Zonal Sea Surface Temperature Gradient Research Articles

Large‐scale indices for assessing typhoon activity around Taiwan

This study quantifies large‐scale indices associated with variations in the western Pacific subtropical high (WPSH) and tropical sea surface temperature (SST) to assess typhoon activity around Taiwan. A WPSH index is constructed for both summer and early autumn as the meridional difference of areal means between the northern (120°–130°E, 25°–30°N) and southern (120°–130°E, 20°–25°N) regions (north minus south) of the 500 hPa height. These are regions north and south of Taiwan. In the high index years, typhoons forming in the tropical western North Pacific tend to be steered by easterly/southeasterly flows at the southwestern boundary of a northwards‐shifted WPSH to move northwestwards towards Taiwan, leading to increased typhoon activity around Taiwan. In the low index years, a southwards‐shifted, eastwards‐retreated WPSH steers typhoons along its western periphery to move northwards away from Taiwan. The northwards shift of the WPSH and consequent enhanced typhoon activity around Taiwan associate with a meridional wave train induced by a zonal SST gradient in summer, but link with a Matsuno‐Gill‐type response induced by tropical SSTs in early autumn. As so, the summer SST index is defined as a zonal gradient of areal‐mean SSTs between the tropical western Pacific (150°–175°E, 10°–20°N) and the Bay of Bengal–South China Sea–Philippine Sea region (80°–130°E, 5°–20°N) (former minus latter). In early autumn, the SST index is averaged from the tropical western Pacific over 130°–165°E, 5°S–5°N. All WPSH and SST indices in summer and early autumn are significantly correlated (at the 99% level) with the number of typhoons affecting Taiwan.

Influence of internal climate variability on Indian Ocean Dipole properties

The Indian Ocean Dipole (IOD) is the dominant mode of interannual variability over the tropical Indian Ocean (IO) and its future changes are projected to impact the climate and weather of Australia, East Africa, and Indonesia. Understanding the response of the IOD to a warmer climate has been largely limited to studies of individual coupled general circulation models or multi-model ensembles. This has provided valuable insight into the IOD’s projected response to increasing greenhouse gases but has limitations in accounting for the role of internal climate variability. Using the Community Earth System Model Large Ensemble (CESM-LE), the IOD is examined in thirty-five present-day and future simulations to determine how internal variability influences properties of the IOD and their response to a warmer climate. Despite small perturbations in the initial conditions as the only difference between ensemble members, significant relationships between the mean state of the IO and the IOD arise, leading to a spread in the projected IOD responses to increasing greenhouse gases. This is driven by the positive Bjerknes feedback, where small differences in mean thermocline depth, which are caused by internal climate variability, generate significant variations in IOD amplitude, skewness, and the climatological zonal sea surface temperature gradient.

Sensitivity of subtropical stationary circulations to global warming in climate models: a baroclinic Rossby gyre theory

Time-mean, zonally asymmetric circulations maintain an intense hydrologic contrast between monsoon regions and subtropical drylands in Earth’s present climate. Climate model simulations suggest that this hydrologic contrast will increase in twenty-first-century global warming scenarios, but the response of the zonally asymmetric circulations to global mean temperature is poorly understood. Here we adapt a simple theory for the strength of time-mean, subtropical, zonally asymmetric circulations (hereafter called stationary circulations) and demonstrate its relevance to summer stationary circulation changes in the Northern Hemisphere in an ensemble of comprehensive climate model simulations of global warming. The theory, which is based on the dynamics of a subtropical Rossby gyre that is in Sverdrup balance and has the vertical structure of a first-baroclinic mode, shows that the weakening of stationary ascent with global warming in the multi-model mean can be represented as a compensation between two processes: a lifting of the tropical tropopause and a decrease of the tropospheric zonal temperature gradient, which respectively require strengthening and weakening of vertical mass flux in the Rossby gyre. A large fraction of the intermodel variance in global warming-induced changes in stationary ascent is associated with intermodel variance in zonal tropospheric temperature gradient changes, which we in turn attribute to intermodel variance in zonal sea surface temperature gradient changes. These results show that much of the sensitivity of subtropical hydrologic contrasts to global mean temperature can be understood in terms of a linear vorticity balance and properties of moist adiabats.

