Anomalous Walker Circulation Research Articles

Interannual relationship of the extreme precipitation dipole mode over South China and Indochina Peninsula with the tropical Pacific-Indian Ocean sea surface temperature anomaly in May

This study investigates the interannual relationship of extreme precipitation days (EPD) over South China (SC) and Indochina Peninsula (ICP) with tropical Pacific-Indian Ocean Sea surface temperature (SST) anomalies during May, utilizing observations, reanalysis, and Coupled-Model Intercomparison Project 6 (CMIP6) model outputs. The results uncover a dipole mode in the interannual variations of May EPD over SC and ICP, characterized by an east–west inverse pattern. This dipole mode is significantly attributed to large-scale circulation anomalies induced by tropical Pacific–Indian Ocean mode (PIM). During the positive phase of PIM, SST anomalies in the tropical east-central Pacific and western Indian Oceans are anomalously warm, contrasted with negative SST anomalies in the western Pacific, leading to anomalous Walker circulations. Accordingly, two robust anticyclones appear in the lower levels of the Northwest Pacific and the Bay of Bengal, respectively. The southwesterlies on the western flank of the anticyclone in the Northwest Pacific cause moisture convergence over SC, favoring the occurrence of extreme precipitation in SC. Conversely, the anticyclonic circulation centered at the Bay of Bengal is not conducive to the transport of warm water vapor from oceans to ICP, resulting in reduced occurrence of extreme precipitation there. The results derived from CMIP6 models unequivocally support the notion that the dipole mode is predominantly shaped by large-scale circulation anomalies induced by PIM. Our results underscore the importance of accounting for the collective impact of oceanic drivers in the tropics, laying a scientific foundation for enhanced precision in simulating extreme precipitation across SC and ICP.

Remote effect of tropical south Atlantic sea surface temperature anomalies on April–June accumulated cyclone energy over the western North Pacific

Global processes and their teleconnections, such as the El Niño-Southern Oscillation (ENSO), have been shown to be a large driver of interannual changes in accumulated cyclone energy (ACE) of western North Pacific (WNP) tropical cyclones (TCs), with higher ACE during El Niño and lower ACE during La Niña. However, it remains uncertain whether interannual changes in WNP TC ACE are modulated by sea surface temperature anomalies (SSTAs) in other oceans. This study finds a significant negative correlation between WNP TC ACE during the early season (April–June) and simultaneous SSTAs over the tropical south Atlantic (TSA) in 1970–2021. On average, in warm TSA years, basinwide April–June ACE is significantly lower, with significant ACE decreases mainly occurring over the region spanning 5°–30°N, 115°–150°E. This is a result of reduced TC frequency, intensity and duration, due to a remote modulation of WNP environmental conditions by TSA SSTAs. In warm TSA years, there are significant decreases in 700–500-hPa relative humidity, 850-hPa relative vorticity and 200-hPa divergence and significant increases in 850–200-hPa vertical wind shear over the portion of the WNP with significant ACE reductions. These environmental changes can be linked to an anomalous Walker circulation induced by TSA SSTAs.

Formation Mechanisms of the Decadal Indian Ocean Dipole Driven by Remote Forcing from the Tropical Pacific

