Cold SST Anomalies Research Articles

Interannual variation in the East Asian summer monsoon-tropical Atlantic SST relationship modulated by the Interdecadal Pacific Oscillation

Tropical Atlantic (TA) SST variability can impact the East Asian summer monsoon (EASM). Here, we find that the interannual EASM–TA relationship exhibits an evident interdecadal variation modulated by the Interdecadal Pacific Oscillation (IPO). The EASM–TA interannual relationship is strong during the positive phase of the IPO (pIPO) but weak during the negative phase of the IPO (nIPO). The pIPO (nIPO)-related warm (cold) SST anomalies in the central tropical Pacific (CTP) intensify (weaken) the convection over the CTP. Therefore, a Matsuno–Gill response of the TA-induced CTP SST change is strong (weak) during the pIPO (nIPO) period. The strong Matsuno–Gill response excites an anomalous anticyclone over the western North Pacific (WNPAC), leading to a significantly positive EASM–TA relationship. However, the WNPAC is absent in the nIPO periods due to the weak Mastono–Gill response, suggesting an insignificant EASM–TA relationship. Understanding the IPO-modulated EASM–TA relationship helps better forecast EASM variability.

Possible factors responsible for the withdrawal process of the South Asian high during summer-to-winter transition

This study investigates the possible factors responsible for interannual variations in the withdrawal process of the South Asian high (SAH) during its summer-to-winter transition. The SAH is more obviously intensified from July to September and lasts longer over the Tibetan Plateau in late-withdrawal years (L_SAH) than early-withdrawal years (E_SAH). The L_SAH tends to appear during the stage when an El Niño has faded away and a La Niña begins to emerge, with warm SST anomalies occurring over the northern Indian Ocean and tropical northwestern Pacific, and cold anomalies existing across the equatorial central eastern Pacific. The influences of these anomalous SST variations on the SAH's withdrawal process take place primarily through two pathways. On the one hand, warm SST anomalies over the northern Indian Ocean and tropical western Pacific can warm the local tropospheric column above via a Matsunuo-Gill response to moisture adjustment and subsequently lead to an intensified and later withdrawal of the SAH. On the other hand, accompanied by warm SST anomalies, an abnormally intensified anticyclone is observed over the northwestern Pacific that shifts southwestwards and brings abundant rainfall to the region between the Yellow River and Yangtze River in July and August via southwesterly water vapor transport. Consequently, latent heating will generate positive geopotential height anomalies on its northwest side in the upper troposphere and ultimately result in an intensified and later withdrawal of the SAH. In addition, warm surface air temperature over the Tibetan Plateau in September also plays a significant role in lengthening the maintenance of the SAH over the Tibetan Plateau. However, the E_SAH tends be weak and lasts for a shorter period over the Tibetan Plateau. The E_SAH tends to occur together with a developing El Niño event, with cold SST anomalies existing over the northern Indian Ocean and tropical northwestern Pacific. Negative SST anomalies, together with a weak and northeastward-shifting subtropical western Pacific high, result in a weak SAH and quicken its withdrawal process from the Tibetan Plateau via cooling the local tropospheric column and reducing rainfall over East Asia, respectively.

Early summer surface air temperature variability over Pakistan and the role of El Niño–Southern Oscillation teleconnections

AbstractEarly summer (May–June) is the season where the surface air temperature (SAT) variability is largest and may result in extreme temperature conditions over Pakistan. Therefore, we analysed the early summer interannual SAT variability over Pakistan for the period 1981–2018 using observational dataset and model simulations. We noted that upper‐level anticyclonic circulation anomalies over Pakistan, which are associated with upper to the middle tropospheric descending motion, favour clear skies with an increase in net shortwave radiation that leads to extreme surface warming over the region. Moreover, the El Niño–Southern Oscillation (ENSO) based on Niño3.4 SST index is negatively associated with SAT anomalies over Pakistan. The cold ENSO phase favours the positive geopotential height anomalies due to the strengthening of the Walker circulation that enhance the sinking motion and result in less cloudiness leading to the extreme higher surface temperature conditions over Pakistan, while the opposite happens in the warm ENSO phase. Moreover, the Saudi‐KAU Atmospheric Global Climate Model (AGCM) simulated large‐scale patterns that are in good agreement with the observations. Sensitivity experiments with the AGCM confirm that cold SST anomalies in the ENSO region significantly favour above‐normal SAT anomalies over Pakistan, while the opposite happens in the warm ENSO phase. These results are important to understand and potentially predict regional heatwaves over the South Asian region, particularly over Pakistan.