A Role for the Equatorial Undercurrent in the Ocean Dynamical Thermostat

AbstractReconstructions of sea surface temperature (SST) based on instrumental observations suggest that the equatorial Pacific zonal SST gradient has increased over the twentieth century. While this increase is suggestive of the ocean dynamical thermostat mechanism of Clement et al., observations of a concurrent weakening of the zonal atmospheric (Walker) circulation are not. Here we show, using heat and momentum budget calculations on an ocean reanalysis dataset, that a seasonal weakening of the zonal atmospheric circulation is in fact consistent with cooling in the eastern equatorial Pacific (EEP) and thus an increase in the zonal SST gradient. This cooling is driven by a strengthening Equatorial Undercurrent (EUC) in response to decreased upper-ocean westward momentum associated with weakening equatorial zonal wind stress. This process can help to reconcile the seemingly contradictory twentieth-century trends in the tropical Pacific atmosphere and ocean. Moreover, it is shown that coupled general circulation models (CGCMs) do not correctly simulate this process; we identify a systematic bias in the relationship between changes in equatorial surface zonal wind stress in the EEP and EUC strength that may help to explain why observations and CGCMs have opposing trends in the zonal SST gradient over the twentieth century.

The Impact of Indian Ocean Mean-State Biases in Climate Models on the Representation of the East African Short Rains

AbstractThe role of the Indian Ocean dipole (IOD) in controlling interannual variability in the East African short rains, from October to December, is examined in state-of-the-art models and in detail in one particular climate model. In observations, a wet short-rainy season is associated with the positive phase of the IOD and anomalous easterly low-level flow across the equatorial Indian Ocean. A model’s ability to capture the teleconnection to the positive IOD is closely related to its representation of the mean state. During the short-rains season, the observed low-level wind in the equatorial Indian Ocean is westerly. However, half of the models analyzed exhibit mean-state easterlies across the entire basin. Specifically, those models that exhibit mean-state low-level equatorial easterlies in the Indian Ocean, rather than the observed westerlies, are unable to capture the latitudinal structure of moisture advection into East Africa during a positive IOD. Furthermore, the associated anomalous easterly surface wind stress causes upwelling in the eastern Indian Ocean. This upwelling draws up cool subsurface waters, enhancing the zonal sea surface temperature gradient between west and east and strengthening the positive IOD pattern, further amplifying the easterly wind stress. This positive Bjerknes coupled feedback is stronger in easterly mean-state models, resulting in a wetter East African short-rain precipitation bias in those models.

Origins of Biases in CMIP5 Models Simulating Northwest Pacific Summertime Atmospheric Circulation Anomalies during the Decaying Phase of ENSO

Abstract The present study documents the biases of summertime northwest Pacific (NWP) atmospheric circulation anomalies during the decaying phase of ENSO and investigates their plausible reasons in 32 models from phase 5 of the Coupled Model Intercomparison Project. Based on an intermodel empirical orthogonal function (EOF) analysis of El Niño–Southern Oscillation (ENSO)-related 850-hPa wind anomalies, the dominant modes of biases are extracted. The first EOF mode, explaining 21.3% of total intermodel variance, is characterized by a cyclone over the NWP, indicating a weaker NWP anticyclone. The cyclone appears to be a Rossby wave response to unrealistic equatorial western Pacific (WP) sea surface temperature (SST) anomalies related to excessive equatorial Pacific cold tongue in the models. On one hand, the cold SST biases increase the mean zonal SST gradient, which further intensifies warm zonal advection, favoring the development and persistence of equatorial WP SST anomalies. On the other hand, they reduce the anomalous convection caused by ENSO-related warming, and the resultant increase in downward shortwave radiation contributes to the SST anomalies there. The second EOF mode, explaining 18.6% of total intermodel variance, features an anticyclone over the NWP with location shifted northward. The related SST anomalies in the Indo-Pacific sector show a tripole structure, with warming in the tropical Indian Ocean and equatorial central and eastern Pacific and cooling in the NWP. The Indo-Pacific SST anomalies are highly controlled by ENSO amplitude, which is determined by the intensity of subtropical cells via the adjustment of meridional and vertical advection in the models.

