterly low-level flow that diverges from an area of anomalous drying in the western North Pacific and converges towards a region with anomalous moistening in the southern central Pacific. Our results show that the SST-induced concurrent spatial shifts of the two circulations are climatically relevant as they can further amplify extratropical precipitation variability on interannual timescales.

Increase in Lower Stratospheric Water Vapor in the Past 100 Years Related to Tropical Atlantic Warming

AbstractLower Stratospheric water vapor (SWV) is one of important drivers of global climate change. Increases and decreases in lower SWV have been found to strengthen and offset global warming effects, respectively. Using several data sets, we find that sea surface temperature (SST) warming in the past 100 years has caused an increase in SWV. SST warming over the tropical Indian Ocean and the western Pacific has resulted in a drier stratosphere. However, tropical Atlantic Ocean warming has resulted in a significantly wetter stratosphere and is the main contributor to the increasing trend of SWV in the past 100 years. The responses of Rossby and Kelvin waves over the Indian Ocean and western Pacific to Atlantic warming have led to a warmer tropopause temperature, resulting in more water vapor entering the stratosphere. This study suggests that SWV trend may simply be the result of a game between warm pool SST and tropical Atlantic SST changes.

Impact of the Indo-Pacific Warm Pool on the Hadley, Walker, and Monsoon Circulations

The Indo-Pacific warm pool (IPWP) is enclosed by a 28 °C isotherm and plays a vital role in controlling atmospheric circulations. However, the effects of changes in regional warm pool sea surface temperatures (SSTs) remain unexplored. We divided the IPWP into the Indian and Pacific sectors and distinguished their responses to natural variability and global warming. Furthermore, we examined the impacts of the interannual variability (IAV) in warm pool SST on the tropical Hadley, Walker, and monsoon circulations. The Hadley circulation was affected by warm pool SST warming, i.e., warmer SSTs over the warm pool strengthened the upward branch of Hadley circulation, whereas the downward branch was respectively weakened and strengthened in the Northern and Southern Hemispheres. Walker circulation was strengthened (weakened) in the warming (natural) mode. Consequently, the Walker circulation is weakened since the natural variability of warm pool SST plays a more dominant role rather than the warming trend of SSTs over the warm pool. Furthermore, our analysis displays that warm pool warming has little impact on the monsoon circulation. Our findings highlight the different roles of the IAV of warm pool regions in each tropical circulation as part of the warming trend and natural variability.

A Concise and Effective Expression Relating Subsurface Temperature to the Thermocline in the Equatorial Pacific

AbstractThe temperature of the subsurface water entrained into the surface mixed layer plays a key role in controlling the sea surface temperature (SST) and its interannual variability in the equatorial Pacific. In this paper, we combine a hyperbolic tangent function bounded by the warm pool SST and centered at the thermocline depth with a variable sharpness parameter to describe the time‐space evolutions of the subsurface temperature. Under simple approximations of the sharpness parameter, this concise expression becomes remarkably efficient in capturing the observed and climate‐model simulated subsurface temperature variability in terms of anomalies of the thermocline depth and SST of the El Niño‐Southern Oscillation (ENSO) phenomenon. The formulations for the subsurface temperature and thermocline sharpness developed in this work should be useful tools for evaluating and understanding the role of the thermocline feedback in ENSO behaviors in both theoretical and comprehensive climate models.

Influences of West Pacific Sea Surface Temperature on Covarying Eurasian Droughts Since the Little Ice Age

The Western Pacific Warm Pool (WP), with the highest sea surface temperature (SST) in the world, has strong impacts on the drought variations in Eurasia. However, since the little ice age (1250–1850, LIA), the co-climatic drought pattern due to WP warming in Eurasia remains unclear. This is a long-term warming background for the current warming period (CWP). In this paper, we use both instrumental data and 1625 tree-ring width records from Eurasia to investigate the drought patterns in both modern and historical periods. This study revealed two seesaw precipitation patterns, namely the Central Asia–Mongolia (CAMO) and Northern Europe–Southern Europe (NESE) patterns. When the Western Pacific Warm Pool sea surface temperature (WPSST) is high, precipitation increases in Central Asia and Northern Europe, and decreases in Mongolia and southern Europe. When the positive (negative) phase event of the El Niño–Southern Oscillation (ENSO) occurs, the WPSST is reduced (increased), and the decreases (increases) of precipitation in Central Asia and Northern Europe and the increases (decreases) in precipitation in Mongolia and southern Europe are more obvious. The CAMO dipole has been strengthened since the LIA. The CAMO dipole is positively correlated with solar radiation and Northern Hemisphere temperature, and negatively correlated with Pacific decadal oscillations (PDO).

