Interdecadal Enhancement of the Walker Circulation over the Tropical Pacific in the Late 1990s

The Walker circulation is one of the major components of the large-scale tropical atmospheric circulation and variations in its strength are critical to equatorial Pacific Ocean circulation. It has been argued in the literature that during the 20th century the Walker circulation weakened, and that this weakening was attributable to anthropogenic climate change. By using updated observations, we show that there has been a rapid interdecadal enhancement of the Walker circulation since the late 1990s. Associated with this enhancement is enhanced precipitation in the tropical western Pacific, anomalous westerlies in the upper troposphere, descent in the central and eastern tropical Pacific, and anomalous surface easterlies in the western and central tropical Pacific. The characteristics of associated oceanic changes are a strengthened thermocline slope and an enhanced zonal SST gradient across the tropical Pacific. Many characteristics of these changes are similar to those associated with the mid-1970s climate shift with an opposite sign. We also show that the interdecadal variability of the Walker circulation in the tropical Pacific is inversely correlated to the interdecadal variability of the zonal circulation in the tropical Atlantic. An enhancement of the Walker circulation in the tropical Pacific is associated with a weakening zonal circulation in the tropical Atlantic and vise versa, implying an inter-Atlantic-Pacific connection of the zonal overturning circulation variation. Whether these recent changes will be sustained is not yet clear, but our research highlights the importance of understanding the interdecadal variability, as well as the long-term trends, that influence tropical circulation.

CMIP5 Simulated Change in the Intensity of the Hadley and Walker Circulations from the Perspective of Velocity Potential

Based on the simulations of 31 global models in CMIP5, the performance of the models in simulating the Hadley and Walker circulations is evaluated. In addition, their change in intensity by the end of the 21st century （2080-2099） under the RCP4.5 and RCP8.5 scenarios, relative to 1986-2005, is analyzed from the perspective of 200 hPa velocity potential. Validation shows good performance of the individual CMIP5 models and the multi-model ensemble mean （MME） in re- producing the meridional （zonal） structure and magnitude of Hadley （Walker） circulation. The MME can also capture the observed strengthening tendency of the winter Hadley circulation and weakening tendency of the Walker circulation. Such secular trends can be simulated by 39% and 74% of the models, respectively. The MME projection indicates that the winter Hadley circulation and the Walker circulation will weaken under both scenarios by the end of the 21st century. The weak- ening amplitude is larger under RCP8.5 than RCP4.5, due to stronger external forcing. The majority of the CMIP5 models show the same projection as the MME. However, for the summer Hadley circulation, the MME shows little change under RCP4.5 and large intermodel spread is apparent. Around half of the models project an increase, and the other half project a decrease. Under the RCP8.5 scenario, the MME and 65% of the models project a weakening of the summer southern Hadley circulation.

Interannual Weakening of the Tropical Pacific Walker Circulation Due to Strong Tropical Volcanism

In order to examine the response of the tropical Pacific Walker circulation（PWC） to strong tropical volcanic eruptions（SVEs）, we analyzed a three-member long-term simulation performed with Had CM3, and carried out four additional CAM4 experiments. We found that the PWC shows a significant interannual weakening after SVEs. The cooling effect from SVEs is able to cool the entire tropics. However, cooling over the Maritime Continent is stronger than that over the central-eastern tropical Pacific. Thus, non-uniform zonal temperature anomalies can be seen following SVEs. As a result, the sea level pressure gradient between the tropical Pacific and the Maritime Continent is reduced, which weakens trade winds over the tropical Pacific. Therefore, the PWC is weakened during this period. At the same time, due to the cooling subtropical and midlatitude Pacific, the Intertropical Convergence Zone（ITCZ） and South Pacific convergence zone（SPCZ） are weakened and shift to the equator. These changes also contribute to the weakened PWC. Meanwhile, through the positive Bjerknes feedback, weakened trade winds cause El Nino-like SST anomalies over the tropical Pacific, which in turn further influence the PWC. Therefore, the PWC significantly weakens after SVEs. The CAM4 experiments further confirm the influences from surface cooling over the Maritime Continent and subtropical/midlatitude Pacific on the PWC. Moreover, they indicate that the stronger cooling over the Maritime Continent plays a dominant role in weakening the PWC after SVEs. In the observations,a weakened PWC and a related El Nino-like SST pattern can be found following SVEs.

