Western North Pacific Subtropical High Research Articles

The Predictability of a Heavy Rainfall Event during the Summer of 2022 Using an All-sky Radiance Assimilation Experiment

AbstractThis paper presents the results of the recent development of the all-sky radiance assimilation system in the Korean Integrated Model (KIM). In the cycled analysis and forecast experiments, the increased coverage of radiance data in cloudy regions improved the quality of initial fields for mass variables, temperature and humidity. The experimental period covered the record-breaking heavy rainfall event on August 9, 2022. We examined the simulation accuracy of the western North Pacific subtropical high (WNPSH) in both clear- and all-sky experiments. In the clear-sky experiment, northward propagation of the WNPSH was restricted. A humid bias exists with clear-sky radiance assimilation over the WNPSH region. Since humid air is lighter than dry air, in this situation, the geopotential height (GPH) should be lower to achieve the same pressure, and a low-pressure bias occurs. All-sky radiance assimilation dries the moisture field, which helps elevate the GPH over the WNPSH region. The expansion of the WNPSH yielded a steeper confrontation in the air between the land and ocean around the southeastern sea of the Korean Peninsula to predict the strength of rainfall events more accurately. A more accurate simulation of the jet stream outlet was also demonstrated in an all-sky experiment. This study shows that the all-sky radiance assimilation can help to more accurately predict extreme rainfall events via proper simulations of large-scale fields.

Characteristics of the East Asian Summer Monsoon Using GK2A Satellite Data

In East Asia, where concentrated summer precipitation often leads to climate disasters, understanding the factors that cause such extreme rainfall is crucial for effective forecasting and preparedness. The western North Pacific subtropical high (WNPSH) is a key driver of summer precipitation variability, and therefore, its monitoring is critical to predicting the wet or dry periods during the East Asian summer monsoon. Using the Geo-KOMPSAT 2A (GK2A) satellite cloud amount data and ERA5 reanalysis data during the years 2020–2023, this study identified three leading empirical orthogonal function (EOF) modes and investigated the associated WNPSH variability at synoptic and subseasonal scales. The analysis includes a linear regression of meteorological fields onto the principal component (PC) time series. All three modes play a role in the spatiotemporal variability of the WNPSH, exhibiting lead–lag relationships. In particular, the second mode is responsible for its northwestward shift and intensification. As the WNPSH moves northwestward, the position of the monsoon rain band also shifts, and its intensity is modulated mainly by the moisture transport along the WNPSH boundary. Our results highlight the potential of high-resolution, real-time data from the GK2A satellite to elucidate WNPSH variability and its impact on the East Asian summer monsoon. By addressing the variability of the WNSPH using GK2A data, we pave the way for the development of a real-time monitoring framework with GK2A, which will improve our predictability and readiness for extreme weather events in East Asia.

The Contrast Precipitation Patterns in Yangtze River Valley Between the Two La Niña Decaying Summers in 2021 and 2022

AbstractAlthough the summers of 2021 and 2022 are both in the two successive La Niña decaying stages and under the same climate background of negative Pacific Decadal Oscillation (PDO) phase and global warming trend, they exhibit significantly different and even opposite precipitation patterns in the Yangtze River Valley (YRV) as well as in the Indian monsoon region (IMR). In contrast to the abundant precipitation and lower temperature in the YRV in summer 2021, in summer 2022 the YRV experiences severe drought and extremely high temperatures, which is also accompanied by Mega‐floods in the IMR. This study identifies the joint influence of sea surface temperature anomalies (SSTAs) in Niño4 and Barents Sea (BS) regions as the underlying cause for the contrast YRV precipitation anomalies in the summers of 2021 and 2022. Specifically, the cold SSTAs in both Niño4 and BS regions in summer 2021 favor stronger and southward shifted western North Pacific subtropical high (WNPSH), leading to more precipitation in the YRV, which is however generally reversed but more intense in summer 2022 because of the synergistic effect of cold Niño4 and warm BS SSTAs. Moreover, the induced extreme precipitation in the IMR in summer 2022, which is absent in summer 2021 due to the offsetting effect of cold SSTAs in both Niño4 and BS regions, in turn further strengthens the anomalous atmospheric circulations via its released large diabatic heating and serves as a relay pathway for the dramatic drought and heat wave in the YRV.