Exploring records of typhoon variability in eastern China over the past 2000 years

AbstractHow climate controls tropical cyclone variability has critical implications for modern human society but is not well understood due to the short length of observational records. To probe this knowledge gap, we present a synthesis of intense typhoon activity from the northwestern Pacific over the past 2000 years, which is supported by a new, well-resolved tidal flat sedimentary record from the Jiangsu coast, eastern China. The record reveals nine intervals of typhoon frequency, indicating that the frequency of intense typhoons has varied on multi-centennial scales over the past 2000 years. Our synthesis shows strong evidence for a seesaw pattern of intense typhoon frequency between southeastern China and Japan and Korea. This pattern can be explained by the El Niño and Southern Oscillation–East Asian Monsoon–sea surface temperature hypothesis, which potentially explains the basin-wide typhoon climate in the northwestern Pacific region. A shift in typhoon activity was identified from 550–280 to 280–50 yr B.P. during the Little Ice Age, when typhoon activity changed from active to quiescent or vice versa. Centennial-scale shifts in Intertropical Convergence Zone and Western Pacific Warm Pool sea surface temperature are likely to be the primary forcing mechanisms driving this shift. Results obtained here provide links between typhoon activity and the El Niño and Southern Oscillation, the East Asian Monsoon, and the Western Pacific Warm Pool sea surface temperature, and therefore improve our ability to fully assess intense typhoon activity in future climate warming.

Improving the dynamical seasonal prediction of western Pacific warm pool sea surface temperatures using a physical–empirical model

AbstractThe western Pacific warm pool (WPWP) has a profound impact on the global climate. In this study, the forecast skill of ENSEMBLES model for predicting the WPWP sea surface temperature (SST) for the period 1960–2006 is evaluated, where a WPWP index (WPWPI) is defined to represent the interannual variability of WPWP SST. The result indicates that the ENSEMBLES exhibit a poor skill in predicting the WPWPI during January–April (2‐ to 5‐month forecasts starting on November 1). To improve the ENSEMBLES‐predicted WPWP SSTs during January–April, a physical–empirical (PE) model is developed based on two predictors, using the year‐to‐year increment method and the linear regression method. The two predictors include the ENSEMBLES‐predicted sea level pressure during January and the observed northern tropical Atlantic SSTs during the preceding August. The mechanisms associated with the two predictors are illuminated. The 1‐year‐out cross‐validation and the independent hindcast indicate that this PE model may notably improve the WPWPI prediction of ENSEMBLES, with a correlation coefficient (CC) above 0.6 between the PE‐model‐predicted WPWPI and the observed WPWPI during January–April. The physical mechanisms expounded in this study and the PE model utilized in this study can be considered to improve the prediction of WPWP SST of numerical models in the future.

Predictability of phases and magnitudes of natural decadal climate variability phenomena in CMIP5 experiments with the UKMO HadCM3, GFDL-CM2.1, NCAR-CCSM4, and MIROC5 global earth system models

Data from decadal hindcast experiments conducted under CMIP5 were used to assess the ability of CM2.1, HadCM3, MIROC5, and CCSM4 Earth System Models (ESMs) to hindcast sea-surface temperature (SST) indices of the Pacific Decadal Oscillation (PDO), the tropical Atlantic SST gradient (TAG) variability, and the West Pacific Warm Pool (WPWP) SST variability from 1961 to 2010. The ESMs were initialized at specific times with observed data to make 10- and 30-year hindcasts/forecasts. Deterministic and probabilistic skill estimates show predictability of detrended WPWP index to 5 years’ lead time and of non-detrended WPWP index to 10 years’ lead time. These estimates also show atypical skill dependence of PDO and TAG indices on lead times, with increasing skill in the middle to end of 10-year hindcasts. The skill of ESMs to hindcast an observed DCV index (signal skill) is surprisingly greater than the skill to hindcast their own DCV index (noise skill) at some lead times. All ESMs hindcast occurrence frequencies of positive and negative phases of the indices, and probabilities of same-phase transitions from one year to the next reasonably well. Four, major, low-latitude volcanic eruptions are associated with phase transitions of all observed and some of the ensemble-average hindcast indices. All ESMs’ WPWP index hindcasts respond correctly to all eruptions as do three observed PDO phase transitions. No one of the ESMs’ hindcasts of the TAG index responds correctly to these eruptions. Some of the ESMs hindcast correct phase transitions in the absence of eruptions also, implying that initializations with observed data are beneficial in predicting phase transitions. The skills of DCV indices’ phase prediction up to at least two years in advance can be used to inform societal impacts adaptation decisions in water resources management and agriculture. The Atlantic region’s responses in these ESMs appear to be fundamentally incorrect.

