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The interannual relationships between the southern subtropical cell (SSTC) in the Indian Ocean (IO) and the El Niño‐Southern Oscillation (ENSO) are examined. The results demonstrate that the impacts of ENSO on the boreal winter IO SSTC display a notable negative skewness, with the amplification of the SSTC during La Niña being far stronger than its reduction during El Niño. This difference is mainly attributed to an asymmetry in anomalous meridional geostrophic transport, characterized by more robust southward (weaker northward) geostrophic transport anomalies during La Niña (El Niño) events. Further analyses reveal that the differing longitudinal positions of the ENSO‐related sea surface temperature anomaly (SSTA) are the primary drivers of the asymmetry in anomalous meridional geostrophic transport via the oceanic and atmospheric teleconnections. Compared to the warm SSTA in El Niño events, the La Niña‐related cool SSTA in the central‐eastern equatorial Pacific displays a greater westward extension, which induces stronger surface easterlies over the western equatorial Pacific, and then initiates a more substantial sea surface height anomaly (SSHA) in the southeast Indian Ocean (SEIO) through oceanic waveguides. This leads to a heightened zonal SSHA gradient over the southern IO, resulting in a larger anomalous meridional geostrophic transport. On the other hand, the westward extension of negative SSTA during La Niña winters triggers anomalous cyclonic northerlies along the western coast of Australia by shifting the Walker circulation westward. These coastal northerlies amplify the positive SSHA signals in the SEIO and enhance the anomalous meridional geostrophic transport, ultimately contributing to a more robust IO SSTC response. Plain Language Summary: The southern subtropical cell (SSTC), driven by wind over the Indian Ocean (IO), is characterized by southward surface flow, subduction in the subtropics, and upwelling in the tropics. This study revealed that the boreal winter SSTC weakens during El Niño and enhances during La Niña, with the latter exhibiting a notably stronger response, contrary to the expected El Niño/La Niña amplitudes. These asymmetric ENSO impacts predominantly result from disparities in anomalous meridional geostrophic transport. In‐depth analysis further uncovered that the distinct longitudinal positions of ENSO‐related sea surface temperature anomalies (SSTA) primarily drive the asymmetry in anomalous meridional geostrophic transport. During La Niña events, the cool SSTA in the central‐eastern equatorial Pacific extends significantly westward compared to the El Niño‐related warm SSTA, which is evident during both the preceding season and the contemporaneous winter. During the preceding season, this westwardly extended SSTA leads to a more substantial anomalous meridional geostrophic transport in the subsequent winters through the propagation of oceanic waveguides. In La Niña winters, the westward extension of negative SSTA further amplifies the anomalous meridional geostrophic transport by shifting the Walker circulation westward. These intensified meridional geostrophic transport anomalies ultimately result in a more pronounced SSTC response during La Niña events. Key Points: The boreal winter southern subtropical cell (SSTC) in the Indian Ocean displays a negative skewness in response to El Niño‐Southern Oscillation (ENSO)Such asymmetric SSTC responses are primarily attributed to an asymmetry in anomalous meridional geostrophic transportDiffering longitudes of ENSO‐related sea surface temperature anomaly cause the asymmetric meridional geostrophic transport via oceanic and atmospheric teleconnections [ABSTRACT FROM AUTHOR]

This study examines the impacts of Eastern Pacific (EP) and Central Pacific (CP) El Niño on summertime extreme precipitation over East Asia during the El Niño decay phase. The findings reveal distinct patterns of extreme precipitation for the two El Niño types. During EP El Niño, more intense extreme precipitation occurs over south of the Yangtze River (SYR), while suppressed extremes are observed over Mei-Yu rainband in China, Baiu in Japan, and Changma in South Korea (MBC). Conversely, CP El Niño leads to weaker (stronger) extreme precipitation over SYR (MBC). This study also differentiates between tropical cyclone (TC) and non-TC related precipitation, showing that TCs have minimal influence on the overall extreme precipitation compared to non-TC related events. Further investigation reveals that EP and CP El Niño distinctly influence atmospheric circulation patterns, thereby causing different distributions of non-TC extreme precipitation. Specifically, during EP El Niño, sea surface temperature (SST) warming signals in the tropical Indian Ocean induce the southward displacement of the South Asia High (SAH), westerly jet (WJ), and Western Pacific subtropical high (WPSH). This amplifies moisture flux convergence, elevating the likelihood of intense extreme precipitation over SYR. Conversely, MBC experiences moisture flux divergence, resulting in fewer precipitation extremes. During CP El Niño, SST warming signals in the Maritime Continent prompt the northward shift of SAH, WJ, and WPSH by modulating local Hadley circulations. These anomalies lead to strengthened moisture convergence (divergence) over MBC (SYR), consequently resulting in a higher (lower) likelihood of intense extreme precipitation over MBC (SYR). [ABSTRACT FROM AUTHOR]