Climate change and La Niña increase the likelihood of the ‘7·20’ extraordinary typhoon‐rainstorm in Zhengzhou, China

AbstractOn 20 July, 2021, an extraordinary rainstorm occurred in Zhengzhou in Central‐North China, which caused hundreds of deaths and serious property damages, and affected 14 million people. The compound anomalous atmospheric circulations were found to be responsible for the occurrence of the extraordinary rainstorm in Zhengzhou, but the influence of large‐scale sea surface temperature anomaly (SSTA) and the contribution of climate change received less attention. This study conducts an attribution analysis based on atmospheric reanalysis and sea surface temperature (SST) observation data combined with Coupled Model Intercomparison Project Phase 6 (CMIP6) model simulation data. The results show that the return period for the extraordinary rainstorm was 250 years, which was caused by the interaction between the abnormally northerly shift of Western North Pacific Subtropical High (WNPSH) that was associated with the negative SSTA over the tropical central‐eastern Pacific (i.e., a moderate La Niña condition) and Typhoon In‐fa and Typhoon Cempaka. A persistent convergence was formed over Zhengzhou by transporting a large amount of water vapour to hinterland. The lagged response of La Niña affected WNPSH and the rainband in China. The attribution analysis shows that climate change due to anthropogenic forcings such as greenhouse gas emissions increased the risk of such a rainstorm event by 12% compared to the conditions under the natural forcings. Under the combined effects of anthropogenic forcings and La Niña, the risk of occurrence increased 47% compared to the effect of natural forcings only on similar precipitation events. This study suggests a synergistic effect of climate change and large‐scale tropical climate variability on compound extremes.

The subseasonal predictability of the western North Pacific subtropical high and the 2020 record-breaking event

The western North Pacific subtropical high (WNPSH), a prominent feature in the North Pacific during the boreal summer, exerts significant socioeconomic consequences by influencing hydrological extremes such as tropical cyclones, the Meiyu front, and summer heat waves over East Asia. Accurately forecasting the characteristics of the WNPSH over extended timescales is crucial, but subseasonal prediction in this specific context is still in its early stages due to the complex dynamics involved. In this study, we investigate the optimal predictable pattern of the WNPSH using linear stochastic dynamics. Our findings reveal that convection over the Philippine/South China Sea and Japan serves as key precursors, where a dipole vorticity pattern leads to maximum growth of the WNPSH on subseasonal timescales, providing a potential source of predictability. Additionally, we examine the role of optimal predictable patterns in the record-breaking 2020 WNPSH event, and we find that the cumulative effect of stochastic forcing helps explain the sustained features of this extreme case.

On the moisture transport regimes for extreme precipitation over North China

A novel clustering method combing the Spearman's rank spatial correlation and the spectral clustering is proposed to investigate the variability in moisture transport. Four moisture transport regimes for the extreme precipitation events (EPEs) over North China are identified through applying the new clustering method to the moisture source distributions of the 232 EPEs over North China during the rainy season (June–September) in 1979–2018. The four regimes comprise approximately 50%, 21%, 20%, and 9% of the EPEs, with counts of 115, 48, 47, and 22, respectively. Each regime exhibits distinctive moisture source pattern. In each of the four regimes, areas with higher moisture sources than the other three (meeting the statistical significance) are: the Indian Ocean and the Indochina Peninsula in regime 1, the South China Sea and the western North Pacific in regime 2, North China and its adjacent areas in regime 3, and the western North Pacific and the areas around the Maritime Continent in regime 4. Furthermore, the atmospheric circulations before the onset of EPEs manifest different patterns across the four regimes. Multiple meteorological phenomena associated with the circulation patterns are identified prior to the EPEs, e.g., the westward extension (observed in regimes 1 and 2) and eastward retreat (regime 4) of the western North Pacific Subtropical High (WNPSH), an upper-level wave train propagating from western Russia to Japan (regimes 1 and 3), extensive atmospheric river (AR) activities (regimes 1 and 4), and tropical cyclones (regime 4). The circulation patterns and the associated meteorological phenomena have critical impacts on the moisture transport and could serve as precursor signals for the EPEs.