Simulations of three natural decadal climate variability phenomena in CMIP5 experiments with the UKMO HadCM3, GFDL-CM2.1, NCAR-CCSM4, and MIROC5 global earth system models

The ability of four Earth System Models (ESMs) to simulate major attributes of sea-surface temperature (SST) manifestations of the Pacific Decadal Oscillation (PDO), the tropical Atlantic SST gradient (TAG) variability, and the West Pacific Warm Pool (WPWP) SST variability was assessed. Data from simulation experiments conducted under the CMIP5 with the CM2.1, HadCM3, MIROC5, and CCSM4 ESMs from 1861 to 2005 were used. A Multi-Model Ensemble was also formed by combining data from the four ESMs. Aerosol optical depths and solar radiation were specified in these experiments. The simulation of PDO spatial pattern, annual cycle, and preferred timescales is the closest to observed attributes in these ESMs. The simulated and observed TAG SST patterns are generally similar, but their average annual cycles are very different and simulated TAG variability does not have a preferred decadal timescale unlike observed variability. No one of the ESMs is able to simulate the WPWP SST pattern. Simulated and observed average annual cycles of the WPWP SST are similar, and the dominant timescale of WPWP variability is close to 8 years in all ESMs and observations. Simulation skills of the three phenomena fluctuate from decade to decade in all ESMs. Major volcanic eruptions substantially influence observed and simulated indices and simulation skills of the three phenomena. The mixing of net surface heat flux anomalies to 200 m depth in the tropical and mid-latitude Pacific, and the WPWP region causes a delayed and multiyear response of the PDO and WPWP indices to volcanic eruptions.

Changing relationship between La Niña and tropical cyclone landfalling activity in South China (La Niña and TC landfalling activity in South China)

ABSTRACTThis study investigates the effect of El Niño–Southern Oscillation and Indo‐Pacific warm pool sea‐surface temperature (SST) on tropical cyclone (TC) landfalling activity along the South China coast and develops a scheme for predicting this activity. In general, landfalling activity tends to be suppressed in El Niño years but a large uncertainty is found for La Niña years. Landfalling activity is generally enhanced in La Niña years before 1997 but suppressed in those after 1997, which may be related to the changes in TC frequency and track pattern over the western North Pacific (WNP) as well as the northward shift in genesis locations. The Indo‐Pacific warm pool SST is found to be related to the TC activity over the WNP and an SST index, defined as the average SST anomalies over the North Indian Ocean and the sea near the Philippines, is used to represent this anomaly. A significant warming of the Indo‐Pacific warm pool is found in the recent decade. The temperature change of this warm pool appears to modulate the TC activity over the WNP and TC landfalling activity in South China in a La Niña year, with an enhancement (a suppression) of TC activity if a cooling (warming) is found. This difference is related to the changes in the strength of monsoon trough, vertical wind shear and steering flow pattern. Based on these results, a schematic prediction scheme of landfalling activity in South China is proposed. Landfalling activity is likely to be normal or below normal if an El Niño event is expected to develop during the TC season. If a La Niña event is expected, and the predicted state of the Indo‐Pacific warm pool (obtained using the persistence forecast) shows a warming (cooling), landfalling activity is likely to be normal or below normal (above normal).

Teleconnection patterns impacting on the summer consecutive extreme rainfall in Central‐Eastern China

ABSTRACTConsecutive extreme rainfall (CER) events can cause enormous damage to human society and the environment in eastern China. This study analyses persistent atmospheric circulation patterns related to dominant CER modes over Central‐Eastern China. Results reveal that the two leading modes together explain 30% of the total CER variance. The circulation patterns related to the first CER mode manifest a barotropic Eurasian (EU) pattern teleconnection 4 days before the CER events occur in the southern Yangtze River. The second CER mode is associated with a negative Pacific‐Japan (PJ) teleconnection pattern, which persists for 5 days until the onset of CER events in the Yangtze River Valley. Furthermore, the stagnant double blocking highs around the Urals and the Sea of Okhotsk and the vapour transport from the western North Pacific (WNP) distinguish the persistent circulations of the second CER mode from the first mode. Therefore, the configuration of persistent circulation patterns is likely responsible for the occurrence of CER events. Further analysis suggests that the Rossby waves, in association with dipolar sea surface temperature (SST) anomalies in the northeastern Atlantic, propagate eastward towards East Asia, generating an EU‐like teleconnection 4 days prior to CER event onset. Furthermore, cold SST anomalies in the warm pool and warm SST anomalies in the tropical WNP‐South China Sea and the western Indian Ocean induce poleward‐propagating Rossby waves, creating a negative PJ teleconnection pattern up to 5 days prior to CER events. These results are supported by ray paths originating from base points of EU and PJ teleconnections.