Modulation of Atlantic Multidecadal Oscillation on the Interdecadal Variation of South Asian High and Somali Jet in Summer

As two important components of the Asian summer monsoon system, the intensities of South Asian High (SAH) and Somali jet (SMJ) in summer exhibit both interannual and decadal variabilities. On the interdecadal timescale, the temporal evolution of the SAH intensity is in phase with that of the SMJ intensity. By comparison, we find that both of them evolve synchronously with the Atlantic Multidecadal Oscillation (AMO), with AMO cold/warm phases corresponding to the weakening/strengthening of SAH and SMJ. Further diagnoses indicate that the interdecadal variabilities of the SAH and SMJ intensities in summer may be modulated by the AMO phase. Mechanistically, this modulation appears to be achieved via an interdecadal Silk Road pattern (SRP)-like wave train along the Asian westerly jet and Matsuno–Gill tropical atmospheric response. The cold SST anomaly over extratropical North Atlantic related to the AMO firstly induces an anomalous high over Western Europe and produces a well-organized wave train between 30°N and 60°N. The anomalous Iranian Plateau low along with the wave train path leads to a weakened SAH. Besides, the AMO-related cold SST anomalies over tropical North Atlantic cool the tropical tropospheric atmosphere through the moist adjustment process and produce a Matsuno–Gill-like atmospheric response covering the tropical Indian Ocean. Due to the Matsuno–Gill response, subsidence motion anomalies over the central tropical Indian Ocean corresponding to a result in increased lower-level divergence and upper-level convergence are excited over the tropical Indian Ocean. Finally, the tropical Indian Ocean divergence in the lower troposphere leads to the weakened summer SMJ, and the tropical Indian Ocean convergence in the upper troposphere results in the decrease and northward displacement of SAH in summer.

Impacts of the Atlantic warm pool on North American precipitation and global sea surface temperature in a coupled general circulation model

The responses of North American precipitation to the Atlantic warm pool (AWP) are investigated by using observational data and the NCAR Community Earth System Model. We show that the responses are controlled by different physical mechanisms in different seasons. In the warm season, a large AWP reduces the North Atlantic subtropical high, consistent with Gill’s physics. The corresponding influence over North America is northerly wind anomalies in the lower atmosphere, which leads to a precipitation suppression in the central United States. However, in the cold season the AWP’s impact on North American precipitation is operated via the teleconnection of cold SST anomalies in the tropical Pacific which are induced by the AWP. A large AWP enhances the local Hadley circulation that moves across the equator to the tropical southeastern Pacific in the boreal summer. This inter-hemispheric process strengthens the atmospheric sinking and then increases the South Pacific subtropical high, resulting in the enhancement of the surface easterly trade wind and thus cold SST anomalies in tropical southeastern Pacific. The wind-evaporation-SST and Bjerknes feedbacks further lead tropical central Pacific SST to a La Nina-like pattern, which is consistent with that the significant and negative correlation between ENSO and the AWP is observed after the 1990s when the AWP leads by 6–9 months. The AWP-induced La Nina-like SST anomalies in the tropical central Pacific further prompts a negative phase of the Pacific North American teleconnection, resulting in decreased precipitation over the southern United States; and vice versa. In addition to the AWP-induced central Pacific-type of ENSO events, the paper also shows the influences of the AWP on SSTs in the North Pacific and Indian Oceans.