Identification of the atmospheric water sources and pathways responsible for the East Asian summer monsoon rainfall

AbstractThe East Asian summer monsoon rainfall provides water security and socio‐economic benefit for over 20% of the global population. However, the sources of this rainfall and how it is carried to the East Asian landmass are still uncertain. To address this, atmospheric water sources and pathways associated with the East Asian summer rainfall are identified and quantified in this study using atmospheric water trajectories, calculated with a novel Lagrangian framework. Evaporated water from the East Asian landmass is found to be the major contributor to East Asian rainfall, amounting to local recycling. The results further indicated that the south Indian Ocean is a major non‐local source for rainfall over southern East Asia during June to August. The role of the south Indian Ocean as a source of atmospheric water is one of the major findings of the study and would help in better understanding and predicting the East Asian summer rainfall. Evaporated waters from the Pacific Ocean (particularly the far‐west Pacific Ocean) dominate the non‐local contribution to precipitation over northern East Asia during June to September and over southern East Asian rainfall during September. The spatial structure of the East Asian rainfall is reported to be determined by the atmospheric waters that are evaporated and transported from the non‐local sources. The role of the north Indian Ocean and the South Asian landmass as a source of water for East Asian precipitation is minimal and restricted to southern East Asia. The cross‐equatorial Somali jet and equatorial trade winds associated with the western North Pacific subtropical high are important pathways for East Asian precipitation sourced over the south Indian Ocean and the Pacific Ocean respectively. In contrast, minor roles are attributed to the Bay of Bengal as a source, and midlatitude westerlies as a transport pathway, for East Asian precipitation.

Evaluation of sub-seasonal prediction skill for an extreme precipitation event in Henan province, China

A severe torrential rain attacked Henan province from July 19 to 21, 2021, resulting in extensive social and economic damages. The models’ sub-seasonal prediction skill for this extreme event remains to be evaluated. Based on the real-time data of 5 models (CMA, ECMWF, NCEP, KMA, and UKMO) from the sub-seasonal to seasonal (S2S) prediction project, our study compared the models’ predictability and explored the possible reasons. Results indicate that most models can predict the spatial distribution of accumulated precipitation for this event 1 week in advance. Two models (NCEP and CMA) still have specific reference values in predicting precipitation intensity 2–3 weeks ahead. However, the predicted maximum rainfall is only about 20% of the observation, and all models cannot catch the extremes of this event. While large-scale atmospheric circulation can be predicted with some accuracy, there are still significant deviations in predicting the location and intensity of the western North Pacific subtropical high (WNPSH) and Typhoon In-Fa. The models predict weaker intensity of the southeast airflow transporting water vapor into the rainstorm area, resulting in significantly weaker precipitation. This is mainly attributed to unsatisfactory predicted typhoon circulation in most models. The model ECMWF and KMA predict a better moisture flux at 925hPa, about 60% of the observations. The characteristics of local high SST centers in the Sea of Japan cannot be caught, resulting in the position of the predicted WNPSH eastward and weak. Therefore, to improve the prediction skill for extreme precipitation events, it is imperative to enhance the interaction mechanisms among atmospheric circulation systems within the model.

Impact of May atmospheric latent heating over the Southeast Asian low-latitude highlands on interannual variability in the Meiyu onset date

The Yangtze-Huai River Basin (YHRB) is an economically developed area and one of the most densely populated regions in China. Based on ERA5 reanalysis and Meiyu monitoring data, this study investigates the relationship between interannual variability in atmospheric latent heating (LH) over the Southeast Asian low-latitude highlands (SEALLH) during early summer and the Meiyu onset date (MOD) in the YHRB. The mean MOD is 9 June, with a quasi-6-year interannual oscillation period. A stronger early summer LH over the SEALLH coincides with earlier MOD; below 500 hPa, LH increases with height and drives an anomalous southwesterly airflow from the Bay of Bengal (BOB) to the SEALLH via β effects. Circulation over the western North Pacific (WNP) is dominated by an anomalous anticyclone, causing the WNP subtropical high (WNPSH) to extend westward and northward. Continuous positive LH anomalies over the SEALLH cause anomalous southwesterly winds to transport moist enthalpy air masses from low latitudes into the YHRB, resulting in abnormal convergence there. Favorable dynamic and thermal conditions drive enhanced precipitation over the YHRB, eventually leading to a MOD that is 13 days earlier than that during periods when early summer LH is relatively weak over the SEALLH. This result potentially supports our capacity to predict the Meiyu in the YHRB.