Strengthening of the Walker Circulation under Global Warming in an Aqua-Planet General Circulation Model Simulation

Most climate models project a weakening of the Walker circulation under global warming scenarios. It is argued, based on a global averaged moisture budget, that this weakening can be attributed to a slower rate of rainfall increase compared to that of moisture increase, which leads to a decrease in ascending motion. Through an idealized aqua-planet simulation in which a zonal wavenumber-1 SST distribution is prescribed along the equator, we find that the Walker circulation is strengthened under a uniform 2-K SST warming, even though the global mean rainfall-moisture relationship remains the same. Further diagnosis shows that the ascending branch of the Walker cell is enhanced in the upper troposphere but weakened in the lower troposphere. As a result, a ＂double-cell＂ circulation change pattern with a clockwise （anti-clockwise） circulation anomaly in the upper （lower） troposphere forms, and the upper tropospheric circulation change dominates. The mechanism for the formation of the ＂double cell＂ circulation pattern is attributed to a larger （smaller） rate of increase of diabatic heating than static stability in the upper （lower） troposphere. The result indicates that the future change of the Walker circulation cannot simply be interpreted based on a global mean moisture budget argument.

A New Index About the Walker Circulation

The Walker circulation(WC)has always been an important issue in atmospheric science research due to the association between the WC and tropical Pacific sea surface temperature(SST),and between the WC and ENSO events.In this paper,a new index-Omega index(OMGI)-is constructed for WC characterization based on the NCEP/NCAR reanalysis data of monthly mean vertical velocity in recent 70 years(1948-2017).Results show that the OMGI can accurately depict the variation characteristics of WC on seasonal,annual and decadal time-scales.There is a significant inverse correlation between the OMGI and equatorial eastern and central Pacific SST.Meanwhile,the peak of the OMGI appears ahead of the ENSO peak,and therefore is able to reflect the SST in the equatorial Pacific.Especially,in 35 ENSO events,the peak of the OMGI appears earlier than Ni?o 3.4 index for 19 times with 2.6 months ahead on average.In 16 El Ni?o events,the peak of the OMGI occurs ahead of the El Ni?o for 9 times with 4 months ahead on average.In19 La Ni?a events,the OMGI peak arises 10 times earlier than the La Ni?a peak,with an average of 1.4 months ahead.OMGI shows obvious leading effect and stability over ENSO events with different strengths and types of single peak and multi peaks:the peak of the OMGI keeps about 2-3 months ahead of the ENSO.Compared with other WC indexes such as UWI and SPLI,OMGI has some advantages in the ability to describe WC changes and present the probability and the time of prediction of ENSO event peaks.

Diagnostic Equations for the Walker Circulation

Two linear partial differential equations are derived in spherical—isobaric coordinates for the numerical simulation of the Walker circulation with the assumption that the meridional motion equation remains in gradient balance. One is for the Walker circulation along the individual latitude in the tropical area, the other for the meridionally—averaged Walker circulation over a tropical zone. Key words Walker circulation - ENSO - Southern Oscillation This work was supported by the “ National key programme for developing basic science” G 1998040900 Part 1 and the National Natural Science Foundation of China (49875021).

Impacts of uncertain cloud-related parameters on Pacific Walker circulation simulation in GAMIL2

To investigate the impacts of uncertain parameters on simulated Pacific Walker circulation （PWC）, a large number of perturbed parameter simulations are conducted using GAMIL2 （the Grid-point Atmospheric Model of IAP/LASG, version 2）, and three different PWC indices are selected.The results show that the influences of some parameters on PWC are dependent on the selected index - a finding supported by the inconsistent responses of different indexes to these parameters. Among the nine parameters, the RH threshold for deep convection （RHCRIT） is the most sensitive in simulating PWC. Increased RHCRIT weakens deep convective heating and stratiform cooling, and strengthens shallow convective heating. Further analysis reveals that uncertain parameters affect the simulated PWC through changing the diabatic heating and vertical motion.