Decadal Variability of the Anticyclone in the Western North Pacific

AbstractThe western North Pacific anomalous anticyclone (WNPAC) significantly affects East Asian climate. Previous studies have elucidated interannual variability of the WNPAC associated with El Niño, but decadal variability of the WNPAC remains unknown. The present paper investigates the dominant modes of decadal variability of the WNPAC by using observational data. The first decadal mode, characterized by an anomalous anticyclone centered over the western North Pacific, is associated with the Pacific decadal oscillation (PDO). The relationship between the first mode and the PDO shifted from in phase to out of phase around 1966. From 1900 to 1966 when the PDO and the first mode are in phase, the anticyclone is maintained by the effects of both the strengthened Aleutian low through meridional atmospheric forcing and Indian Ocean warming through enhanced zonal Walker circulation. From 1967 to 2012, the anticyclone is induced by cold SST anomalies over the central equatorial Pacific when the PDO and the first mode are out of phase. The second decadal mode is characterized by an anomalous anticyclone extending from southeastern China to the Philippine Sea and is associated with the Maritime Continent (MC). This anticyclone resides in the sinking branch of the local Hadley circulation, triggered by enhanced convection associated with the MC warming from 1900 to 2012. The finding of the decadal WNPAC in this paper may provide a new way to explain East Asian climate on a decadal time scale.

Roles of Wind Stress and Subsurface Cold Water in the Second-Year Cooling of the 2017/18 La Niña Event

After the strong 2015/16 El Nino event, cold conditions prevailed in the tropical Pacific with the second-year cooling of the 2017/18 La Nina event. Many coupled models failed to predict the cold SST anomalies (SSTAs) in 2017. By using the ERA5 and GODAS (Global Ocean Data Assimilation System) products, atmospheric and oceanic factors were examined that could have been responsible for the second-year cooling, including surface wind and the subsurface thermal state. A time sequence is described to demonstrate how the cold SSTAs were produced in the central-eastern equatorial Pacific in late 2017. Since July 2017, easterly anomalies strengthened in the central Pacific; in the meantime, wind stress divergence anomalies emerged in the far eastern region, which strengthened during the following months and propagated westward, contributing to the development of the second-year cooling in 2017. At the subsurface, weak negative temperature anomalies were accompanied by upwelling in the eastern equatorial Pacific, which provided the cold water source for the sea surface. Thereafter, both the cold anomalies and upwelling were enhanced and extended westward in the centraleastern equatorial Pacific. These changes were associated with the seasonally weakened EUC (the Equatorial Undercurrent) and strengthened SEC (the South Equatorial Current), which favored more cold waters being accumulated in the central-equatorial Pacific. Then, the subsurface cold waters stretched upward with the convergence of the horizontal currents and eventually outcropped to the surface. The subsurface-induced SSTAs acted to induce local coupled air-sea interactions, which generated atmospheric-oceanic anomalies developing and evolving into the second-year cooling in the fall of 2017.

Southeastern China Boreal Winter Precipitation Anomalies are Dependent on Intensity of El Ni\xf1o

Previous studies reported that boreal winter precipitation in southeastern China (SEC) tends to increase during El Niño. In this study, however, we find that most weak El Niño events are accompanied by below-normal precipitation in SEC, although strong El Niño events are accompanied by above-normal precipitation in SEC for both eastern Pacific El Niño and central Pacific El Niño. Both the cold SST anomalies in the western North Pacific (WNP) and the warm SST anomalies in the central tropical Pacific are important for the formation of anomalous anticyclone over the WNP, which favors above-normal precipitation over SEC by transporting more water vapor to SEC. The cold SST anomalies in the WNP only excite a weak anomalous anticyclone locally when the weak warm SST anomalies in the central tropical Pacific are accompanied by weak enhanced convection anomalies. In such condition, El Niño does not affect precipitation in SEC apparently.

Modulation of the Northern Winter Stratospheric El Niño–Southern Oscillation Teleconnection by the PDO