Multi‐Objective Ensemble‐Processing Strategies to Optimize the Simulation of the Western North Pacific Subtropical High in Boreal Summer

AbstractThe western North Pacific Subtropical High (WNPSH) in boreal summer is a major atmospheric player affecting East Asian climate, but its simulation in state‐of‐the‐art climate models is still largely biased. Here we use a multi‐objective optimization strategy, the Pareto optimality, to incorporate multiple physical constraints in processing multi‐model simulations provided by the Coupled Model Intercomparison Project Phase 6. We aim to improve the simulation of WNPSH by this practice. Sea surface temperatures from three tropical oceanic basins are found highly related to WNPSH, and thus used as constraints. We also present an ameliorated strategy, which takes a subset of the raw Pareto optimality by imposing conditions of smallest errors. Results show that the overestimate of WNPSH is effectively corrected. The two multi‐objective optimization schemes both perform better than the traditional approach, revealing the importance of implementing physically based links in processing multi‐model ensemble simulations.

40-Year Statistics of Warm-Season Extreme Hourly Precipitation over Southwest China

Abstract Southwest China (SWC) possesses complicated topography with frequent geological activities, where heavy precipitation occurs frequently in warm seasons. Few previous studies on extreme precipitation were carried out at hourly scales. In this study, spatiotemporal variations of the extreme hourly precipitation (EHP) over SWC during the warm season of 1981–2020 and the involved mechanisms are investigated. Results show that the threshold and intensity of EHP present similar spatial distribution—lower (higher) in the west (east) part of SWC, while the EHP frequency is opposite. The long-term trend of EHP amount shows a more significant positive tendency than that of hourly precipitation (HP) amount due to synchronous increases in intensity and frequency. The significant increasing trend of EHP occurs in areas above 500-m terrain height, with a weak increasing trend below 500 m (e.g., Chongqing and eastern Sichuan). EHP appears mainly from June to August and exhibits a bimodal distribution in diurnal variation. The mechanism analysis demonstrates that occurrences of EHP are generally accompanied by positive anomalies of temperature, humidity, and geopotential height. Anomalous cyclonic circulation can also be found in the low-level wind field. The westward and northward extension of the western North Pacific subtropical high (WNPSH) as well as temperature rise may be the primary reason for the increase of EHP. For Chongqing and eastern Sichuan, the anticyclone circulation in low-level and the significantly weakened water vapor flux convergence cause poor moisture and dynamic conditions, inhibiting the growth of EHP. Significance Statement Heavy precipitation occurs frequently during the warm season in Southwest China (SWC), often causing severe impacts on human safety and economic property. This study analyses spatiotemporal variations of the extreme hourly precipitation (EHP) over SWC during the warm season of 1981–2020 and the involved mechanisms. The increasing trend of EHP far exceeds that of hourly precipitation (HP), especially in areas above 500 m. The westward and northward extension of the western North Pacific subtropical high (WNPSH) and temperature rise may be the main reason for the increase of EHP. For areas below 500 m (e.g., Chongqing and eastern Sichuan), poor moisture and dynamic conditions inhibited the growth of EHP.

Interdecadal Variability in the Interface Between the Indian Summer Monsoon and the East Asian Summer Monsoon

AbstractThis study investigates the interdecadal variability (IDV) of the interface between the Indian summer monsoon and the East Asian summer monsoon (IIE). Results suggest that the IDV of the IIE is characterized by a uniform zonal movement associated with seesaw variations between the Indian summer monsoon (ISM) and the East Asian summer monsoon (EASM). The IDV of the IIE is closely linked to two air–sea coupled modes: one resembles the Asian–Pacific Oscillation (APO) and the other resembles the North Atlantic tripole (NAT) pattern. Both the APO‐like and NAT‐like patterns facilitate an eastward shift of the IIE during their positive phase. The APO‐like mode strengthens the western North Pacific subtropical high (WNPSH), which further enhances the climatological southwesterly wind over East Asia and leads to an intensification of the EASM. The simultaneous anticyclonic anomaly over the India and Bay of Bengal (BOB) region weakens the ISM. The NAT‐like mode forces a wave train of alternating cyclonic and anticyclonic anomalies across Eurasia, causing southwesterly anomalies over East Asia that favors a stronger EASM. The atmospheric response is reversed during the negative phases of the APO‐like and NAT‐like patterns. Recent decades have seen a weakening influence of the APO‐like mode, but a growing impact of the NAT‐like mode on the IIE‐IDV. The interdecadal transition of the IIE–APO and IIE–NAT relationships has a smaller influence on the IIE than that of the phase change of the APO‐like and NAT‐like patterns. Modeling results confirm the influence of the APO‐like and NAT‐like patterns on the IIE‐IDV.