The Energy Cycle Associated to the Pacific Walker Circulation and Its Relationship to ENSO

In this paper we study the Lorenz energy cycle of the Walker circulation associated with ENSO. The robust formulation of the energetics allows drawing a clear picture of the global energy and conversion terms associated with the three dimensional domains appropriate to qualify the large scale transfers that influence, and are influenced by, the anomalies during ENSO. A clear picture has emerged in that El Nino and La Nina years have approximately opposite anomalous energy fluxes, regardless of a non-linear response identified in the potential energy fields (zonal and eddy). During El Ninos the tropical atmosphere is characterized by an increase of zonal available potential energy, decrease of eddy available potential energy and decrease of kinetic energy fields. This results in weaker upper level jets and a slowingdown of the overall Walker cell. During La Ninas reversed conditions are triggered, with an acceleration of the Walker cell as observed from the positive anomalous kinetic energy. The potential energy in the Walker circulation domain during the cold phase is also reduced. An equally opposite behavior is also experienced by the energy conversion terms according to the ENSO phase. The energetics-anomalous behavior seem to be triggered at about the same time when ENSO starts to manifest for both the positive and negative phases, suggesting a coupled mechanism in which atmospheric and oceanic anomalies interact and feed back onto each other.

A Tri-mode of Mock-Walker Cells

This work uses cloud-resolving simulations to study mock-Walker cells driven by a specified sea surface temperature(SST).The associated precipitation in the mock-Walker cells exhibits three different modes,including a single peak of precipitation over the SST maximum(mode 1),symmetric double peaks of precipitation straddling the SST maximum(mode 2),and a single peak of precipitation on one side of the SST maximum(mode 3).The three modes are caused by three distinct convective activity center migration traits.Analyses indicate that the virtual effect of water vapor plays an important role in differentiating the three modes.When the SST gradient is large,the virtual effect may be strong enough to overcome the temperature effect,generating a low-level low-pressure anomaly below the ascending branch of the Walker cell off the center.The results here highlight the importance of the virtual effect of water vapor and its interaction with convection and large-scale circulation in the Walker circulation.

Interactions between the 30－60 Day Oscillation，the Walker Circulation and the Convective Activities in the Tropical Western Pacific and Their Relations to the Interannual Oscillation

In this paper, interactions between the 30-60 day oscillation, the Walker circulation and the convective activitiesin the tropical western Pacific during the Northern Hemisphere summer are analyzed by using the observed data ofwind fields and high-cloud amounts for the period from 1980 to 1989.The analyzed results show that the 30-60 day oscillation (hereafter called LFO) may be largely affected by theconvective activities in the tropical western Pacific. The LFO in the tropical western Pacific during the strongconvective activities around the Philippines stronger than those during the weak convective activities around thePhilippines. Moreover. in the case of strong convective activities around the Philippines, the LFO in the tropical western Pacific and tropical eastern Indian Ocean generally propagates westward, and it is intensified by the LFO with awestward propagating center of maximum oscillation trom the east to 140°E. However, in the case of weakconvective activities around the Philippines, the LFO gradually becomes stronger with a eastward propagating centerof maximum oscillation from the eastern Indian Ocean to the tropical western Pacific.Corresponding to the 30-60 day oscillation, the Walker circulation is also in oscillation over the tropical Pacificand its circulation cell seems to shift gradually westward from the tropical western Pacific to the tropical eastern Indian Ocean with strong convective activities around the Philippines. This may maintain the intensitication ofconvective activities there. However, during the weak convective activities around the Philippines, the Walker circulation gradually moves eastward and an ascending flow may appear in the equatorial central Pacific. This may causeconvective activities to be intensified over the equatorial central Pacific.The analyzed results also show that the LFO in the tropical western Pacific and East Asia may be associated withthe interannual oscillation of the SST anomaly in the tropical western Pacific.

Changes of the tropical Pacific Walker circulation simulated by two versions of FGOALS model

Here we assessed the performances of IAP/LASG climate system model FGOALS-g2 and FGOAS-s2 in the simulation of the tropical Pacific Walker circulation （WC）. Both models reasonably reproduce the climatological spatial distribution features of the tropical Pacific WC. We also investigated the changes of WC simulated by two versions of FGOALS model and discussed the mechanism responsible for WC changes. Observed Indo-Pacific sea level pressure （SLP） reveals a reduction of WC during 1900-2004 and 1950-2004, and an enhancement of WC during 1982-2004. During the three different time spans, the WC in FGOALS-g2 shows a weakening trend. In FGOALS-s2, tropical Pacific atmospheric circulation shows no significant change over the past century, but the WC strengthens during 1950-2004 and 1982-2004. The simulated bias of the WC change may be related to the phase of the multi-decadal mode in coupled models, which is not in sync with that in the observations. The change of WC is explained by the hydrological cycle constraints that precipitation must be balanced with the moisture trans- porting from the atmospheric boundary layer to the free troposphere. In FGOALS-g2, the increasing amplitude of the relative variability of precipitation （AP/P） is smaller （larger） than the relative variability of moisture （Aq/q） over the tropical western （eastern） Pacific over the three time spans, and thus leads to a weakened WC. In FGOALS-s2, the convective mass exchange fluxes increase （decrease） over the tropical western （eastern） Pacific over the past 53 a （1950-2004） and the last 23 a （1982- 2004）, and thus leads to a strengthened WC. The distributions of sea surface temperature （SST） trends dominate the change of WC. Over the past 55 a and 23 a, tropical Pacific SST shows an E1 Nifto-like （a La Nifia-like） trend pattern in FGOALS-g2 （FGOALS-s2）, which drives the weakening （strengthening） of WC. Therefore, a successful simulation of the tropical Pacific SST change pattern is necessary for a reasonable simulation of WC change in climate system models. This idea is further sup- ported by the diagnosis of historical sea surface temperature driven AGCM-simulations.