Abstract On decadal time scales, a zonal SST dipole dominates the tropical Indian Ocean in boreal late summer and fall, called the decadal Indian Ocean dipole (D-IOD). The D-IOD has a spatial pattern different from the traditional interannual IOD, with its eastern pole located off Java, rather than the whole Sumatra–Java coasts as the latter. Here, we show that the D-IOD is generated by both the remote tropical Pacific decadal variability (TPDV) forcing and the decadal modulation of interannual IODs, but with its distinctive spatial pattern and seasonality mainly shaped by the former. In August–September (AS), due to the seasonal strengthening of trade winds, the descending branch of TPDV-induced Walker circulation moves westward into the eastern Indian Ocean relative to June–July, which stimulates equatorial easterly anomalies and oceanic upwelling Kelvin waves, causing subsurface cooling off Java. The subsurface cooling just occurs within the time window of climatological coastal upwelling so that subsurface cold anomalies are brought into the surface by mean upwelling and further transported offshore by mean flows, forming the D-IOD eastern pole. The subsurface cooling is only generated near Java but not Sumatra, because the former is closer to the exit of the Indonesian Throughflow (ITF). Weakened ITF during positive TPDV inhibits the growth of subsurface warming off Java prior to the establishment of AS equatorial easterly anomalies, whereas this ITF effect is not observed off Sumatra. Moreover, warming of the D-IOD western pole might be associated with off-equatorial Rossby waves induced by TPDV-related wind stress curls. Significance Statement The decadal Indian Ocean dipole (D-IOD) is one of the leading decadal modes of the tropical Indian Ocean SST variations. Its formation mechanisms, especially those related to its spatiotemporal characteristics, are not well understood. Based on observations and reanalysis, we show that the tropical Pacific decadal variability (TPDV) mainly accounts for the distinctive spatial pattern and seasonality of the D-IOD via the seasonal migration of the anomalous Walker circulation and ensuing oceanic subsurface dynamics. Our results highlight that the TPDV is an important source of D-IOD’s predictability, and it might be beneficial for operational decadal predictions for the tropical Indian Ocean.

Impact of Arctic sea ice on the boreal summer intraseasonal oscillation

This study investigates the relationship between sea ice concentration (SIC) in the Arctic Ocean and the Boreal Summer Intraseasonal Oscillation (BSISO) from 1991 to 2020 and its underlying mechanism. A significantly positive (negative) correlation was found between the frequency of phase 7 (3) of BSISO1 (30–60 d) and the preceding winter SIC, which is located the north of the East Siberian-Beaufort Sea (ESBS). Compared with low-SIC years, the conditions including northeasterly vertical wind shear, an enhanced ascending branch of the anomalous Walker circulation, an eastward water vapour transport channel, and an increased humidity gradient induce active convection over the Philippine Sea in high-SIC years, which benefits (hinders) to phase 7 (3) of BSISO1. The positive SIC anomaly during the transition from winter to spring influences local temperature and pressure through anomalous local sensible heat flux. This anomaly induces wave activity flux from the ESBS, which converges over the Bering Sea, enhancing the Aleutian Low (AL). Subsequently, the AL triggers an anomalous subtropical anticyclone through wave-mean flow interaction in the North Pacific. Due to southerly wind stress and increased sea surface heat flux, positive sea surface temperature anomalies near Japan persist in the summer, heating the lower troposphere and increasing baroclinicity. Significant positive geopotential heights and anticyclone anomalies occur over Japan, accompanied by a negative vorticity anomaly. The enhanced ascending motion over the Philippine Sea, facilitated by Ekman pumping, favours convection and influences the frequency of phases 7 and 3.

Enhanced Impacts of ENSO on the Southeast Asian Summer Monsoon Under Global Warming and Associated Mechanisms

AbstractBased on outputs of 28 coupled models from the Phase 6 of the Coupled Model Intercomparison Project (CMIP6), we show that the response of the Southeast Asian summer monsoon to the El Niño‐Southern Oscillation (ENSO) during post‐ENSO summer will likely strengthen in a warmer climate, which can be attributed to concurrently weakened sea‐surface temperature anomalies (SSTAs) in the western equatorial Pacific (WEP). The weakened WEP SSTAs are primarily caused by enhanced latent heat damping due to increased surface wind speed anomalies, which are associated with the eastward shift of the El Niño‐induced anomalous Walker circulation due to El Niño‐like sea surface temperature change in the tropical Pacific under global warming. Besides, the climatological zonal ocean currents will slow down due to the weakening of climatological Walker circulation, which also acts to weaken the WEP SSTAs via reducing the advection of anomalous temperature by the mean current.

Interannual impact of tropical southern Atlantic SST on surface air temperature over East Asia during boreal spring

Using reanalysis data and simulations, this study revealed a pronounced negative interannual relationship between tropical southern Atlantic (TSA) sea surface temperature (SST) and East Asian surface air temperature (SAT) during boreal spring (March–May). Results confirm that the March–May TSA–SST anomaly can be considered an independent tropical driver unrelated to El Niño–Southern Oscillation. A possible mechanism linking TSA–SST and East Asian spring SAT involves an atmospheric wave train, energy conversion, and potential vorticity (PV)–θ dynamics. The anomalous TSA–SST induces an anomalous Walker circulation, which initiates a wave train that extracts energy from a westerly jet and propagates toward East Asia. Subject to PV–θ dynamics, the East Asian PV anomaly embedded within this wave train leads to bowed isentropes and resultant notable anomalous East Asian SAT. In particular, the bootstrapping results suggest that TSA–SST anomaly can cause an approximately sevenfold increase in the occurrence probability of extreme East Asian spring SAT.