Influences of the North Pacific Victoria Mode on the South China Sea Summer Monsoon

Using the reanalysis data and the numerical experiments of a coupled general circulation model (CGCM), we illustrated that perturbations in the second dominant mode (EOF2) of springtime North Pacific sea surface temperature (SST) variability, referred to as the Victoria mode (VM), are closely linked to variations in the intensity of the South China Sea summer monsoon (SCSSM). The underlying physical mechanism through which the VM affects the SCSSM is similar to the seasonal footprinting mechanism (SFM). Thermodynamic ocean–atmosphere coupling helps the springtime SST anomalies in the subtropics associated with the VM to persist into summer and to develop gradually toward the equator, leading to a weakened zonal SST gradient across the western North Pacific (WNP) to central equatorial Pacific, which in turn induces an anomalous cyclonic flow over the WNP and westerly anomalies in the western equatorial Pacific that tend to strengthen the WNP summer monsoon (WNPSM) as well as the SCSSM. The VM influence on both the WNPSM and SCSSM is intimately tied to its influence on ENSO through westerly anomalies in the western equatorial Pacific.

Nonlinear Meridional Moisture Advection and the ENSO‐Southern China Rainfall Teleconnection

AbstractIn the boreal cooler months of 2015, southern China (SC) experienced the largest rainfall since 1950, exceeding 4 times the standard deviation of SC rainfall. Although an El Niño typically induces a positive SC rainfall anomaly during these months, the unprecedented rainfall increase cannot be explained by the strong El Niño of 2015/2016, and the dynamics is unclear. Here we show that a nonlinear meridional moisture advection contributes substantially to the unprecedented rainfall increase. During cooler months of 2015, the meridional flow anomaly over the South China Sea region, which acts on an El Niño‐induced anomalous meridional moisture gradient, is particularly large and is supported by an anomalous zonal sea surface temperature gradient over the northwestern Pacific, which recorded its largest value in 2015 since 1950. Our study highlights, for the first time, the importance of the nonlinear process associated with the combined impact of a regional sea surface temperature gradient and large‐scale El Niño anomalies in forcing El Niño rainfall teleconnection.

Revisiting the intraseasonal, interannual and interdecadal variability of tropical cyclones in the western North Pacific

This paper reviews the recent progress and research on the variability of tropical cyclones (TCs) at different time scales. Specific focus is placed on how different types of external forcings or climatic oscillations contribute to TC variability in the western North Pacific (WNP). At the intraseasonal scale, recent advances on the distinctive impacts of the Madden–Julian Oscillation (MJO), the Quasi-biweekly Oscillation, and the asymmetric MJO modulation under different El Niño–Southern Oscillation (ENSO) states, as well as the influences of the Pacific–Japan teleconnection, are highlighted. Interannually, recent progress on the influences of the ENSO cycle, different flavors of ENSO, and impacts of Indian Ocean warming is presented. In addition, the uncertainty concerning interdecadal TC variations is discussed, along with the recently proposed modulation mechanisms related to the zonal sea surface temperature gradient, the North Pacific Gyre Oscillation, and the Pacific Decadal Oscillation (PDO). It is hoped that this study can deepen our understanding and provide information that the scientific community can use to improve the seasonal forecasting of TCs in the WNP.

ENSO‐Like Modulated Tropical Pacific Climate Changes Since 2.36 Myr and Its Implication for the Middle Pleistocene Transition

AbstractEl Niño/Southern Oscillation (ENSO) activity and the Pacific Walker Circulation are controlled by the zonal sea surface temperature (SST) gradient between the western and Eastern Equatorial Pacific (EEP) and the corresponding barometric difference. Variations in the zonal SST gradient since the early Pleistocene have primarily been triggered by changes in the SST in the Eastern Equatorial Pacific. However, the response of the ENSO‐like long‐term state to the cooling of the EEP and its coupling role with tropical Pacific climate changes are still not well established. Here we present a high‐resolution grain‐size record spanning the last 2.36 Myr, obtained from marine core sediment located in the West Philippine Sea in order to decipher the tropical pacific climate changes and reveal its controlling mechanism. By combining our data with other long‐term climatic records from the Equatorial Pacific, we demonstrate that the cooling of SST and enhanced upwelling in the EEP resulted in the development of the Walker Circulation and increased monsoon precipitation in Luzon from 2.2 to 1.6 Myr, from 1.2 to 0.8 Myr, and since 0.2 Myr ago. The progressive cooling of the high‐latitudes in the Quaternary may be responsible for our observation here. A newly identified 100 kyr dominant period between 2.2 and 1.6 Myr in the ENSO‐like modulated Pacific climate records indicates that the ENSO‐like system may play a key role in facilitating or responding to the global climate changes.