The behavior of deep convective clouds over the warm pool and connection to the Walker circulation

As the deep convective clouds(DCCs) over the western Pacific and Indian Ocean warm pool may play different roles in the climate system, variations in DCC properties over these two sectors are investigated and compared. The DCC intensity and area varies more significantly in the Indian Ocean than the western Pacific sector, while the DCC frequency is comparable in both sectors at the seasonal scale. Although the Indian Ocean sector is strongly dominated by the seasonal evolution, the interannual variations in the two sectors are comparable for all three DCC properties(frequency, intensity, and area). Besides,Walker circulation is closely correlated with the interannual variability of DCCs in both sectors. The Walker circulation strengthens(weakens) as the DCCs shift eastward(westward) over the Indian Ocean sector and westward(eastward) over the western Pacific sector. When more or stronger DCCs occur over the Indian Ocean sector(western Pacific sector), the Walker circulation becomes stronger(weaker) and shifts westward(eastward). Interestingly, the response of the Walker circulation to DCC variability over the warm pool is asymmetry. The asymmetry response of the Walker circulation to the negative and positive DCC anomaly may be related to the non-linearity internal variability of the atmosphere. DCCs over the Indian Ocean sector have a much weaker nonlinear correlation with the Walker circulation than DCCs over the western Pacific sector.

Impact of tropical Atlantic warming on the Pacific Walker circulation with numerical experiments of CGCM

The Pacific Walker circulation(PWC)was weak in the 20th century,but its strength increased in an interdecadal scale in the late 1990s.Previous studies have suggested that it could be caused by the warming of the tropical Atlantic Ocean,or induced by the warming of the tropical Indian Ocean.The tropical Atlantic Ocean would not only directly affect the PWC through the equatorial east Pacific to the west,but also produce an indirect effect to the east through the equatorial west Indian Ocean.Using a coupled general circulation model,we designed a series of tropical Atlantic Heating and Heating_Shut experiments with different heating rates,to detect the mechanism of the impact of tropical Atlantic warming on the PWC.Results show that the tropical Atlantic heating weakens the Atlantic Walker circulation but strengthens the PWC.Diagnostics of multiple physical variables with coherent lowereupper troposphere structure show the responses of the Indian Ocean to the Atlantic heating play a critical role in the strengthening of the PWC.The Atlanticelinked atmosphere over the tropical Indian Ocean exerts a significantly positive heat flux onto the ocean there,greatly warming the tropical Indian Ocean,especially on the west part.This produces strong convectively ascending at the equatorial West Indian Ocean,but descending at the East-central Indian Ocean,corresponding to a‘Walker’circulation and an‘antieWalker’circulation situated at the West and East equatorial Indian Ocean respectively.Meanwhile,the convergence(divergence)of the lower(upper)troposphere over the Indo‒Pacific region is also strengthened.In this way,the tropical Atlantic heating is linked to the PWC through the circulation over the equatorial Indian Ocean.This study serves as a preliminary step to understand the impact of tropical Atlantic warming on the PWC,more Atlantic heating sensitivity studies with multi-model experiments are required to further reveal the linkage of the Pacific and Atlantic.