The formation mechanisms of the Indian Ocean Dipole (IOD) have been studied extensively. However, few studies have considered the influence of the South Pacific atmosphere and sea surface temperature (SST) variability on IOD formation. In this study, reanalysis data, CMIP6 model outputs, and linear baroclinic model experiments were used to investigate this influence. The results revealed a close connection between the preceding boreal winter–spring Southwest Pacific Dipole (SWPD) SST anomaly (SSTA) pattern and the subsequent IOD and associated physical mechanisms. Facilitated by the local air–sea positive feedback, the positive SWPD, coupled with low-level cyclonic circulations, persists from the preceding winter to early summer. During early summer, the below-normal precipitation and diabatic heating associated with the SWPD western pole cooling strengthens and induces an anticyclonic circulation on its western side as the climatological southeasterly winds encroach northward. This anticyclonic circulation extends from southern Australia into the tropical southeastern Indian Ocean, favoring enhanced southeasterly winds and cooling off Java-Sumatra. Subsequently, the IOD develops in summer–autumn with the aid of wind–evaporation–SST and wind–thermocline–SST positive feedback. The westward downwelling Rossby wave and eastward upwelling Kelvin wave associated with surface wind anomalies also contribute to the IOD western pole warming and eastern pole cooling, respectively. Furthermore, the IOD interacts with El Niño through Walker circulation and increases its amplitude. These findings are supported by the results obtained from the linear baroclinic model and CMIP6 historical simulations and can potentially improve the predictability of the IOD. [ABSTRACT FROM AUTHOR]

The Indian Ocean dipole (IOD) is the dominant mode of interannual variability in the tropical Indian Ocean (TIO), characterized by warming (cooling) in western TIO and cooling (warming) in eastern TIO during its positive (negative) phase. Observed IOD events exhibit distinct amplitude asymmetry in relation to negative nonlinear dynamic heating. Nearly all models in phase 5 of the Coupled Model Intercomparison Project (CMIP) simulate a less-skewed IOD than observed, but 6 out of 20 CMIP6 models can reproduce realistic high skewness. Analysis of less-skewed models indicates that the positive IOD-like biases in the mean state, which can be traced back to their weaker simulations of the preceding Indian summer monsoon, reduce the convective response to positive sea surface temperature anomalies in the western TIO, resulting in a weaker zonal wind response and weaker nonlinear zonal advection during positive IOD events. Besides, ocean stratification in the eastern TIO influences the IOD skewness: stronger stratification leads to larger mixed-layer temperature response to thermocline changes, contributing to larger anomalous vertical temperature gradient, larger nonlinear vertical advection, and thus stronger positive IOD skewness. Our findings underscore the importance of reducing Indian summer monsoon biases and eastern TIO stratification biases, for properly representing the IOD in Earth system models. [ABSTRACT FROM AUTHOR]

In 2022, South China suffered severe flooding due to unexpected persistent heavy rainfall in the pre-flood season. The in situ cumulative precipitation amount during May–June in South China broke the record established in 1981. The present study ascribed this record-breaking event to extreme warm sea surface temperature anomalies (SSTAs) in the Northeast Pacific (NP) and the Maritime Continent (MC) region. In May, the NP warming associated with La Niña and the negative phase of the Pacific Decadal Oscillation intensified extratropical diabatic heating and induced a cyclonic anomaly over Northeast Asia, which shifted the East Asian trough (EAT) southward. Subsequently, the westerly wind accelerated to the south of the anomalous EAT and strengthened the western North Pacific subtropical high, which led to increase in rainfall over South China. In June, the rainfall over South China remained above normal owing to anomalies of the meridional monsoon circulation manifested as a response to the development of a warm SSTA in the MC region. The anomalous warming in the deep tropics suppressed the Asian summer monsoon convection through anomalous atmospheric descent over South Asia, accompanied by enhanced atmospheric ascent and increased water vapor convergence over South China. Numerical sensitivity experiments validated the above processes and indicated that historic warm SSTAs in the NP and MC regions could increase the probability of extreme persistent above-normal pre-flood rainfall over South China. The results support improved predictability of pre-flood rainfall over South China on the interannual timescale outside the equatorial central–eastern Pacific. [ABSTRACT FROM AUTHOR]