Low‐Frequency Variability and the Unusual Indian Ocean Dipole Events in 2015 and 2016

AbstractAn unusual positive Indian Ocean Dipole (IOD) event occurred in 2015 associated with the 2015/2016 extreme El Niño. Unlike the canonical IOD, sea surface temperature (SST) warming in the west central tropical Indian Ocean (TIO) dominated the strong zonal SST gradient as cooling off Sumatra‐Java was weak. Over the southeastern TIO, deeper thermocline has suppressed the upwelling cooling since 2012. Such deepened thermocline related to a low‐frequency adjustment and curtailed cool anomalies in the 2015 positive IOD but favored warm anomalies in the 2016 negative IOD. Based on statistical analyses, ocean assimilation data confirm that an IOD‐like pattern exists in the TIO on decadal timescale. During a negative decadal IOD‐like phase, thermocline is deeper in the southeastern TIO; the thermocline‐SST feedback is unfavorable for positive IOD occurrence and intensity, but conducive to negative IOD events. Thus, we propose that the 2015–2016 IOD events are modulated by the low‐frequency variability of thermocline.

Hydrological variability in the western tropical Pacific over the past 700 kyr and its linkage to Northern Hemisphere climatic change

El Niño/Southern Oscillation (ENSO) events and latitudinal shifts of the Intertropical Convergence Zone (ITCZ) cause large scale hydrologic changes in the western tropical Pacific (WTP). However, there is still a debate on the role of ENSO and ITCZ shifts on the hydroclimate in the WTP and their linkage to Northern Hemisphere climate change on glacial-interglacial timescales. For that reason, we present ice volume-corrected seawater δ18O (δ18Osw-ice, a proxy for salinity) and sea surface temperature (SST) records from IMAGES Core MD06-3047B recovered from the northern WTP to investigate the hydroclimate in the WTP during the last 700kyr. Planktic foraminifera Globigerinoides ruber based Mg/Ca-SST and δ18Osw-ice records reveal distinct glacial-interglacial variations. The δ18Osw-ice record shows that our study site is characterized by fresher conditions and thus higher precipitation during glacials than during interglacials. Our results are in good agreement with numerical model simulations, which also suggest a glacial freshening in the northern WTP. A glacial freshening of the WTP is most likely related to a substantial weakening of the Atlantic Meridional Overturning Circulation (AMOC) and/or exposure of the Maritime Continent due to sea level changes which resulted in an enhanced Pacific Walker circulation and/or southward shift of the ITCZ. In addition, SST records from the WTP and the eastern tropical Pacific (ETP) suggest an enhanced tropical Pacific zonal SST gradient and thus La Niña-like conditions, which may have contributed to fresher conditions in the WTP during glacial periods. Besides, our rainfall record is in antiphase to the Chinese speleothem δ18O records, supporting the idea of an opposing trend in precipitation changes between the WTP and East Asia on orbital timescales.

Influences of two types of El Niño event on the Northwest Pacific and tropical Indian Ocean SST anomalies