Different ENSO Teleconnections over East Asia in Early and Late Winter: Role of Precipitation Anomalies in the Tropical Indian Ocean and Far Western Pacific

Abstract This study aims to better understand the ENSO impacts on climate anomalies over East Asia in early winter (November–December) and late winter (January–February). In particular, the possible mechanisms during early winter are investigated. The results show that ENSO is associated with a Rossby wave train emanating from the tropical Indian Ocean toward East Asia (denoted as tIO-EA) in early winter. This tIO-EA wave train in El Niño (La Niña) is closely related to a weakening (strengthening) of the East Asian trough, and thereby a weakened (strengthened) East Asian winter monsoon and warm (cold) temperature anomalies over northeastern China and Japan. By using partial regression analysis and numerical experiments, we identify that the formation of tIO-EA wave train is closely related to precipitation anomalies in the tropical eastern Indian Ocean and western Pacific (denoted as eIO/wP). In addition, the ENSO-induced North Atlantic anomalies may also contribute to formation of the tIO-EA wave train in conjunction with the eIO/wP precipitation. The response of eIO/wP precipitation to ENSO is stronger in early winter than in late winter. This can be attributed to the stronger anomalous Walker circulation over the Indian Ocean, which in turn is caused by higher climatological SST and stronger mean precipitation state in the Indian Ocean during early winter.

Revisiting the ENSO–monsoonal rainfall relationship: new insights based on an objective determination of the Asian summer monsoon duration

Traditionally, the boreal summer monsoon season is regarded as a fixed period of June–September (JJAS). In fact, the monsoon commencement and termination are not fixed to the annual cycle, thus the monsoon duration exhibits remarkable variations. Based on the multivariable empirical orthogonal function analysis, the commencement, termination, and duration of the Asian summer monsoon (ASM) are objectively determined in this study. The ASM duration is shown to be closely linked to the sea surface temperature anomalies in the equatorial Pacific Ocean, which tends to be shorter during an El Niño and longer during a La Niña year. Through the anomalous Walker circulation and the equatorial Rossby wave response, a developing La Niña event is generally associated with advanced commencement and delayed termination, thus a longer ASM duration. The importance of monsoon duration is illustrated further by revisiting the relationship between El Niño-southern oscillation (ENSO) and the monsoonal rainfall, which is the total rainfall within monsoon duration (from commencement to termination) rather than within JJAS. The relationship between the JJAS rainfall over South Asia and ENSO has exhibited remarkable interdecadal changes, which becomes weak since the 1980s and is recovered after the early 2000s. In sharp contrast, the linkage between the monsoonal rainfall over South Asia and ENSO remains significant and robust over the past four decades. Via attaching the rainfall during the transition periods (advanced commencement and delayed termination), the longer ASM duration during La Niña shall increase the total rainfall, which can reinforce the canonical ENSO-monsoonal rainfall relationship. The above results suggest that a reasonable definition of ASM duration may help us better understand the monsoon phenomenon and teleconnections.

Modulation of western North Pacific tropical cyclone formation by central Pacific El Niño–Southern Oscillation on decadal and interannual timescales