Interannual variability of surface mixed‐layer near‐inertial energy (NIE, representing the intensity of near‐inertial waves) in the South China Sea and western North Pacific (WNP) is investigated using satellite‐tracked surface drifter data set. It is found that NIE in the study region correlates negatively with El Niño‐Southern Oscillation (ENSO) with a correlation coefficient of R = −0.44 and a time lag of 5 months, mainly because the variation of local wind stress lags behind El Niño by 4 months. By separating summer and winter seasons, the correlation is significantly improved. The summer NIE correlates positively with El Niño (R = 0.62), since tropical cyclones over the WNP tend to be stronger and longer‐lived during the El Niño developing phase. The winter NIE correlates negatively with El Niño (R = −0.65), since the winter monsoon is weakened by the ENSO‐related WNP anomalous anticyclone. This is the first time that interannual variability of NIE is studied by direct current velocity observations. Plain Language Summary: Interannual climate variability, especially El Niño‐Southern Oscillation (ENSO), can trigger a series of flood, drought, and wildfires, resulting in a huge loss of people's property around the world. It has been demonstrated that ENSO can modulate large‐scale oceanic and atmospheric processes, such as sea surface temperature, circulation, monsoon, tropical cyclones (TCs), etc. Near‐inertial waves (NIWs) are ubiquitous in the upper ocean and usually contain the most energy of internal waves. Since NIWs are generated by time‐varying wind stress, they could also be influenced by ENSO. Using current velocity observations from satellite‐tracked surface drifter data set with hourly resolution, interannual variability of the surface mixed‐layer near‐inertial energy (NIE) is investigated. It is found that NIE in the western North Pacific (WNP) is indeed modulated by ENSO, with the monthly NIE lagging behind ENSO by 5 months. Since the monsoon reverses direction seasonally, summer and winter NIEs are discussed separately. The summer NIE is mainly generated by TCs while the winter NIE is mainly generated by winter monsoon, which are all tightly connected to and modulated by ENSO. Since NIWs play an important role in upper‐ocean turbulent mixing and air‐sea interaction, their interannual variations have important implications for the climate system. Key Points: Mixed‐layer near‐inertial energy in the study region correlates negatively with El Niño‐Southern Oscillation (ENSO) with a time lag of 5 monthsSummer near‐inertial energy is mainly generated by tropical cyclones and correlates positively with ENSOWinter near‐inertial energy is mainly generated by the winter monsoon and correlates negatively with ENSO [ABSTRACT FROM AUTHOR]

Marine heatwaves (MHWs) in the South China Sea (SCS) have dramatic impacts on local ecosystems, fisheries, and aquacultures. Our results show that SCS MHWs were strongly regulated by El Niño–Southern Oscillation (ENSO) with a distinct life cycle during 1982–2018. Based on the ENSO-associated sea surface temperature anomaly (SSTA) warming peaks in the SCS, we can classify SCS MHWs into three categories: El Niño-P1 during the first warming peak of El Niño from September to the following February, El Niño-P2 during the second warming peak of El Niño from the following June to September, and La Niña-P1 during the single warming peak of La Niña from the following February to May. The three types of SCS MHWs are all affected by the lower-level enhanced anticyclone over the western North Pacific (WNP), but their physical mechanisms are quite different. In El Niño-P1, SCS MHWs are mostly induced by enhanced net downward shortwave radiation and reduced latent heat flux loss over the southwestern and northern SCS, respectively. In El Niño-P2, SCS MHWs are primarily attributed to weaker entrainment cooling caused by a local enhanced anticyclone and stronger Ekman downwelling in the central-northern SCS. However, in La Niña-P1, SCS MHWs are mainly contributed by the reduced latent heat loss due to the weaker WNP anticyclone centered east of the Philippines on the pentad time scale. The distinct spatial distributions of MHWs show phase locking with ENSO-associated SCS SSTA warming, which provides a potential seasonal forecast of SCS MHWs according to the ENSO phase. [ABSTRACT FROM AUTHOR]