Record-Breaking heavy rainfall around Henan Province in 2021 and future projection of extreme conditions under climate change

A record-breaking extreme precipitation event that occurred in July 2021 over Henan Province is investigated through observations and reanalysis data. Meanwhile, future extreme precipitation conditions are projected through an ensemble projection model using climate model outputs as input. The long duration, the wide range of influence, the extensive amount of accumulated precipitation, and the high intensity of precipitation are the four major characteristics of this event. Further, the physical causes of this event are explored. First of all, the abnormal northward movement of the western North Pacific Subtropical High is quite significant. Secondly, two slow-moving typhoons are found, which intensify inland moisture transport. Thirdly, after encountering the windward slope, the water vapor is violently lifted and continuously accumulated. Future projections of multiple extreme indices show that more extreme precipitation conditions are expected across the entire Henan Province. In addition, the contribution of various sources to the uncertainty of the ensemble projection are quantified. The global climate model is the primary contributing factor. Meanwhile, the effects explained by the interactive effects between global climate models and shared social-economic pathways cannot be neglected. Our study has constructed an analysis framework called the “Four-in-One” (spatial–temporal feature analysis, causal analysis, future projection, and uncertainty quantification). It provides further insight into changes in local extreme precipitation conditions. The overall framework of this study is equally applicable to investigating other types of extreme events.

Record-breaking marine heatwave in northern Yellow Sea during summer 2018: Characteristics, drivers and ecological impact

Globally, marine heatwaves (MHWs) are becoming more common, more intense, and longer-lasting. They could have a large ecological and societal impact when compounded by low oxygen concentrations or high acidity. Here, using a high-resolution satellite product and reanalysis datasets, we investigated the characteristics of the MHW at northern Yellow Sea (NYS) during mid-summer 2018 and the driving mechanisms of large-scale atmospheric circulations. Results showed that the MHW in mid-summer 2018 (lasting from 26 July to 18 August 2018) had been the most intense since 1982, reaching an anomaly peak of 5.15 °C. For the 2018 MHW, the onset rate was 0.49 °C/day, indicating that the reaction window was relatively short and hard to take mitigation measures, while the decline rate was 0.19 °C/day, meaning the coping window was long and easy to push an already stressed system. The synergy of the two large-scale dynamic systems, i.e., the northward-shifted western north Pacific subtropical high (WNPSH) and the northeastward-expanded South Asia high (SAH), was likely responsible for establishment and maintenance of the hot-weather conditions. These high-pressure systems could result in stronger descending motion, less cloud cover, more solar radiation, and smaller wind speeds which in combination aggravated the MHW. We further found that the unprecedented MHW was actually also impacted by terrestrial heatwave. From 14 July to 15 August 2018, Northeast China was affected by an exceptionally long and intense atmospheric heat wave (AHW). The AHW had impacted on the MHW through warm advection transportation and may significantly contribute to the record-breaking intensity of the MHW, in addition to the impact of abnormal atmospheric circulations. Finally, we showed that a mass mortality of sea cucumbers in the study region during mid-summer 2018 was highly likely caused by the MHW through severe heat stress.

Predictability of the Western North Pacific Subtropical High and Associated East Asian Monsoon Rainfall in APCC Multi‐Models

AbstractUsing selected models with better performance in simulating the Western North Pacific Subtropical High (WNPSH) variability can improve predictions for the East Asia Monsoon (EAM) precipitation. The selected model is clearly superior to the EAM rainfall prediction with the responses to sea surface temperature anomalies (SSTA) forcing and coupling in the atmospheric circulation between the East Asian subtropical Jet and WNPSH, which are similar to those in the observations. The findings of this study reveal that the “poor” model for EAM rainfall prediction is highly correlated with SSTA regardless of El Niño‐Southern Oscillation phases. The “poor” model shows the responding circulation pattern to the El Niño persistent SSTA in the central‐eastern Pacific until JJA, which induces more bias in the prediction of WNPSH variability and EAM rainfall. It is also noticeable that the “poor” model exhibits stronger barotropic response between the WNPSH and EASJ compared to observations, and therefore, it induces stronger relation between the EAJS and EAM rainfall. As a result, both atmospheric circulation itself over the EAM region and atmospheric‐ocean interactions are important to enhance the predictability of the WNPSH and EAM rainfall.