Using the CMIP5 multimodel ensemble (MME) historical experiments, the modulation of the stratospheric El Niño–Southern Oscillation (ENSO) teleconnection by the Pacific decadal oscillation (PDO) is investigated in this study. El Niño (La Niña) significantly impacts the extratropical stratosphere mainly during the positive (negative) PDO in the MME. Although the composite tropical ENSO SST intensities are similar during the positive and negative PDO in models, the Pacific–North American (PNA) responses are only significant when the PDO and ENSO are in phase. The local SST anomalies in the North Pacific can constructively (destructively) interfere with the tropical ENSO forcing to influence the extratropical eddy height anomalies when the PDO and ENSO are in (out of) phase. The difference between the positive and negative PDO in El Niño or La Niña winters filters out the tropical SST forcing, permitting the deduction of the extratropical SST contribution to the atmospheric response. The composite shows that the cold (warm) SST anomalies in the central North Pacific associated with the positive (negative) PDO have a similar impact to that of the warm (cold) SST anomalies in the tropical Pacific, exhibiting a positive (negative) PNA-like response, enhancing (weakening) the upward propagation of waves over the western coast of North America. The composite difference between the positive and negative PDO in El Niño or La Niña winters, as well as in eastern Pacific ENSO or central Pacific ENSO winters, presents a highly consistent atmospheric response pattern, which may imply a linear interference of the PDO’s impact with ENSO’s.

Influence of Tropical SSTs on the Interannual Variation of the Summer Monsoon Break over the Western North Pacific

AbstractThe break of the western North Pacific (WNP) summer monsoon (WNPSM) occurs climatologically in early August and is accompanied by a remarkable suppression of convection over the ocean east of the Mariana Islands (10°–20°N, 140°–160°E). This suppression of convection is sandwiched between two convection peaks in late July and mid-August. Two types of monsoon break are identified in the interannual variation of the WNPSM break in the period 1979–2015, exhibiting a distinct subseasonal evolution of convection that is either in phase or out of phase with the climatological evolution. The preceding SST anomalies in the tropical WNP during early and mid-July are responsible for the interannual variation of the monsoon break. Warm (cold) SST anomalies induce an advanced (delayed) evolution of the WNPSM, with the establishment of strong convection in late July (early August) followed by a monsoon break in early August (mid-August). The subseasonal evolution of convection is therefore in phase (out of phase) with that of the climatological mean. The above SST anomalies mainly result from the local wind–evaporation–SST positive feedback during spring and summer. This local air–sea interaction is still robust after the linear regression components related to the variability of ENSO are excluded from the original fields, indicating that it is, to a large extent, independent of ENSO. The ENSO decaying phases have a secondary role in modulating the SST anomalies related to the WNPSM break.

SST variability in the eastern intertropical Indian Ocean – On the search for trigger mechanisms of IOD events

This study focusses on the variability of sea surface temperature in the eastern intertropical Indian Ocean off the coasts of Sumatra and Java - a region that is central to the Indian Ocean Dipole (IOD). Using multiplatform satellite and ARGO data, here we show that the appearance of vast areas of anomalously cold surface water off the southwest Sumatran coast during positive IOD (pIOD) events is preconditioned by upwelling favourable winds along Sumatra's central west coast (1–3°S). A simple theory of coastal upwelling along coasts of finite horizontal extent, developed in this work, can explain this SST response. While this theory is consistent with the pIOD events of the years 1994, 1997 and 2006, it uncovers the existence of another strong upwelling event in the year 2011 that is missed when calculating average SST anomalies in the wider region (i.e. the Dipole Mode Index). In addition, we also derived the surface circulation establishing in the study region during pIOD events. This circulation pattern indicates that equatorward transport of colder water by a classical upwelling jet (that we call the South West Sumatra Current) reinforces negative (cold) SST anomalies off southwest Sumatra.

Atmospheric energetics over the tropical Indian Ocean during Indian Ocean dipole events

The evolution of atmospheric perturbation potential energy (PPE) over the tropical Indian Ocean is analyzed during a composite positive IOD event using reanalysis datasets for the period 1948–2015. The IOD modulates the variation in PPE, which then affects perturbation kinetic energy (PKE) through energy conversion. The PPE anomalies in the lower troposphere (1000–850 hPa) as the dominant layer of the PPE in the whole troposphere (1000–150 hPa) present a dipole pattern corresponding to the anomalous variation in SST during the IOD event. When cold SST anomalies (SSTAs) first appear in the eastern Indian Ocean (IOD-E), they reduce the atmospheric PPE in the lower troposphere rapidly. The negative PPE anomalies lead to less energy conversion to PKE, restraining the surface wind convergence over the IOD-E and weakening the climatological Walker circulation. Meanwhile, the surface easterly wind anomalies strengthen, which depresses the thermocline to the west and gives warmer SSTAs in the western Indian Ocean (IOD-W). The PPE anomalies and energy conversion (CK) over the IOD-W are opposite to those over the IOD-E, the anomalous easterly wind continues to develop, and the positive SSTAs in the IOD-W reach a peak. Thus, the response of the Walker circulation over the Indian Ocean provides a positive feedback during the IOD event and explains the delayed effect of IOD-E SSTAs on IOD-W SSTAs.