In this study, future change of El Niño-related East Asian (EA) rainfall and the diversity of this relationship are investigated on the basis of the historical and representative concentration pathway 8.5 (RCP 8.5) simulations taken from the Coupled Model Intercomparison Project phase 5 (CMIP5). By evaluating the East Asian Summer Monsoon (EASM) climatology and interannual variations in simulations contributing to CMIP5, nine models are verified to be capable of reproducing El Niño diversity and EASM simultaneously. Six of these models are selected for projecting the multi-model ensemble (MME) mean of two types of El Niño-related EA/western North Pacific (WNP) rainfall patterns and low-level atmospheric circulations under global warming, considering the realism in their simulated El Niño and EASM phenomena. It was found that, under a warmer background climate, the general patterns of anomalous circulation and rainfall will persist, but with amplification of the rainfall intensity during mature boreal winter and decaying summer for both Eastern-Pacific (EP) and Central-Pacific (CP) El Niño. Amplification of CP type-related rainfall seems to be stronger than that for EP type El Niño. Further analyses show that a moister atmosphere tends to always strengthen the rainfall variations for both El Niño flavors, regardless of how the El Niño-related circulation amplitude is modulated in various seasons. However, in boreal summer during the El Niño decaying phase, strengthened anomalous circulation also enhances the rainfall variability, with an effect comparable to the background moisture increase. Some of these atmospheric circulation changes might be associated with modified sea surface temperature anomalies (SSTA) of El Niño and its diversity, under global warming. Our results indicate the importance of better preparedness and higher resilience in the EA region to enhanced El Niño-induced hydrological variations under a warming climate. [ABSTRACT FROM AUTHOR]

The Mid‐to‐Lower Reaches of the Yangtze River (MLRYR) suffered an extreme drought in the post‐monsoon season of 2019, contemporaneous with a strong Central Pacific (CP) El Niño and a super positive Indian Ocean Dipole (pIOD) event. The present work shows that CP El Niño‐related Pacific Sea Surface Temperature Anomalies (SSTAs) weakened the western North Pacific anticyclone to prevent moisture transport to the MLRYR and explained 60% of the drought intensity. The super pIOD with an extremely cold SSTA in the tropical Southeastern Indian Ocean contributed 40% of the drought amplitude via an atmospheric teleconnection. The Indian Ocean cold SSTAs first strengthened the post‐monsoon rainfall and enhanced diabatic heating over South Asia, leading to baroclinic circulation anomalies with induced descending motion over the MLRYR. The aggravated dry conditions there ultimately broke the historical drought record for the period since 1979. Plain Language Summary: In the post‐monsoon season of 2019, the Mid‐to‐Lower Reaches of the Yangtze River (MLRYR) experienced a record‐breaking drought, which severely disrupted water supplies and affected the planting of crops. At the same time, a super positive Indian Ocean Dipole (pIOD) event occurred, along with a central Pacific (CP) El Niño in the tropical Pacific. The present study indicates that in addition to the CP El Niño, the extremely cold SSTAs in the tropical Southeastern Indian Ocean associated with the super pIOD event was also an important factor in the record‐breaking drought event. This factor first shifted the intertropical convergence zone northward to intensify the post‐monsoon rainfall and its released condensation heating over South Asia. Then, a vertically baroclinic circulation was stimulated to strengthen a descending motion over the MLRYR via an atmospheric teleconnection. On the other hand, the tropical Pacific warm SSTAs related to the strong CP El Niño weakened the western North Pacific anticyclone, which reduced the moisture supply to the MLRYR. In this way, both the pIOD and CP El Niño events jointly resulted in the record‐breaking MLRYR drought in 2019 and explained approximately 40% and 60% of this extreme drought, respectively. Key Points: Drought over the mid‐to‐lower reaches of the Yangtze River (MLRYR) broke historical records in 2019 during the post‐monsoon seasonA strong central Pacific El Niño contributed ∼60% of the extreme MLRYR drought intensity due to the tropical Pacific air‐sea interactionAnother 40% of this MLRYR drought can be attributed to a super positive Indian Ocean Dipole via a teleconnection effect over South Asia [ABSTRACT FROM AUTHOR]