Based on the HadISST1 and NCEP datasets, we investigated the influences of the central Pacific El Nino event (CP-EL) and eastern Pacific El Nino event (EP-EL) on the Sea Surface Temperature (SST) anomalies of the Tropical Indian Ocean. Considering the remote effect of Indian Ocean warming, we also discussed the anticyclone anomalies over the Northwest Pacific, which is very important for the South China precipitation and East Asian climate. Results show that during the El Nino developing year of EP-EL, cold SST anomalies appear and intensify in the east of tropical Indian Ocean. At the end of that autumn, all the cold SST anomaly events lead to the Indian Ocean Dipole (IOD) events. Basin uniform warm SST anomalies exist in the Indian Ocean in the whole summer of EL decaying year for both CP-and EP-ELs. However, considering the statistical significance, more significant warm SST anomalies only appear in the North Indian Ocean among the June and August of EP-EL decaying year. For further research, EP-EL accompany with Indian Ocean Basin Warming (EPI-EL) and CP El Nino accompany with Indian Ocean Basin Warming (CPI-EL) events are classified. With the remote effects of Indian Ocean SST anomalies, the EPI-and CPI-ELs contribute quite differently to the Northwest Pacific. For the EPI-EL developing year, large-scale warm SST anomalies arise in the North Indian Ocean in May, and persist to the autumn of the El Nino decaying year. However, for the CPI-EL, weak warm SST anomalies in the North Indian Ocean maintain to the El Nino decaying spring. Because of these different SST anomalies in the North Indian Ocean, distinct zonal SST gradient, atmospheric anticyclone and precipitation anomalies emerge over the Northwest Pacific in the El Nino decaying years. Specifically, the large-scale North Indian Ocean warm SST anomalies during the EPI-EL decaying years, can persist to summer and force anomalous updrafts and rainfall over the North Indian Ocean. The atmospheric heating caused by this precipitation anomaly emulates atmospheric Kelvin waves accompanied by low level easterly anomalies over the Northwest Pacific. As a result, a zonal SST gradient with a warm anomaly in the west and a cold anomaly in the east of Northwest Pacific is generated locally. Furthermore, the atmospheric anticyclone and precipitation anomalies over the Northwest Pacific are strengthened again in the decaying summer of EPI-EL. Affected by the local Wind-Evaporation-SST (WES) positive feedback, the suppressed East Asian summer rainfall then persists to the late autumn during EPI-EL decaying year, which is much longer than that of CPI-EL.

Impacts of the Boreal Spring Indo-Pacific Warm Pool Hadley Circulation on Tropical Cyclone Activity over the Western North Pacific

AbstractThis study investigated the impacts of the interannual variability in the boreal spring regional Hadley circulation over the Indo-Pacific warm pool (IPWP) on the tropical cyclone (TC) activity over the western North Pacific (WNP). The principal modes of the interannual variability in the IPWP Hadley circulation were calculated using empirical orthogonal function (EOF) analysis. The leading mode (EOF-1) features cross-equatorial southerly wind anomalies over the Indian Ocean and Maritime Continent and has an evident impact on WNP TC activity during summer. In the summer following a positive phase of the EOF-1, a cyclonic circulation anomaly, with upward motion, positive relative vorticity anomalies, and weak sea level pressure, dominates the WNP, and this favors increased TC genesis. However, large positive vertical wind shear anomalies over the South China Sea and Philippine Sea inhibit the TC intensification. A positive wind–sea surface temperature (SST)–precipitation feedback was found to facilitate the ability of the signal of the EOF-1 to persist until the summer. The westerly wind anomalies converge around 10°N over the WNP, thus increasing precipitation, and this increased precipitation enhances the westerly wind anomalies via a Gill-type response. The strengthened westerly wind anomalies increase total wind speeds, which in turn cool the SST in the Bay of Bengal and the South China Sea, and warm the SST in the eastern WNP, increasing the zonal SST gradient. Consequently, this increased zonal SST gradient further enhances the westerly wind anomalies, strengthens the monsoon trough, and increases the WNP precipitation further. Therefore, the WNP precipitation anomalies are sustained into the summer.

Are Simulated and Observed Twentieth Century Tropical Pacific Sea Surface Temperature Trends Significant Relative to Internal Variability?

AbstractHistorical trends in the tropical Pacific zonal sea surface temperature gradient (SST gradient) are analyzed herein using 41 climate models (83 simulations) and 5 observational data sets. A linear inverse model is trained on each simulation and observational data set to assess if trends in the SST gradient are significant relative to the stationary statistics of internal variability, as would suggest an important role for external forcings such as anthropogenic greenhouse gasses. None of the 83 simulations have a positive trend in the SST gradient, a strengthening of the climatological SST gradient with more warming in the western than eastern tropical Pacific, as large as the mean trend across the five observational data sets. If the observed trends are anthropogenically forced, this discrepancy suggests that state‐of‐the‐art climate models are not capturing the observed response of the tropical Pacific to anthropogenic forcing, with serious implications for confidence in future climate projections. There are caveats to this interpretation, however, as some climate models have a significant strengthening of the SST gradient between 1900 and 2013 Common Era, though smaller in magnitude than the observational data sets, and the strengthening in three out of five observational data sets is insignificant. When combined with observational uncertainties and the possibility of centennial time scale internal variability not sampled by the linear inverse model, this suggests that confident validation of anthropogenic SST gradient trends in climate models will require further emergence of anthropogenic trends. Regardless, the differences in SST gradient trends between climate models and observational data sets are concerning and motivate the need for process‐level validation of the atmosphere‐ocean dynamics relevant to climate change in the tropical Pacific.