Long-Term Changes, Synoptic Behaviors, and Future Projections of Large-Scale Anomalous Precipitation Events in China Detected by a Deep Learning Autoencoder

Abstract The frequency of large-scale anomalous precipitation events over China has increased during 1961–2018. However, it remains challenging to understand the mechanisms associated with these anomalous events showing different spatial patterns. Here, we applied an autoencoder technique to identify large-scale anomalous precipitation events for both observations and model simulations, which were then classified into several patterns through a self-organizing map method. The synoptic behavior and atmospheric circulation background of different anomalous patterns were also analyzed using simultaneous composite analyses. Results show that occurrences of different anomalous precipitation patterns have increased significantly, except those centered in North China, Northeast China, and the Yangtze River basin. The anomalous precipitation patterns manifest various intraseasonal distributions, which are linked to zonal oscillations of the western North Pacific subtropical high (WNPSH) and meridional displacements of East Asia westerly jet (EAJ). Accompanied by the westward movement of WNPSH, anticyclonic systems transport warm moist air from the Indian Ocean and the South China Sea to converge with the cold air caused by anomalous cyclones over the northwest flank of the WNPSH, leading to large-scale anomalous precipitation in these regions. Besides WNPSH, the northward and southward displacements of EAJ also favor the occurrence of anomalous precipitation events in northern and southern China, respectively. Our study also illustrates that the occurrence frequency of anomalous precipitation events is projected to increase remarkably under the Shared Socioeconomic Pathway 5–8.5 (SSP585) scenario by rates of twofold to fourfold.

Distinctive Features of Monsoon-TC Joint Rainfall over Western North Pacific and its Relationship with the Maritime Continent Thermal Condition

ABSTRACT The East Asian summer monsoon (EASM) and tropical cyclones (TCs) in the Western North Pacific (WNP) are both responsible for the East Asian summer rainfall, yet most studies only examine their rainfall separately. In this study, the East Asian summer rainfall for the past 39-years (1983-2021, May to September) is divided into three categories: monsoon rainfall without TCs’ influence (monsoon-only rainfall), TC rainfall independent of monsoon (TC-only rainfall) and monsoon-TC joint (MS-TC) rainfall. Compared with the other two categories, MS-TC rainfall exhibits distinctive features. During strong MS-TC years, a distinct cyclonic anomaly centre prevails over tropical WNP with anomalous southeasterlies extending from the tropics to the subtropics. Large rainfall centres are located at the west edge of the northern Philippines, the Philippine Basin, and the Korean Peninsula. The WNP Subtropical High (WNPSH) withdrawals eastward, with an eastward extension of the monsoon trough. These circulation configurations provide favourable environmental conditions for more northward movements of TCs, including low-level positive relative vorticity and enhanced vertical motion in WNP. Observational and theoretical analysis results show that anomalous thermal conditions in the southern Maritime Continent (MC) (97.5°−112.5°E, 8°−18°S) in early spring (March to April) can be a precursor for anomalous MS-TC rainfall. Strong MS-TC rainfall is usually preceded by depressed convection and anomalous westerlies near the MC. These anomalies can persist through the following summer and induce the positive feedback of cooling sea surface temperature (SST) in MC and cyclonic anomalies in tropical WNP through the wind-evaporation-SST (WES) effect and local Hadley circulation anomalies. The linear baroclinic model (LBM) experiments demonstrate that enhanced convection in tropical WNP further maintains the anomalous cyclone through Gill's response, which in turn modulates the distribution and amount of MS-TC rainfall.