We investigated the effects of monsoon onset vortex (MOV) on the mixed layer heat budget in the Bay of Bengal (BOB) in spring 2003 using the reanalysis datasets. The results suggest that the solar radiation flux penetrating the mixed layer and the existence of barrier layer are both able to modulate the effects of MOV on the evolution of sea surface temperature (SST) in the BOB. Prior to the formation of BOB MOV, the local SST raised quickly due to mass of solar radiation reaching the sea surface under the clear-sky condition. Meanwhile, since the mixed layer was shallow before the onset of the Asian summer monsoon (ASM), some solar radiation flux could penetrate to directly heat the deeper water, which partly offset the warming effect of shortwave radiation. On the other hand, the in-situ SST started to cool due to the upwelling of cold water when the MOV generated over the BOB, along with the rapidly increased surface wind speed and its resultant deeper mixed layer. As the MOV developed and moved northward, the SST tended to decrease remarkably because of the strong upward surface latent heat flux over the BOB ascribed to the wind-evaporation mechanism. However, the MOV-related precipitation brought more fresh water into the upper ocean to produce a thicker barrier layer, whose thermal barrier effect damped the cooling effect of entrainment upwelling on the decrease tendency of the BOB SST. In other words, the thermal barrier effect could slow down the decreasing trend of the BOB SST even after the onset of ASM, which facilitated the further enhancement of the MOV. [ABSTRACT FROM AUTHOR]

Based on a transient simulation of the Community Earth System Model, we identified two anomalous "zonal triple-pole type" annual cycles in the equatorial Pacific sea surface temperature (SST), which were induced by precessional evolution of the summer-minus-winter insolation difference and the autumn-minus-spring insolation difference, respectively. For example, due to the increased summer–winter insolation contrast, a zonal positive–negative–positive pattern of equatorial SST anomalies was detected after subtracting basin-scale summer SST warming. The positive SST anomalies were associated with anomalous upward air flows over the western Pacific and eastern Pacific, whereas the negative SST anomalies in the central Pacific were coupled with anomalous downward air flows, oceanic upwelling, and thermocline cooling. These central Pacific anomalies were due to multiple air–sea interactions, particularly zonal advection feedback and Bjerknes feedback. This anomalous annual cycle also included winter equatorial air–sea coupled anomalies with similar spatial patterns but opposite signs. The annual mean equatorial rainfall was significantly increased west of 135°E but decreased between 135°E and 160°W in response to the moderately intensified Walker circulation west of 160°W. The autumn–spring insolation contrast induced similar seasonal reversed anomalies during autumn and spring, but the annual means were only weakly enhanced for the Walker circulation and the rainfall anomalies had smaller magnitudes east of 160°E. These distinct responses of the annual mean climate indicated different seasonal biases in terms of the equatorial SST and associated Walker circulation anomalies due to forcing by the two seasonal insolation contrasts, and these findings had meaningful implications for paleoceanographic studies. [ABSTRACT FROM AUTHOR]

In this study, we assess the performance of state-of-the-art coupled models in reproducing El Niño diversity and its impacts on the East Asian Summer Monsoon (EASM), based on simulations taken from the Coupled Model Intercomparison Project phase 5 (CMIP5). All of the 17 models used in this study are identified to give reasonable eastern-Pacific (EP) El Niño-related sea surface temperature anomalies (SSTA) and anomalous Walker circulation compared with observations, whereas one-third of the models fail to reproduce the Walker circulation anomalies for central-Pacific (CP) El Niño. In these models, negative SSTA over the eastern-to-central Pacific are found to be too strong, while positive SSTA shift to the western Pacific during CP El Niño. There is thus sinking (rising) motion over the central (western) part of the Pacific basin, implying westward displaced anomalous Walker cells, relative to the observed. This results in simulated low-level cyclonic flow and positive rainfall anomalies over the far western north Pacific/South China Sea during and after the peak phase of CP El Niño, which are opposite to the observed anomalous climate state. The other two-thirds of models, with realistic CP El Niño-related anomalous Walker cell, can better reproduce the corresponding wind circulation and rainfall anomalies over East Asian (EA) region. The contrasting CP El Niño-related teleconnection among models mainly arises from the different simulated CP El Niño SSTA, which in turn determine the circulation anomalies through a Gill-type response. Further analyses reveal that, in the mean state of models with erroneous teleconnection, there exists a large cold SST bias over the eastern Pacific. The stronger Walker circulation, and possibly the stronger Bjerknes feedback, might be responsible for the overly negative SSTA associated with CP El Niño in the vicinity. Our results highlight the sensitivity of El Niño–EA climate linkage to the oceanic state in models, especially for CP El Niño events. [ABSTRACT FROM AUTHOR]