Assessing the Relationship between October-November-December Rainfall and Indian Ocean Dipole in Recent Decades over Tanzania Following the 2011 Abrupt Change

The present study explored how the Indian Ocean Dipole (IOD) influences October-November-December (OND) rainfall over Tanzania in recent decade following the 2011 abrupt change. The study spans 50 years, from 1973 to 2022. Notable abrupt changes were observed in 1976 and 2011, leading us to divide our study into two periods: 1976-2010 and 2011-2022, allowing for a close investigation into the existing relationship between OND IOD and OND rainfall and their associated large-scale atmospheric circulations. It was found that the relationship between OND IOD and OND rainfall strengthened, with the correlation changed from +0.73 during 1976-2010 to +0.81 during 2011-2022. Further investigation revealed that, during 1976-2010, areas that received above- normal rainfall during positive IOD experienced below-normal during 2011- 2022 and vice versa. The same pattern relationship was observed for negative IOD. Spatial analysis demonstrates that the percentage departure of rainfall across the region mirrors the standardized rainfall anomalies. The study highlights that the changing relationship between OND IOD and OND rainfall corresponds to the east-west shift of Walker circulation, as well as the north-south shift of Hadley circulation. Analysis of sea surface temperature (SST) indicates that both positive and negative IOD events strengthened during 2011-2022 compared to 1976-2010. Close monitoring of this relationship across different timescales could be useful for updating OND rainfall seasonal forecasts in Tanzania, serving as a tool for reducing socio-economic impacts.

Interdecadal Variability of the East Asian Summer Monsoon and Associated Atmospheric Circulations

Based on the National Centers for Environmental Prediction and National Center for Atmospheric Research （NCEP/NCAR） reanalysis data from 1950-1999, interdecadal variability of the East Asian Summer Monsoon （EASM） and its associated atmospheric circulations are investigated. The EASM exhibits a distinct interdecadal variation, with stronger （weaker） summer monsoon maintained from 1950-1964 （1976-1997）. In the former case, there is an enhanced Walker cell in the eastern Pacific and an anti-Walker cell in the western Pacific. The associated ascending motion resides in the central Pacific, which flows eastward and westward in the upper troposphere, descending in the eastern and western ends of the Pacific basin. At the same time, an anomalous East Asian Hadley Cell （EAHC） is found to connect the low-latitude and mid-latitude systems in East Asia, which strengthens the EASM. The descending branch of the EAHC lies in the west part of the anti-Walker cell, flowing northward in the lower troposphere and then ascending at the south of Lake Baikal （40°-50°N, 95°- 115°E） before returning to low latitudes in the upper troposphere, thus strengthening the EASM. The relationship between the EASM and SST in the eastern tropical Pacific is also discussed. A possible mechanism is proposed to link interdecadal variation of the EASM with the eastern tropical Pacific SST. A warmer sea surface temperature anomaly （SSTA） therein induces anomalous ascending motion in the eastern Pacific, resulting in a weaker Walker cell, and at the same time inducing an anomalous Walker cell in the western Pacific and an enhanced EAHC, leading to a weaker EASM. Furthermore, the interdecadal variation of summer precipitation over North China is found to be the south of Lake Baikal through enhancing and reducing strongly regulated by the velocity potential over the regional vertical motions.

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Nino Events

The inverse relationship between the warm phase of the El Nino Southern Oscillation(ENSO)and the Indian Summer Monsoon Rainfall(ISMR)is well established.Yet,some El Nino events that occur in the early months of the year(boreal spring)transform into a neutral phase before the start of summer,whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season.This study investigates the distinct influences of an exhausted spring El Nino(springtime)and emerging summer El Nino(summertime)on the regional variability of ISMR.The two ENSO categories were formulated based on the time of occurrence of positive SST anomalies over the Nino-3.4 region in the Pacific.The ISMR’s dynamical and thermodynamical responses to such events were investigated using standard metrics such as the Walker and Hadley circulations,vertically integrated moisture flux convergence(VIMFC),wind shear,and upper atmospheric circulation.The monsoon circulation features are remarkably different in response to the exhausted spring El Nino and emerging summer El Nino phases,which distinctly dictate regional rainfall variability.The dynamic and thermodynamic responses reveal that exhausted spring El Nino events favor excess monsoon rainfall over eastern peninsular India and deficit rainfall over the core monsoon regions of central India.In contrast,emerging summer El Nino events negatively impact the seasonal rainfall over the country,except for a few regions along the west coast and northeast India.