Weakening of Northwest Pacific Anticyclone Anomalies during Post–El Niño Summers under Global Warming

The northwest Pacific anticyclone (NWPAC) anomalies during post–El Niño summers are a key predictor of the summer climate in East Asia and the northwestern Pacific (NWP). Understanding how this will change under global warming is crucial to project the changes in the variability of the northwest Pacific summer monsoon. Outputs from 18 selected coupled models from phase 5 of the Coupled Model Intercomparison Project show that the anomalous NWPAC response to El Niño will likely be weakened under global warming, which is attributed to the decreased zonal contrast between the tropical Indian Ocean (TIO) warming and the NWP cooling during post–El Niño summers. Under global warming, the NWPAC anomalies during the El Niño mature winter are weakened because of decreased atmospheric circulation in response to El Niño–Southern Oscillation (ENSO), which leads to the weakening of local air–sea interaction and then decreases the cold NWP SST anomalies. Furthermore, the decreased surface heat flux anomalies, the weakened anticyclone anomalies over the southeastern Indian Ocean, and the slackened anomalous easterlies over the north Indian Ocean weaken the warm TIO SST anomalies. However, the strengthened tropospheric temperature anomalies could enhance the anomalous TIO warming. Although the changes in TIO SST anomalies are indistinctive, the weakening of the SST anomaly gradient between the TIO and the NWP is robust to weaken the NWPAC anomalies during post–El Niño summers. Moreover, the positive feedback between the TIO–NWP SST anomalies and the NWPAC anomalies will enhance the weakening of NWPAC under global warming.

Intrinsic precursors and timescale of the tropical Indian Ocean Dipole: insights from partially decoupled numerical experiment

The intrinsic precursors and timescale of the tropical Indian Ocean Dipole (IOD) are examined with the help of a partially coupled global experiment with decoupled SST over the tropical Pacific. The IOD does exist in the absence of sea surface temperature interannual variability in the tropical Pacific in our modeling framework, but has weaker amplitude and damped Bjerknes feedback. However, IOD variability is much more biennial in the absence than presence of El Nino Southern Oscillation, especially in the eastern equatorial Indian Ocean (IO). Such biennial rhythm results mainly from two mechanisms internal to the IO. The tropical ocean dynamics play a key role in the biennial anomalies during boreal winter with a sudden reversal of thermocline anomalies in the eastern equatorial IO forced by intraseasonal disturbances reminiscent of the Madden–Julian Oscillation (MJO). However, this preconditioning is not sufficient for triggering IOD events in the next boreal spring per se. The main trigger for pure IODs relates to tropical–extratropical interactions within the IO. Convection and diabating heating associated with negative IODs promote a Gill-type tropical response, excite mid-latitude wave-trains and subtropical blocking in the Southern Hemisphere that trigger positive subtropical IOD events during boreal winter. The latter promotes cold SST and anticyclonic circulation anomalies over the southeast IO that persist and migrate northeastward, triggering positive IOD events during the next boreal spring. Accounting for the complementary influence of tropical ocean dynamics coupled to MJO and tropical-extratropical ocean–atmosphere interactions may thus help improving IOD predictability.