AbstractThis study investigates the impact of central Pacific (CP) El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) frequency over the western North Pacific (WNP) on decadal and interannual timescales. The CP ENSO‐TC frequency relationship is strong and significant only on decadal timescales, with enhanced TC formation over most of the WNP associated with warm CP ENSO phases. TC formation changes, driven by CP ENSO, exhibit a southeast–northwest dipole pattern on interannual timescales, similar to the typical pattern induced by eastern Pacific ENSO. Associated with warm CP ENSO phases on decadal timescales are increases in TC formation east of 125°E, while increases in TC formation east of 140°E are observed with warm CP ENSO phases on interannual timescales. These increases in TC activity are primarily caused by favourable dynamic conditions, including increased 850‐hPa relative vorticity and 200‐hPa divergence and decreased 850–200‐hPa vertical wind shear. The main TC formation difference on decadal and interannual timescales is concentrated over the longitudinal band of 125°–140°E, where reduced TC formation is observed on interannual timescales during CP El Niño years, predominately due to suppressed thermodynamic factors, for example, reduced maximum potential intensity and 700–500‐hPa relative humidity. We find insignificant (significant) changes in thermodynamic conditions between 125°E and 140°E due to weak (strong) descending motion in the western cell of the anomalous Walker circulation on decadal (interannual) timescales. This can be explained by different patterns of CP ENSO‐induced sea surface temperature anomalies on different timescales.

How can the positive Indian Ocean Dipole events co‐occur with La Niña?

AbstractPositive/negative Indian Ocean Dipole (IOD) tends to co‐occur with El Niño/La Niña. Five positive IODs, however, are observed under La Niña condition in 1961, 1966, 1967, 2007, and 2011. This study uses the monthly ocean temperature and atmospheric reanalysis datasets from ECMWF to investigate how these positive IOD events occurred. It is found that the five positive IODs are consistently accompanied by a moderately strong positive subsurface IOD (Sub‐IOD) event that initiated earlier. The Sub‐IOD‐related oceanic dynamical processes including oceanic Kelvin and Rossby waves help to promote the development of the positive IODs. More specifically, the equatorial easterly anomalies in the Tropical Indian Ocean (TIO) that are crucial for the development of the positive Sub‐IODs and IODs in 1961/2011 are triggered by an anticyclonic curl over South Indian Ocean that is induced by a Southern Hemispheric Indian Ocean Subtropical Dipole mode. The crucial equatorial easterly anomalies in 1967 are triggered by a thermocline warming in the southwestern TIO induced by westward downwelling oceanic Rossby waves, while in 1966/2007, the equatorial easterly anomalies are connected with the anomalous Walker circulation resulting from the certain sea surface temperature anomaly pattern over the equatorial Pacific. On the other hand, the remote effects of La Niña on TIO are relatively weaker, being overwhelmed by the above triggering processes.

What Determine the Performance of the ENSO‐East Asian Winter Monsoon Relationship in CMIP6 Models?

AbstractThis study evaluates the relationship between El Niño‐Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) in 26 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. Of particular concern is the plausible factors for the model's capability of simulating the ENSO‐EAWM relationship. Results show that the model's ability of simulating the ENSO‐EAWM relationship is more dependent upon the longitudinal extension of ENSO‐related equatorial Pacific sea surface temperature (SST) anomalies than the amplitude of the equatorial central‐eastern Pacific SST anomalies. The influence of the amplitude of ENSO on the simulation of the ENSO‐EAWM relationship depends on the westward extension of ENSO‐related equatorial Pacific SST anomalies. Another factor for the model's ability of simulating the ENSO‐EAWM relationship is the SST anomalies in the tropical western North Pacific (WNP). A westward extension of the equatorial Pacific SST anomalies shifts the west branch of anomalous Walker circulation too far westward, which causes westward displaced anomalous ascending (descending) motion around the Philippines through modulating regional meridional vertical circulation in El Niño (La Niña) years. The weak SST anomalies in the tropical WNP lead to the failure of inducing anomalous lower‐level anticyclone (cyclone) over the Philippine Sea through a Rossby wave response in El Niño (La Niña) years. The accompanying weak anomalous lower‐level southwesterly (northeasterly) winds along the west flank of the anomalous anticyclone (cyclone) account for the weak ENSO‐EAWM relationship.