ENSO Indices and Analyses

New ENSO indices were developed and the spatial variability and temporal evolution of ENSO were analyzed based on the new indices and modeling experiments, as well as multiple data resources. The new indices, after being defined, were validated with their good diagnostic characteristics and correlation with wind and SST. In the analysis after the definition and validation of the new indices, ENSO feedbacks from wind, heat fluxes, and precipitation were spatially and temporally examined in order to understand ENSO variability and evolution with some emphasized points such as the interaction among the feedbacks, the role of westerly wind bursts and the transformation between zonal and meridional circulations in an ENSO cycle, and the typical pattern of modern ENSO.

Influence of solar wind energy flux on the interannual variability of ENSO in the subsequent year

Previous studies have tended to adopt the quasi-decadal variability of the solar cycle （e.g.sunspot number （SSN） or solar radio flux at 10.7 cm （F10.7） to investigate the effect of solar activity on El Ni（n）o-Southern Oscillation （ENSO）.As one of the major terrestrial energy sources,the effect of solar wind energy flux in Earth＇s magnetosphere （Ein） on the climate has not drawn much attention,due to the big challenge associated with its quantitative estimation.Based on a new Ein index estimated by three-dimensional magnetohydrodynamic simulations from a previous study,this study reveals that Ein exhibits both quasi-decadal variability （periodic 11-year） and interannual （2-4 years） variability,which has rarely before been detected by SSN and F10.7.A significant interannual relationship between the annual mean Ein and subsequent early-winter ENSO is further revealed.Following high Ein,the sea level pressure in the subsequent early winter shows significant positive anomalies from Asia southward to the Maritime Continent,and significant negative anomalies over the Southeast and Northeast Pacific,resembling the Southern Oscillation.Meanwhile,significant upper-level anomalous convergence and divergence winds appear over the western and eastern Pacific,which is configured with significant lower-level anomalous divergence and convergence,indicating a weakening of the Walker circulation.Consequently,notable surface easterly wind anomalies prevail over the eastern tropical Pacific,leading to El Ni（n

Long-term change in low-cloud cover in Southeast China during cold seasons

Southeast China has comparable stratus cloud to that over the oceans,especially in the cold seasons(winter and spring),and this cloud has a substantial impact on energy and hydrological cycles.However,uncertainties remain across datasets and simulation results about the long-term trend in low-cloud cover in Southeast China,making it difficult to understand climate change and related physical processes.In this study,multiple datasets and numerical simulations were applied to show that low-cloud cover in Southeast China has gone through two stages since 1980—specifically,a decline and then a rise,with the turning point around 2008.The regional moisture transport plays a crucial role in low-cloud cover changes in the cold seasons and is mainly affected by the Hadley Cell in winter and the Walker Circulation in spring,respectively.The moisture transport was not well simulated in CMIP6 climate models,leading to poor simulation of the low-cloud cover trend in these models.This study provides insights into further understanding the regional climate changes in Southeast China.

LINKAGE BETWEEN INDIAN OCEAN DIPOLE AND TWO TYPES OF El NI?O AND ITS POSSIBLE MECHANISMS

After compositing three representative ENSO indices,El Nio events have been divided into an eastern pattern(EP) and a central pattern(CP).By using EOF,correlation and composite analysis,the relationship and possible mechanisms between Indian Ocean Dipole(IOD) and two types of El Nio were investigated.IOD events,originating from Indo-Pacific scale air-sea interaction,are composed of two modes,which are associated with EP and CP El Ni o respectively.The IOD mode related to EP El Nio events(named as IOD1) is strongest at the depth of 50 to 150 m along the equatorial Indian Ocean.Besides,it shows a quasi-symmetric distribution,stronger in the south of the Equator.The IOD mode associated with CP El Nio(named as IOD2) has strongest signal in tropical southern Indian Ocean surface.In terms of mechanisms,before EP El Nio peaks,anomalous Walker circulation produces strong anomalous easterlies in equatorial Indian Ocean,resulting in upwelling in the east,decreasing sea temperature there;a couple of anomalous anticyclones(stronger in the south) form off the Equator where warm water accumulates,and thus the IOD1 occurs.When CP El Nio develops,anomalous Walker circulation is weaker and shifts its center to the west,therefore anomalous easterlies in equatorial Indian Ocean is less strong.Besides,the anticyclone south of Sumatra strengthens,and the southerlies east of it bring cold water from higher latitudes and northerlies west of it bring warm water from lower latitudes to the 15° to 25°S zone.Meanwhile,there exists strong divergence in the east and convergence in the west part of tropical southern Indian Ocean,making sea temperature fall and rise separately.Therefore,IOD2 lies farther south.