Dynamical downscaling of East Asian winter monsoon changes with a regional ocean–atmosphere coupled model

This study investigated the impacts of regional air–sea coupling on the downscaling of historical and future winter climate changes under the Representative Concentration Pathway (RCP) 8.5 scenario over the Coordinated Regional Climate Downscaling Experiment (CORDEX) East Asia domain. Two sets of dynamical downscaling results were compared using a regional ocean–atmosphere coupled model (FROALS) and the corresponding atmosphere‐only regional climate model (RegCM3) at a horizontal resolution of 50 km. Driven by the reanalysis data, the inclusion of the local air–sea coupling process has little impact on the simulation of surface climate over most land regions of East Asia. However, driven by the global climate model, the cold SST anomalies simulated by FROALS were much larger than those driven by the reanalysis data, which favoured a stronger and northward extended low‐level anomalous anticyclone. The improved low‐level winter monsoon flow partly reduced the dry biases (cold biases) over the Yangtze River valley (north China and Mongolia) in RegCM3. Under the RCP8.5 scenario, a weakened Aleutian Low and relatively weak increase of northeasterly over the southern coast of China were found in the FROALS simulation, which were consistent with those from the driving global climate system model. However, the atmosphere‐only RCM exhibited extended regions with decreases in precipitation over east China, and a weaker surface warming over southern China. These projected changes in the atmosphere‐only RCM were followed by the evidently enhanced northerly flow in east China, which was mainly caused by the exaggerated decrease in sea‐level pressure over the western North Pacific as a result of strong responses to the local SST warming. Our results indicated that a regional air–sea coupled model could be a useful tool for the dynamical downscaling of present and future winter climate changes over East Asia.

Relationship between Winter Rainfall in South China and Preceding Heat Content Anomaly over Western Pacific Warm Pool and Its Possible Mechanism

By using the historical SST data provided by Japan Meteorological Agency,NCEP / NCAR reanalysis data,SST data from Hadley center,and the daily precipitation at wintertime in 1979-2010 from 753 stations in China,the relationship between winter rainfall of South China and the preceding heat content anomalies over western Pacific warm pool is studied. The results showthat winter rainfall in South China is negatively related to the heat content anomalies at 0 ~ 200 m over the key area of 6. 5°N-11. 5°N,160. 5°E-172. 5°E in June and July,thus the heat content over the key area could be regarded as a precursor of the winter rainfall. The difference field of the atmospheric circulation in winter between the cold years and the warm years resembles the regressed atmospheric circulation field against the sign reversal of heat content index defined within warm pool. It is found that there is an anomalous teleconnection on the East Asian coast in the 500 h Pa height field,with anomalous lowin South China accompanied by cyclonic circulation on 850 h Pa and divergence in upper troposphere,and with anomalous anticyclonic circulation near Philippines which significantly enhances water vapor transport to South China. It indicates that the winter rainfall in South China is more( less) in cold( warm) years. In summer,the strong westerly anomalies near the equator in the northern hemisphere results in the development of cold( warm) SST to the west( east) of key area. In late autumn,an anomalous anticyclone centered in Philippines is formed on the west of the cold SST anomalies,which is strengthened during the winter. The anomalous enhanced warm air on its northwest side transport water vapor to South China. At the same time,the anomalous Walker circulation,excited by the warm SSTA in the central and eastern Pacific,has sinking branch in the equatorial western Pacific,strengthening the Philippines anticyclone and rising motion in South China. It is the above processes that lead to more( less) winter rainfall in South China in the years of cold( warm) SST over the key area.

Different impacts of various El Niño events on the Indian Ocean Dipole

Our early work (Wang and Wang in J Clim 26:1322-1338, 2013) separates El Nio Modoki events into El Nio Modoki I and II because they show different impacts on rainfall in southern China and typhoon landfall activity. The warm SST anomalies originate in the equatorial central Pacific and subtropical northeastern Pacific for El Nio Modoki I and II, respectively. El Nio Modoki I features a symmetric SST anomaly distribution about the equator with the maximum warming in the equatorial central Pacific, whereas El Nio Modoki II shows an asymmetric distribution with the warm SST anomalies extending from the northeastern Pacific to the equatorial central Pacific. The present paper investigates the influence of the various groups of El Nio events on the Indian Ocean Dipole (IOD). Similar to canonical El Nio, El Nio Modoki I is associated with a weakening of the Walker circulation in the Indo-Pacific region which decreases precipitation in the eastern tropical Indian Ocean and maritime continent and thus results in the surface easterly wind anomalies off Java-Sumatra. Under the Bjerknes feedback, the easterly wind anomalies induce cold SST anomalies off Java- Sumatra, and thus a positive IOD tends to occur in the Indian Ocean during canonical El Nio and El Nio Modoki I. However, El Nio Modoki II has an opposite impact on the Walker circulation, resulting in more precipitation and surface westerly wind anomalies off Java-Sumatra. Thus, El Nio Modoki II is favorable for the onset and development of a negative IOD on the frame of the Bjerknes feedback.