Heat transport variation due to change of North Pacific subtropical gyre interior flow during 1993–2012

Applying segment-wise altimetry-based gravest empirical mode method to expendable bathythermograph temperature, Argo salinity, and altimetric sea surface height data in March, June, and November from San Francisco to near Japan (30∘ N, 145∘ E) via Honolulu, we estimated the component of the heat transport variation caused by change in the southward interior geostrophic flow of the North Pacific subtropical gyre in the top 700 m layer during 1993–2012. The volume transport-weighted temperature (T I) is strongly dependent on the season. The anomaly of T I from the mean seasonal variation, whose standard deviation is 0.14∘C, was revealed to be caused mainly by change in the volume transport in a potential density layer of 25.0−25.5σ 𝜃 . The anomaly of T I was observed to vary on a decadal or shorter, i.e., quasi-decadal (QD), timescale. The QD-scale variation of T I had peaks in 1998 and 2007, equivalent to the reduction in the net heat transport by 6 and 10 TW, respectively, approximately 1 year before those of sea surface temperature (SST) in the warm pool region, east of the Philippines. This suggests that variation in T I affects the warm pool SST through modification of the heat balance owing to the entrainment of southward transported water into the mixed layer.

High-Resolution Simulations of Decadal Climate Variability Impacts on Water Yield in the Missouri River Basin with the Soil and Water Assessment Tool (SWAT)

ABSTRACT The Missouri River basin (MRB) is the largest river basin in the United States and is one of the most important agricultural regions in the world. Three decadal climate variability (DCV) phenomena—the Pacific decadal oscillation (PDO), the tropical Atlantic sea surface temperature (SST) gradient variability (TAG), and the west Pacific warm pool SST variability (WPWP)—substantially influence hydrometeorology in the MRB. The authors report on a simulation study with the Soil and Water Assessment Tool (SWAT) to estimate impacts on water availability in response to realistic values of PDO, TAG, and WPWP indices in approximately 13 500 hydrologic unit areas covering the MRB. SWAT, driven by hydrometeorological anomalies associated with positive and negative phases of PDO and TAG, indicated major impacts on water yields and streamflows, as much as ±40% of the average in many locations. Impacts of the WPWP index were smaller. Consistent with observations during 1949–2010, SWAT showed water flow increases of as much as 80% of the average, causing very wet periods when the positive phase of the PDO and the negative phase of the TAG at extreme amplitudes were superposed. Water flows decreased by a similar amount, resulting in severe to extreme droughts when the negative phase of the PDO and the positive phase of the TAG at extreme amplitudes were superposed. Thus, the combined and cumulative effects of these DCV phenomena on water flows, droughts, and wet periods in the MRB can be dramatic, with important consequences for all water-consuming sectors as well as for feedbacks to the climate system.

Asian summer monsoon variability during the late glacial and Holocene inferred from the stable carbon isotope record of black carbon in the sediments of Muge Co, southeastern Tibetan Plateau, China

A sediment core from an alpine lake (Muge Co) from the southeastern margin of the Tibetan Plateau was analyzed for sediment grain size distribution and the carbon isotopic composition of black carbon in order to reconstruct Asian summer monsoon variability over the last 12 ka. Five major climatic stages were identified: (1) From 12 to 10.5 cal. ka BP, a cold and dry climate prevailed, corresponding to the Younger Dryas in the North Atlantic; (2) from 10.5 to 9.2 cal. ka BP, precipitation increased rapidly but was interrupted by a pronounced cold and dry phase at about 10 cal. ka BP; (3) from 9.2 to 6.2 cal. ka BP, the Holocene climatic optimum was reached and maintained; (4) from 6.2 to 2.5 cal. ka BP, the climate gradually became cooler and drier; (5) from 2.5 cal. ka BP to the present, a climatic reversal toward wetter conditions occurred. The trend of our reconstructed Asian summer monsoon precipitation record is similar on orbital and millennium timescales to that of other records from the Asian monsoon region. Although the maximum monsoon intensity recorded in the Muge Co sediments lagged the peak of Northern Hemisphere insolation by ~3.0 ka, the strengthening trend was in phase with the rise in the Western Pacific Warm Pool sea surface temperature. Therefore, we suggest that the tropical ocean temperatures may have been the major driver of Asian summer monsoon precipitation in the southeastern Tibetan Plateau. In addition, other factors such as sea-level rise and temperature in the North Atlantic region may have played an important role in the evolution of the Asian summer monsoon since the last deglaciation. However, human activity may have affected the monsoon proxies during the last 2.5 ka, and further studies are needed to distinguish the effects of climate and human disturbance.