Climatological characteristics of the monsoon breaks during the southwest monsoon season of the Philippines

AbstractThis study investigates the climatological characteristics of the monsoon breaks during the southwest monsoon season (June–September) of the Philippines from 1979 to 2020. The monsoon break is defined as a period when the average rainfall across the eight western coast stations of the Philippines decreases below 5 mm·day−1 for at least three consecutive days. About 208 monsoon break events were identified, with 131 short‐ (3–4 days; 63% of the total monsoon break events), 71 medium‐ (5–9 days; 34%) and 6 long‐duration (≥10 days; 3%) monsoon break events. The short‐ and medium‐duration monsoon breaks were found to be more frequent during June and less frequent during August. Lag composite analyses of the time series of rainfall, low level zonal wind (UWND850hPa), vertically integrated moisture flux convergence (VIMFC) and mid‐tropospheric geopotential height (HGT500hPa) relative to the starting date of the monsoon break events show that the monsoon break is characterized by decrease in rainfall, weakening of westerly winds, increase in HGT500hPa and suppression of VIMFC over Luzon Island. Further analyses of the large‐scale circulation features show the appearance of an anticyclonic circulation over Luzon Island that suppresses the rainfall in the region and westward extension of the western North Pacific Subtropical High (WNPSH). The occurrence of the monsoon break events appears to be modulated by the 30–60‐day mode of the boreal summer intraseasonal oscillation (BSISO). About 39%, 55% and 82% of the short‐, medium‐ and long‐duration monsoon break events occur between Phases 1 and 4 during significant suppressed phases of the BSISO (i.e., amplitude >1), respectively. Convective activities and tropical cyclone activities are generally suppressed in these phases. The suppression of convective activities near the Philippines may facilitate the westward expansion of WNPSH, which further suppresses the rainfall and weaken the southwest monsoon.

Characteristics and Formation of Two Leading Marine Heatwave Modes in the North Indian Ocean during Summer and Their Implications for Local Precipitation

Abstract Marine heatwaves (MHWs) are extreme climatic events that can have a significant impact on marine ecosystems and their services across the world. We examine the spatiotemporal variation of summer MHWs in the north Indian Ocean (NIO) and find that the whole NIO Basin exhibits a pronounced spatial variability as well as a significant increasing trend in MHW frequency. We show that the NIO has two leading MHW modes linked to two distinct sea surface temperature (SST) patterns during summer. The first MHW mode is associated with basinwide warming, which is preconditioned by a decaying El Niño–Southern Oscillation (ENSO) and sustained throughout the summer by anomalous northeasterlies extending from the anticyclonic circulation of the western North Pacific subtropical high (WNPSH). The combined effect of thermocline warming due to downwelling oceanic planetary waves, decreased wind-induced evaporative cooling, and enhanced insolation cause basinwide summer MHWs. The second MHW mode exhibits a zonal dipole pattern, which has unfavorable cooling conditions in the previous seasons. The second MHW mode is associated with a phase change of ENSO and is greatly influenced by the formation of an interhemispheric pressure difference (IHPD) due to strengthening of the Australian high (AH) and weakening of the WNPSH. The IHPD induces cross-equatorial southerly winds across the eastern Indian Ocean. These winds favor the transformation of basinwide cooling conditions into zonal SST patterns via wind–evaporation–SST and thermocline–SST feedback, causing MHWs with a zonal dipole pattern. These MHW modes have a significant influence on the distribution and intensity of summer precipitation in the NIO.

Dominant modes and mechanisms of atmospheric rivers in East Asia

Atmospheric rivers (ARs), the long and narrow conveyors of intense moisture transport, are a unique mover of moisture from the low latitudes to the mid to high latitudes and have a considerable impact on hydrometeorological events (i.e., extreme precipitation). Using the top-hat by reconstruction (THR) algorithm and ERA5 reanalysis data, this study analyzes the climatological characteristics of ARs over East Asia (EA) in summer (June to August) during 1979–2020. According to the four leading modes of EOF analysis, ARs events occur mainly in the Yangtze River valley, the Huaihe River valley, and the coastal region of South China. The sea surface temperature anomalies (SSTAs) related to the first EOF mode (EOF1) show the warming of the central and eastern Pacific and Indian Oceans. The northward movement of the westerly jet stream maintains the western North Pacific subtropical high (WNPSH) and guides water vapor transport to China. EOF2 corresponds to the positive phase of EAP (East Asia-Pacific) pattern. SSTAs related to EOF2 show an El Niño recession-like pattern, which modulates precipitation over southern China through anticyclones over the western North Pacific. SSTAs related to EOF3 show a La Niña-like pattern, which regulates the precipitation in the Yangtze-Huaihe valley through upper-level convective heating on the north side of the WNPSH. For EOF4, the circulation system affecting ARs is dispersed, resulting in smaller scale ARs. ARs of EOF4 are associated with cyclones, mainly showing the characteristics of coastal landing ARs. These results can provide a new forecasting source for EA summer precipitation.