In‐phase transition from the winter monsoon to the summer monsoon over East Asia: Role of the Indian Ocean

Analysis of observations shows that the weak East Asian winter monsoon (EAWM) to weak East Asian summer monsoon (EASM) transitions mainly occur in El Niño decaying years whereas strong EAWM (denoting stronger northerly winds along East Asian coast) to strong EASM (denoting less precipitation along the Meiyu‐Baiu rainband) transitions all occur in non‐El Niño–Southern Oscillation (ENSO) years during the period 1979–2009. This new finding implies that ENSO is not indispensable to the in‐phase EAWM to the EASM transitions. The present study reveals an important role of the Indian Ocean in the strong EAWM to strong EASM transitions and proposes a possible mechanism for these transitions. A strong EAWM induces more precipitation over the Maritime Continent, and the associated anomalous heating excites a Gill‐Matsuno type pattern in the tropics. The resultant wind and cloud changes enhance latent heat flux and reduce downward shortwave radiation over the northwestern Indian Ocean in winter, which leads to SST cooling. The cold SST anomalies persist to summer and excite an anomalous cyclone over the subtropical western North Pacific, leading to a strong EASM. The above processes also operate in the weak EAWM to weak EASM transitions during which El Niño impacts dominate but with additional contributions from the EAWM. The results of observational analysis are confirmed by numerical experiments with a coupled model.

Extreme wet years over southern Africa: Role of Indian Ocean sea surface temperatures

Southern Africa is a predominantly semiarid region with a high degree of interannual rainfall variability. Although much of the recent climate research has focused on the causes of drought events, the region has also experienced extremes of above average rainfall, the most recent examples being the major flooding episodes that devastated Mozambique during 2000 and 2001. This paper investigates extremely wet years over southern Africa during the twentieth century. Focusing on the two most extreme years, 1974 and 1976, we show that while ENSO serves as an important control on rainfall variability, a specific pattern of SSTs in the SW Indian Ocean, with warm anomalies in the subtropical SW Indian Ocean and cool anomalies in the northern SW Indian Ocean that is statistically independent of ENSO, plays a crucial role in generating extreme conditions. To do this, we use a series of multimodel experiments, to demonstrate first the importance of global sea surface temperatures. Through additional idealized experiments with HadAM3, we then isolate the role of SST anomalies in the Indian Ocean. The anomalies are based on the observed SSTs with the ENSO signal linearly removed. The critical influence is tied to cold SST anomalies in the Mascarene region which induce an anomalous anticyclonic circulation driving an anomalous low‐level easterly moisture flux along 10–20°S into eastern southern Africa. This results in enhanced moist convective uplift, conducive to enhanced rainfall, over a large part of southern Africa. Near surface humidity and 500‐hPa omega fields extend from eastern southern Africa into the Agulhas region in a tropical‐temperate cloud band like structure. The similarity between the reanalysis fields for the extreme years and the model experiments is striking.

Improvement of ENSO prediction using a linear regression model with a southern Indian Ocean sea surface temperature predictor

This study presents a detailed comparison between three ENSO precursors which can predict across the spring persistence barrier: the anomalous equatorial Pacific upper ocean heat content, the zonal equatorial wind stress anomaly in the far‐western Pacific and SST anomalies in the South‐East Indian Ocean (SEIO) during the late boreal winter. A new correlation analysis confirms that El Niño (La Niña) onsets are preceded by significant cold (warm) SST anomalies in the SEIO during the late boreal winter after the 1976–77 climate regime shift. Thus, the objective is to examine the respective potential of these three ENSO precursors to predict ENSO events across the boreal spring barrier during recent decades. Surprisingly, in this focus, cross‐validated hindcasts of the linear regression models based on the lagged relationship between Niño3.4 SST and the predictors suggest that SEIO SST anomalies during the late boreal winter is the more robust ENSO predictor.