The amplitude of El Niño, measured by the Nino3.4 sea surface temperature anomaly (SSTA) index, has exhibited an interdecadal change with a weakening trend since the late 1990s, characterized by the distinct Bjerknes stability index between 1980–1998 and 1999–2014. Statistical results suggest that this was primarily induced by the attenuation of the zonal wind stress and low‐level wind anomalies in response to the zonal equatorial SSTA gradient. The weakened atmospheric responses to the zonal equatorial SSTA gradient were associated with the obvious westward extension of the negative sea level pressure anomalies (SLPA) over the tropics. This was mainly attributed to the transition of the dominant atmospheric circulation over the North Pacific, where the Aleutian Low (AL) mode was replaced by the North Pacific Oscillation (NPO) mode after the late 1990s. Numerical experiments from the long‐term historical simulations of the GFDL‐CM3 model indicate that both of the AL and NPO were dominant over the North Pacific and alternated on the interdecadal time‐scale. When the NPO mode was dominant, it became more effective at playing a role in triggering an El Niño in the central Pacific via the seasonal footprinting mechanism over the subtropical northeastern Pacific. Such a change might cause the westward shift of the tropical SLPA and the attenuated atmospheric responses to the zonal SSTA gradient, ultimately resulting in the weakening of the El Niño amplitude. [ABSTRACT FROM AUTHOR]

The impacts of the eastern-Pacific (EP) and central-Pacific (CP) El Niño-Southern Oscillation (ENSO) on the southern China wintertime rainfall (SCWR) have been investigated. Results show that wintertime rainfall over most stations in southern China is enhanced (suppressed) during the EP (CP) El Niño, which are attributed to different atmospheric responses in the western North Pacific (WNP) and South China Sea (SCS) during two types of ENSO. When EP El Niño occurs, an anomalous low-level anticyclone is present over WNP/the Philippines region, resulting in stronger-than-normal southwesterlies over SCS. Such a wind branch acts to suppress East Asian winter monsoon (EAWM) and enhance moisture supply, implying surplus SCWR. During CP El Niño, however, anomalous sinking and low-level anticyclonic flow are found to cover a broad region in SCS. These circulation features are associated with moisture divergence over the northern part of SCS and suppressed SCWR. General circulation model experiments have also been conducted to study influence of various tropical sea surface temperature (SST) patterns on the EAWM atmospheric circulation. For EP El Niño, formation of anomalous low-level WNP anticyclone is jointly attributed to positive/negative SST anomalies (SSTA) over the central-to-eastern/ western equatorial Pacific. However, both positive and negative CP Niño-related-SSTA, located respectively over the central Pacific and WNP/SCS, offset each other and contribute a weak but broad-scale anticyclone centered at SCS. These results suggest that, besides the vital role of SST warming, SST cooling over SCS/WNP during two types of El Niño should be considered carefully for understanding the El Niño-EAWM relationship. [ABSTRACT FROM AUTHOR]

South China Sea (SCS) experienced an exceptionally long-lived marine heatwave (MHW) event in summer 2020, which broke the historical records in terms of its time duration since 1982. This long-lasting MHW was primarily attributed to the combined effects of the seasonal cycle (SC) and 30–60-day intraseasonal oscillation (ISO) components of anomalous atmospheric circulations. During the SC developing phase, the superimposed anticyclonic circulation regimes of the SC and 30–60-day ISO jointly contributed to the first warming sea surface temperature (SST) peak over the central SCS in July, via the combined effects of net downward shortwave radiation (SWR) and latent heat flux (LHF) anomalies. In contrast, during the SC decaying phase, the SCS MHW-related warming SST tendency was mostly due to 30–60-day ISO signals. During the growth phase of ISO, the joint warming effect of the SWR and LHF anomalies was the dominant factor maintaining the SCS MHW. In the decaying phase of 30–60-day ISO, however, the second MHW-related warming peak around Beibu Gulf in September was mainly contributed by the sharply reduced LHF loss due to strong southwesterly anomalies over the northern SCS on the 30–60-day timescale. The varying SC component plays a crucial role in the occurrence and maintenance of MHWs, while the 30–60-day ISO can effectively regulate the development and pattern of SCS MHWs. These results suggest that, besides the vital role of the SC component, the 30–60-day ISO should also be considered for fully understanding the extreme MHW events. • An extremely long-lived, record-breaking MHW occurred in SCS during 2020 boreal summer. • Jointed effects of seasonal changes and ISO modulations in surface heat fluxes, contributed to the record-breaking MHW2020. • Combined effects of the seasonal cycle and ISO components on SCS summertime MHWs are common in historical records. [ABSTRACT FROM AUTHOR]