Different Seasonal Precipitation Anomaly Patterns in Central Asia Associated With Two Types of El Niño During 1891–2016

In this study, we examine the different seasonal precipitation anomaly patterns in Central Asia associated with the cold-tongue (CT) El Niño and warm-pool (WP) El Niño from the El Niño developing autumn to the decaying spring based on the Global Precipitation Climatology Centre (GPCC) full data reanalysis version 2018 (GPCC V2018) data set. Overall, El Niño are associated with more precipitations over Central Asia, but significant discrepancies can be found in the precipitation anomaly spatial patterns associated with the two types of El Niño from the El Niño developing autumn to the decaying spring. The precipitation associated with CT El Niño is mostly concentrated in the plains and hilly areas of Central Asia and is more dispersed in space. Whereas the precipitation associated with WP El Niño is mostly concentrated along Pamirs and Tian Shan Mountains with consistency throughout the autumn before El Niño peaks to the spring when El Niño decays. Also, the strength of the positive precipitation anomaly associated with WP El Niño is significantly stronger than that of CT El Niño. The analysis of anomalous atmospheric circulation caused by two types of El Niño shows that the interconfiguration of anomalous high pressure in the south side of Central Asia at low and middle latitudes and anomalous low pressure and anomalous high pressure in the high latitudes of Eurasia affects the southwest water vapor flux and north side water vapor flux in Central Asia, thus causing different effects of different types of El Niño on precipitation in Central Asia at different stages. The spatial consistency of the WP El Niño effect on precipitation in Central Asia over three seasons may be related to the upward branch of the anomalous Walker circulation over the Indian Ocean induced by it.

Mechanisms Determining Diversity of ENSO-Driven Equatorial Precipitation Anomalies

Abstract The longitudinal location of precipitation anomalies over the equatorial Pacific shows a distinctive feature with the westernmost location for La Niña, the easternmost location for eastern Pacific (EP) El Niño, and somewhere between for central Pacific (CP) El Niño, even though the center of the sea surface temperature anomaly (SSTA) for La Niña is located slightly east of that of CP El Niño. The mechanisms for such a precipitation diversity were investigated through idealized model simulations and moisture and moist static energy budget analyses. It is revealed that the boundary layer convergence anomalies associated with the precipitation diversity are mainly induced by underlying SSTA through the Lindzen–Nigam mechanism, that is, their longitudinal locations are mainly controlled by the meridional and zonal distributions of the ENSO SSTA. The westward shift of the precipitation anomaly center during La Niña relative to that during CP El Niño is primarily caused by the combined effects of nonlinear zonal moist enthalpy advection anomalies and the Lindzen–Nigam mechanism mentioned above. Such a zonal diversity is further enhanced by the “convection–cloud–longwave radiation” feedback, the SST-induced latent heat flux anomalies, and the advection of mean moist enthalpy by anomalous winds. This diversity in the longitudinal location of precipitation anomalies has contributions to the diversities in the longitudinal locations of anomalous Walker circulation and western North Pacific anomalous anticyclone/cyclone among the three types of ENSO.

Strengthening impacts of spring sea surface temperature in the north tropical Atlantic on Indian Ocean dipole after the mid-1980s

It is well known that the Indian Ocean dipole (IOD) is closely related to El Niño-Southern Oscillation (ENSO). In this study, it is found that spring–summer north tropical Atlantic (NTA) sea surface temperature (SST) anomalies can contribute to the development of the IOD since the mid-1980s as well as ENSO. After the mid-1980s, an anticyclonic circulation over the subtropical northeastern Pacific could be excited by cool NTA SST anomalies in spring and summer due to a Gill-type Rossby-wave response. A low-level cyclonic circulation appears in the west Pacific in turn. The anomalous lower-level southwesterlies on the southeastern flank of the cyclonic circulation reduce the climatological wind speed, and thus, warm SST anomalies appear and extend to the central tropical Pacific via the wind-evaporation-SST (WES) feedback. As a result, Walker circulation over the Indo-Pacific region is changed, with anomalous descending in the eastern tropical Indian Ocean and ascending in the central tropical Pacific. The descending branch of anomalous Walker circulation generates surface southeasterly wind anomalies along the coast off Sumatra and lifts the thermocline in the eastern tropical Indian Ocean, which produces a positive IOD event in autumn. Such a cross-basin mechanism is supported by a coupled model experiment with warm SST perturbations over the NTA. In contrast, although cold NTA SST anomalies can induce warm SST anomalies in the eastern equatorial Pacific through interhemispheric meridional circulation before the mid-1980s, the responses in the tropical Indian Ocean are rather weak. In addition, the enhancement in the NTA-IOD relationship after the mid-1980s is suggested to result from the changes in the SST mean state under the context of global warming, which is confirmed by two coupled model experiments with different mean SST backgrounds.