Highly variable Pliocene sea surface conditions in the Norwegian Sea

Abstract. The Pliocene was a time of global warmth with small sporadic glaciations, which transitioned towards the larger-scale Pleistocene glacial–interglacial variability. Here, we present high-resolution records of sea surface temperature (SST) and ice-rafted debris (IRD) in the Norwegian Sea from 5.32 to 3.14 Ma, providing evidence that the Pliocene surface conditions of the Norwegian Sea underwent a series of transitions in response to orbital forcing and gateway changes. Average SSTs are 2 °C above the regional Holocene mean, with notable variability on millennial to orbital timescales. Both gradual changes and threshold effects are proposed for the progression of regional climate towards the Late Pliocene intensification of Northern Hemisphere glaciation. Cooling from 4.5 to 4.3 Ma may be linked to the onset of poleward flow through the Bering Strait. This cooling was further intensified by a period of cool summers due to weak obliquity forcing. A 7 °C warming of the Norwegian Sea at 4.0 Ma suggests a major increase in northward heat transport from the North Atlantic, leading to an enhanced zonal SST gradient in the Nordic Seas, which may be linked to the expansion of sea ice in the Arctic and Nordic Seas. A warm Norwegian Sea and enhanced zonal temperature gradient between 4.0 and 3.6 Ma may have been a priming factor for increased glaciation around the Nordic Seas due to enhanced evaporation and precipitation at high northern latitudes.

Changes in equatorial zonal circulations and precipitation in the context of the global warming and natural modes

The strengthening and westward shift of Pacific Walker Circulation (PWC) is observed during the recent decades. However, the relative roles of global warming and natural variability on the change in PWC unclearly remain. By conducting numerical atmospheric general circulation model (AGCM) experiments using the spatial SST patterns in the global warming and natural modes which are obtained by the multi-variate EOF analysis from three variables including precipitation, sea surface temperature (SST), and divergent zonal wind, we indicated that the westward shift and strengthening of PWC are caused by the global warming SST pattern in the global warming mode and the negative Interdecadal Pacific Oscillation-like SST pattern in the natural mode. The SST distribution of the Pacific Ocean (PO) has more influence on the changes in equatorial zonal circulations and tropical precipitation than that of the Indian Ocean (IO) and Atlantic Ocean (AO). The change in precipitation is also related to the equatorial zonal circulations variation through the upward and downward motions of the circulations. The IO and AO SST anomalies in the global warming mode can affect on the changes in equatorial zonal circulations, but the influence of PO SST disturbs the changes in Indian Walker Circulation and Atlantic Walker Circulation which are affected by the anomalous SST over the IO and AO. The zonal shift of PWC is found to be highly associated with a zonal gradient of SST over the PO through the idealized numerical AGCM experiments and predictions of CMIP5 models.

Variations in the western Pacific warm pool across the mid-Pleistocene: Evidence from oxygen isotopes and coccoliths in the West Philippine Sea