An advance in the timing of the onset of the South China Sea (SCS) summer monsoon (SCSSM) during the period 1980-2014 can be detected after 1993/94. In the present study, the interannual variability of the SCSSM onset is classified into two types for the periods before and after 1993/94, based on their different characteristics of vertical coupling between the upper- and lower-tropospheric circulation and the differences in their related sea surface temperature anomalies (SSTAs). On the interannual time scale, type-I SCSSM onset is characterized by anomalous low-level circulation over the northern SCS during 1980-93, whereas type-II SCSSM onset is associated with anomalies of upper-level circulation in the tropics during 1994-2014. The upper-tropospheric thermodynamic field and circulation structures over the SCS are distinct between the two types of SCSSM onset, and this investigation shows the importance of the role played by the spring SSTAs in the southern Indian Ocean (SIO) and that of ENSO events in type-I and type-II SCSSM onset, respectively. In the early episode, the warming SIO SSTAs can induce an anomalous low-level anticyclone over the northern SCS that affects local monsoonal convection and rainfall over land to its north, demonstrating a high sensitivity of subtropical systems in type-I SCSSM onset. However, in type-II SCSSM onset during the later episode, the winter warm ENSO events and subsequent warming in the tropical Indian Ocean can influence the SCSSM onset by modulating the spring tropical temperature and upper-level pumping effect over the SCS. [ABSTRACT FROM AUTHOR]

South China experienced a series of unprecedented extreme heat wave (EHW) events in early summer (June–July) 2020, which broke the historical record for the frequency of EHW events since 1979. Observational analyses showed that the prolonged EHW events were primarily induced by an anomalous tropospheric circulation over South China with a barotropic structure. This was attributed to the exceptional westward displacement of the western Pacific subtropical high (WPSH) and the eastward-extended upper-level South Asia high (SAH). In addition, the value of the SAH–WPSH index, which describes the relative displacement of the SAH and WPSH, also broke the historical record. Further analysis revealed that the EHW events in South China were closely associated with warming of the tropical Indian Ocean (TIO) sea surface temperature (SST), which affected both the SAH and WPSH via atmospheric teleconnections. Sensitivity and pacemaker experiments using an atmosphere-only and a coupled global climate model, respectively, suggested that the anomalous warming of the SST in the TIO directly caused the significant southeastward displacement of the intensified SAH as a result of diabatic heating in the troposphere. This heating also led to a westward-displaced WPSH via an induced mid- to lower tropospheric Kelvin wave and the associated Ekman divergence. In summary, warming in the TIO sustained exceptional shifts in both the SAH and WPSH, which favored the formation of a stationary anomalous high over South China that ultimately contributed to the record-breaking EHW events in early summer 2020. • Prolonged EHWs over SC broke historical records in 2020 during the early summer • The EHWs event was directly attributed to the exceptionally westward-displaced WPSH and eastward-extended SAH • The unusually TIO warming played the critical role in sustaining the exceptional shifts of both SAH and WPSH [ABSTRACT FROM AUTHOR]

We investigate the variations of subsurface ocean temperature (SOT) based on the monthly-Simple Ocean Data Assimilation (SODA) during 1958-2007, and discuss the linkage between the variations of SOT and the eastern and central Pacific ENSO (EP and CP-ENSO) events. The wavelet analyses suggest that the variation of the EP and CP-ENSO events shows the 2-7 and the 10-15 years oscillation in the tropical sea surface temperature (SST), and coupled with a zonal dipole mode and a tripole mode in the SOT anomalous field reveled by the singular value decomposition (SVD) analysis. During the mature phase of CP-ENSO, the positive center of SOT at the subsurface layer locates in the west of dateline, which results in the increase of SOT in the Niño4 region and causes the CP-ENSO event. Statistical analysis implies that, the eastern and central Pacific subsurface indices which are defined by the expansion coefficients of the first and third SVD mode for SOT have shown the capabilities in disguising the EP and CP-ENSO events, respectively. In addition, corresponding to the increase of the SOT amplitude on the 10-15 years time scale, we found that the frequency and intensity of CP-El Niño events has exhibited an upward trend after the 1980s, which suggests that the CP-ENSO event has shown an enhanced impact on the global climate in the past decades. [ABSTRACT FROM AUTHOR]