Time-Spatial Features of Mix El Niño

The diversity of El Niño is a critical field of the climate research. The eastern Pacific (EP) and central Pacific (CP) types of El Niño have been identified in the previous studies. However, the extreme El Niño event that occurred in 2015–2016 is quite different from both the EP and CP El Niño events. The sea surface temperatures anomalies (SSTA) for this event widely spread in both the central and eastern Pacific and have a small zonal gradient in the central-eastern Pacific. Many researchers regarded this event as a mixed type of El Niño. Using the regression-EOF method, the Mix El Niño pattern is extracted from the tropical Pacific SSTA field during the period from 1900 to 2019. Here, we reveal that the Mix El Niño is a very usual rather than a new type of El Niño, it is just that the EP and CP El Niño events are more frequent since the 1980s, while the Mix El Niño events frequently appear before the 1980s. The time-spatial features of the Mix El Niño are further investigated. The results demonstrate a unique westward propagation of the maximum SSTA for the Mix El Niño from the far eastern Pacific to the central Pacific. In contrast, the SSTA center is locked in the far eastern Pacific region for the EP El Niño and the central Pacific region for the CP El Niño. The evolutions of subsurface ocean temperature anomalies and sea surface height anomalies are also examined to support this. The ocean–atmosphere interaction plays an important role in the evolution of the Mix El Niño. The anomalous atmospheric Walker circulation for the Mix El Niño is mainly in the western and central Pacific as well as very weak in the eastern Pacific. In contrast, there are significant westerlies/easterlies in the eastern Pacific for the EP/CP El Niño. The small gradient of SSTA in the central-eastern Pacific for the Mix El Niño leads to weak zonal wind anomalies, which further weaken the zonal gradient of SSTA. All this suggests that the Mix El Niño is not unusual and fundamentally different from the EP and CP El Niño with important implications for global climate effects.

Rapid changes in northeastern tropical Pacific Ocean surface salinity due to trans-basin moisture transport in recent decades

Argo observations revealed dramatic sea surface salinity (SSS) changes in the northeastern tropical Pacific Ocean (NETPO) during the past one and half decades, with a steady increase during 2004–2012 and a sharp decrease during 2013–2016. These changes mainly result from anomalous precipitation associated with the Walker Circulation through moisture transport across Central America. The anomalous Walker Circulation is caused by changes in trans-basin sea surface temperature (SST) gradients, especially those in the eastern tropical Pacific. Results using a longer reanalysis indicate that the upward trend of SSS in the NETPO started from the mid-1990s as the moisture transport across Central America decreased in response to reduced trans-basin SST gradients associated with the Interdecadal Pacific Oscillation and Atlantic Multidecadal Oscillation.

Performance of the IPCC AR6 models in simulating the relation of the western North Pacific subtropical high to the spring northern tropical Atlantic SST

AbstractThis study examines the relationship between the spring northern tropical Atlantic (NTA) sea surface temperature (SST) and its following summer western North Pacific subtropical high (WNPSH) in historical simulations of 20 coupled models of the IPCC AR6. These models well simulate the spring NTA SST, as well as the WNPSH‐related atmospheric and precipitation anomalies over the Asian monsoon region. Ensemble mean of these models reproduces the observed relation and processes linking the spring NTA SST to the summer WNPSH. In the ensemble mean, spring NTA SST warming persists to the following summer, and induces an anomalous Walker circulation with an ascent over the tropical Atlantic and a descent over the tropical central Pacific. The associated precipitation decrease over the tropical central Pacific induces an anomalous low‐level anticyclone over the WNP via a Rossby wave atmospheric response. Meanwhile, spring NTA SST warming leads to easterly wind anomalies over the Indian Ocean (IO) via a Kelvin wave atmospheric response and causes SST warming there via reducing wind speeds. Then, the precipitation anomalies induced by the IO SST warming result in an enhancement of the WNPSH. There exists a large diversity of the NTA SST‐WNPSH relationship among the models. This diversity is related to the difference in the climatological mean IO precipitation. The models with large mean IO precipitation provide a favourable condition for the IO SST warming to bring large local precipitation anomalies and trigger strong easterly wind anomalies over the tropical WNP, and thus exert a strong impact on the WNPSH.