We present planktonic foraminifera oxygen isotope and Florisphaera profunda abundance data from the International Marine Global Change Study Program (IMAGES) Core MD06-3050, which was collected in the West Philippine Sea on the margin of the Western Pacific Warm Pool (WPWP). Our records reveal marked changes in the upper water structure in the Kuroshio source region as early as 1.5Ma, continuing through the mid-Pleistocene climate transition (MPT). These changes are evidenced by reconstructions of the thermocline (based on planktonic foraminiferal oxygen isotope differences) and the nutricline (based on the relative abundance of the coccolithophore species Florisphaera profunda) in the Kuroshio source region. The evolution of the thermocline/nutricline featured three prominent phases: 1) phase I (1.5–1.1Ma), which was characterized by low-amplitude variations in a shallow thermocline; 2) phase II (1.1–0.8Ma), which was characterized by high-amplitude variations in a relatively shallow thermocline/nutricline; and 3) phase III (0.8–0.5Ma), which was characterized by low-amplitude variations in a deep thermocline/nutricline. Combined with the zonal sea surface temperature gradient in the tropical Pacific, the evolution of the upper water structure can be linked to tropical climatic changes across the MPT, including the intensification of atmospheric circulation and a gradual cessation of the long-term El Niño-like state. The upper water structure of the WPWP was asymmetric before 0.8Ma, with a shallow thermocline/nutricline along the northern margin and a deep thermocline/nutricline in the center of the WPWP and South China Sea (SCS). The modern WPWP was established by approximately 0.8Ma, and the upper water structure has changed synchronously as a whole since that time. A comparison of the δ18O values derived from Globigerinoides ruber tests from the MD06–3050 core with those from other cores in the western Pacific suggests that the lower glacial/interglacial G. ruber δ18O values in MD06-3050 before 0.8Ma could be related to the enhancement of the WPWP and to changes in atmospheric circulation patterns.

Linkages between the South and East Asian summer monsoons: a review and revisit

The relationship between the South Asia monsoon (SAM) and the East Asia monsoon (EAM) possibly modulated by both external forcings and internal dynamics has been a long-standing and controversial issue in climate sciences. This study reviews their linkages as revealed in modern records and model simulations during the past, present and future, and provides a comprehensive explanation of the key mechanisms controlling the diversity of the SAM–EAM relationship. Particular attention is paid to several external forcings that modulate the relationship, including El Niño and Southern Oscillation, Indian Ocean Dipole mode (IODM), boreal summer teleconnections, and Eurasian snow extent on intraseasonal to interdecadal timescales. The major focus is placed on two integral views of the inter-connection between the two monsoon systems: one is the positive inter-correlation, which is associated with decaying El Niño and developing Indian Ocean sea surface temperature (SST) warming anomalies; the other is the negative inter-correlation, resulting from developing El Niño and western Pacific SST cooling. The IODM mode also has a delayed impact on the negative connection by modulating Eurasian snow cover. The observed evidence reveals that the recent intensification of the negative relationship is attributable to the strengthening of the zonal SST gradient along the Indian Ocean, western Pacific, and eastern Pacific. Analysis of experiments in the fifth phase of the Coupled Model Intercomparison Project further indicates a possibility for the negative linkage to be further enhanced under anthropogenic global warming with considerable interdecadal modulation in mid and late twenty-first century.

Onset of the Bay of Bengal summer monsoon and the seasonal timing of ENSO's decay phase

ABSTRACTBased on multiple sources of atmospheric and oceanic data, this study demonstrates a close relationship between the onset of the Bay of Bengal (BOB) summer monsoon (BOBSM) and the seasonal timing of El Niño–Southern Oscillation (ENSO)'s decay phase. Through distinguishing ‘later‐decay’ and ‘normal‐decay’ ENSO events, it is found that a later/earlier onset of the BOBSM following El Niño/La Niña is mainly caused by later‐decay ENSO events, while no significant changes in BOBSM onset can be identified between normal‐decay El Niño and normal‐decay La Niña events. Diagnosis of the related dynamic and thermodynamic processes further confirms that, for later‐decay ENSO events that remain active until mid‐April, persistent ENSO‐induced ‘atmospheric‐bridge’ processes can yield a stronger vertical coupling of circulation between the upper and lower troposphere, and thus an anomalously earlier (later) BOBSM onset following a later‐decay La Niña (El Niño). In the lower troposphere, the persisting zonal sea surface temperature (SST) gradient between the Indian Ocean and the western Pacific following later‐decay ENSO events can significantly modulate the lower tropospheric barotropic instability over the northern BOB, which in turn overwhelms the local SST conditions to dominate the development of monsoon convections. In the upper troposphere, later‐decay ENSO alters the position of the South Asian high and the upper atmospheric divergence‐pumping through the anomalous Walker circulation. In contrast, due to the earlier damping of normal‐decay ENSO, the BOBSM onset processes are barely modulated.