Quantifying the Intra-Regional Precipitation Variability in Northwestern China over the Past 1,400 Years.

There has been a surge of paleo-climatic/environmental studies of Northwestern China (NW China), a region characterized by a diverse assortment of hydro-climatic systems. Their common approach, however, focuses on “deducing regional resemblance” rather than “exploring regional variance.” To date, efforts to produce a quantitative assessment of long-term intra-regional precipitation variability (IRPV) in NW China has been inadequate. In the present study, we base on historical flood/drought records to compile a decadal IRPV index for NW China spanned AD580–1979 and to find its major determinants via wavelet analysis. Results show that our IRPV index captures the footprints of internal hydro-climatic disparity in NW China. In addition, we find distinct ~120–200 year periodicities in the IRPV index over the Little Ice Age, which are attributable to the change of hydro-climatic influence of ocean-atmospheric modes during the period. Also, we offer statistical evidence of El Niño Southern Oscillation (Indo-Pacific warm pool sea surface temperature and China-wide land surface temperature) as the prominent multi-decadal to centennial (centennial to multi-centennial) determinant of the IRPV in NW China. The present study contributes to the quantitative validation of the long-term IRPV in NW China and its driving forces, covering the periods with and without instrumental records. It may help to comprehend the complex hydro-climatic regimes in the region.

Pliocene warmth and gradients

The Pliocene epoch (5.3–2.6 Ma) generates continued debate as an example of a warm climate with external forcing similar to the present day. O'Brien et al. presented new multi-proxy sea surface temperature (SST) reconstructions from the South China Sea, adding to this debate. Based on their records, and a hypothesized seawater chemistry adjustment to temperature reconstructions previously derived from the Mg/Ca ratios of planktonic foraminifera, they suggest that the western Pacific warm pool was “2 °C warmer than today” in the Pliocene. This contradicts previous evidence of long-term stability in warm pool SSTs, but possibly reconciles temperature reconstructions and climate model simulations. Here we raise several points contrary to those conclusions.

Dynamical changes in the tropical Pacific warm pool and zonal SST gradient during the Pleistocene

AbstractIn the late Pleistocene, the atmospheric carbon dioxide concentration (pCO2) is thought to be a primary driving force for tropical sea surface temperature (SST) change because glacial‐interglacial changes in tropical Pacific SST covary with pCO2. However, if the regional radiative effects of pCO2 were the only agent of change, tropical SST gradients should have remained similar as pCO2 varied with time. Instead, a new record of SST from the west Pacific shows that tropical SST gradients were different, even reversed, in the past, suggesting an important role for dynamical circulation changes. Specifically, changes in the temperature of upwelled source water, in addition to local pCO2 forcing, influenced tropical Pacific SST. These dynamical changes, rather than pCO2, may have shifted the background state of the tropics and even helped set the stage for the mid‐Pleistocene transition.

Multidecadal variability in East African hydroclimate controlled by the Indian Ocean

The recent decades-long decline in East African rainfall suggests that multidecadal variability is an important component of the climate of this vulnerable region. Prior work based on analysing the instrumental record implicates both Indian and Pacific ocean sea surface temperatures (SSTs) as possible drivers of East African multidecadal climate variability, but the short length of the instrumental record precludes a full elucidation of the underlying physical mechanisms. Here we show that on timescales beyond the decadal, the Indian Ocean drives East African rainfall variability by altering the local Walker circulation, whereas the influence of the Pacific Ocean is minimal. Our results, based on proxy indicators of relative moisture balance for the past millennium paired with long control simulations from coupled climate models, reveal that moist conditions in coastal East Africa are associated with cool SSTs (and related descending circulation) in the eastern Indian Ocean and ascending circulation over East Africa. The most prominent event identified in the proxy record--a coastal pluvial from 1680 to 1765--occurred when Indo-Pacific warm pool SSTs reached their minimum values of the past millennium. Taken together, the proxy and model evidence suggests that Indian Ocean SSTs are the primary influence on East African rainfall over multidecadal and perhaps longer timescales.