Differential Influences of Teleconnections from the Indian and Pacific Oceans on Rainfall Variability in Southeast Asia

This study investigates the individual and combined impacts of El Niño and the positive Indian Ocean Dipole (IOD) on the Southeast Asia (SEA) rainfall variability. Using composite and partial correlation techniques, it is shown that both inter-annual events have individually distinct impacts on the SEA rainfall anomaly distribution. The results showed that the impacts of the co-occurrence of El Niño and IOD events are significant compared to the individual effects of pure El Niño or pure IOD. During June-July-August and September-October-November, the individual impacts of the pure El Niño and IOD events are similar but less significant. Both events caused negative impacts over the southern part of SEA during June-July-August (JJA) and propagated northeastward/eastward during September-October-November (SON). Thus, there are significant negative impacts over the southern part of SEA during the co-occurrence of both events. The differential impacts on the anomalous rainfall patterns are due to the changes in the sea surface temperature (SST) surrounding the region. Additionally, the differences are also related to the anomalous regional atmospheric circulations that interact with the regional SST. The anomalous Walker circulation that connects the Indian Ocean and tropical Pacific Ocean also plays a significant role in determining the regional anomalous rainfall patterns.

Differences in Western North Pacific Tropical Cyclone Activity among Three El Niño Phases

AbstractThe impacts of El Niño on tropical cyclone (TC) activity over the western North Pacific (WNP) are examined through investigation of three types of tropical Pacific warming episodes according to where the maximum sea surface temperature (SST) anomalies occur in the equatorial Pacific: the eastern Pacific El Niño (EPE), the central Pacific El Niño (CPE), and the mixed El Niño (ME). More TCs form over the eastern part of the WNP in all three El Niño types, whereas the frequency of TCs over the western part of the WNP increases as the peak SST anomalies migrate from east to west. Although TCs more frequently recurve at higher latitudes during EPE and CPE, the most frequent region for recurving is much closer to the East Asian continent in CPE years than in EPE years. In contrast, more TCs track westward and threaten the Philippines in ME years. The increased TC genesis over the western part of the WNP can be explained by enhanced low-level relative vorticity, reduced vertical wind shear, and increased maximum potential intensity during CPE and increased midlevel moisture during EPE and ME. This increase is further related to updraft anomalies near the date line driven by an anomalous Walker circulation and an anomalous low-level cyclonic circulation over the WNP. The TC track differences among the different El Niño types are linked to the east–west shift of the western Pacific subtropical high, possibly caused by an anomalous Hadley circulation from 120° to 130°E that is strongly coupled with the anomalous Walker circulation.

The Recent Decline and Recovery of Indian Summer Monsoon Rainfall: Relative Roles of External Forcing and Internal Variability

AbstractThe Indian summer monsoon (ISM) rainfall affects a large population in South Asia. Observations show a decline in ISM rainfall from 1950 to 1999 and a recovery from 1999 to 2013. While the decline has been attributed to global warming, aerosol effects, deforestation, and a negative-to-positive phase transition of the interdecadal Pacific oscillation (IPO), the cause for the recovery remains largely unclear. Through analyses of a 57-member perturbed-parameter ensemble of model simulations, this study shows that the externally forced rainfall trend is relatively weak and is overwhelmed by large internal variability during both 1950–99 and 1999–2013. The IPO is identified as the internal mode that helps modulate the recent decline and recovery of the ISM rainfall. The IPO induces ISM rainfall changes through moisture convergence anomalies associated with an anomalous Walker circulation and meridional tropospheric temperature gradients and the resultant anomalous convection and zonal moisture advection. The negative-to-positive IPO phase transition from 1950 to 1999 reduces what would have been an externally forced weak upward rainfall trend of 0.01 to −0.15 mm day−1decade−1during that period, while the rainfall trend from 1999 to 2013 increases from the forced value of 0.42 to 0.68 mm day−1decade−1associated with a positive-to-negative IPO phase transition. Such a significant modulation of the historical ISM rainfall trends by the IPO is confirmed by another 100-member ensemble of simulations using perturbed initial conditions. Our findings highlight that the interplay between the effects of external forcing and the IPO needs be considered for climate adaptation and mitigation strategies in South Asia.