Western North Pacific Subtropical High Research Articles

Climate projection of tropical cyclone lifetime in the western North Pacific basin

Abstract In this study, the potential changes of tropical cyclone (TC) lifetime in the western North Pacific basin are examined for different future climates. Using homogeneous 9 km-resolution dynamical downscaling with the Weather Research and Forecasting (WRF) model, we show that TC averaged lifetime displays insignificant change under both low and high greenhouse-gas concentration scenarios. However, more noticeable changes in the tails of TC lifetime statistics are captured in our downscaling simulations, with more frequent long-lived TCs (lifetime of 8-11 days) and less short-lived TCs (lifetime of 3-5 days). Unlike present-day simulations, it is found that the correlation between TC lifetime and the Niño index is relatively weak and insignificant in all future downscaling simulations, thus offering little explanation for these changes in TC lifetime statistics based on the El Niño-Southern Oscillation. More detailed analyses of TC track distribution in the western North Pacific basin reveal, nevertheless, a noticeable shift of TC track patterns towards the end of the 21st century. Such a change in TC track climatology results in an overall longer duration of TCs over the open ocean, which is consistent across future scenarios and periods examined in this study. This shift in the TC track pattern is ultimately linked to changes in the Western North Pacific Subtropical High, which retreats to the south during July and to the east during August-September. The results obtained in this study provide new insights into how large-scale circulations can affect TC lifetime in the western North Pacific basin in warmer climates.

Inhomogeneous Sea‐Salt Aerosols—A New Strengthening Mechanism for the Western North Pacific Subtropical High

AbstractThe western North Pacific Subtropical High (WNPSH) significantly influences East Asian weather. In the Northwest Pacific where sea‐salt aerosols (SSAs) are abundant and the large‐scale environment is dominated by the dry subsidence of the WNPSH during summer, inhomogeneous SSAs form as a product of the environment. However, the extent to which inhomogeneous SSAs affect the WNPSH remains unclear. This study investigates the radiative effects of SSAs through numerical simulations, revealing a novel mechanism for the strengthening of the WNPSH. The results demonstrate that inhomogeneous SSAs enhance the WNPSH by generating diabatic cooling in the upper troposphere and associated unstable subsidence motion. Further considering the radiative hysteresis effects of inhomogeneous SSAs, the WNPSH further strengthens under the combined dynamic and thermodynamic influences associated with upper‐level radiative cooling. Inhomogeneous SSAs not only enhance the WNPSH but also influence the location where the central area of high pressure intensifies.

Effect of atmospheric response induced by preceding typhoon on movement of subsequent typhoon over Northwestern Pacific

This study investigates the atmospheric interactions between two closely located typhoons in 2019. Typhoons in the Western Pacific significantly impact Eastern and Southeastern Asian countries, leading to various damages. As global warming is expected to increase typhoon intensity, accurate track forecasting becomes crucial for coastal disaster management. Despite the existing knowledge about the influence of typhoon activities on the atmospheric background, limited research addresses the atmospheric response between two typhoons. The study focuses on the cases of LEKIMA and KROSA, occurring simultaneously in 2019, and utilizes the Weather Research and Forecasting (WRF) model for simulations. The experimental setup involves comparing two scenarios: one with both typhoons and one with LEKIMA removed. Results reveal LEKIMA-induced distinctive atmospheric responses, including the closure of the western North Pacific subtropical high (WNPSH) boundary and the formulation of a wave train, influencing KROSA’s stagnation. The absence of LEKIMA allows KROSA to move more freely along the steering flow. Furthermore, the study highlights the potential of atmospheric models for understanding typhoon effects at regional to mesoscale levels. A comprehensive analysis of similar cases could enhance typhoon predictions, contributing to better damage mitigation strategies.

Global convection-permitting model improves subseasonal forecast of plum rain around Japan

Abstract In 2020 early summer, a historically severe rainy season struck East Asia, causing extensive damage to life and property. Subseasonal forecast of this event challenges the limits of rainy season predictability. Employing the integrated Atmospheric Model Across Scales (iAMAS) and the Sunway supercomputer, we conducted ensemble one-month forecasts at global 3 km, variable 4-60 km, and global 60 km resolutions. The global convection-permitting forecast accurately captures the rainband, while other forecasts exhibited northward and weaker shifts due to the northward shifts of the atmospheric rivers (ARs) over Japan, attributed to intensified Western North Pacific Subtropical High (WNPSH). Further, the double-ITCZ-like tropical rainfall pattern in Western Pacific (D-ITCZ-WP) in global convection-permitting forecast contributes to a more accurate WNPSH and rainband. In contrast, other forecasts show a single-ITCZ-like pattern in Western Pacific (S-ITCZ-WP), leading to a northward-shifted WNPSH and rainband, advocating the importance of accurately representing tropical convections, as they can significantly affect mid-/high-latitude weather and climate.

A newly integrated model for synthesizing multiple external forcing signals to predict the ridge line of the western North Pacific subtropical high in summer

The western North Pacific subtropical high (WNPSH) is a critical atmospheric circulation system influencing weather and climate patterns in eastern China. Accurately predicting its activity is essential for anticipating and managing natural disasters such as droughts and floods in the region. In this study, we employ the decision tree (DT) as the fundamental model and devise a compatible framework to create an integrated model known as the Decision Tree Integration Model for Synthesizing Multiple External Forcing Signals (DTI-MS), to perform classification prediction of the WNPSH ridge line in summer. Within the construction of DTI-MS, the interpretability advantage of DT model is fully exploited. Three significant sources of external forcing signals, namely sea surface temperature in the Pacific, Tropical Indian Ocean and Atlantic Ocean, and snow depth across the Eurasian continent, are identified from distinct DT models, with clear physical meanings. Through the voting strategy, these signals are integrated, laying the groundwork for DTI-MS to synthesize multiple external forcing signals. Regarding prediction performance, DTI-MS achieves a high classification accuracy of 91% in the training set and 93% in the testing set, with recalls for the southerly and northerly positions of the WNPSH in the testing set also exceeding 80%. Compared with the commonly used machine learning models such as DT and Random Forests constructed through general modeling approaches, DTI-MS also demonstrates higher classification accuracy. Overall, DTI-MS not only shows promising potential in predicting the WNPSH ridge line, but its integration framework also provides a new paradigm for short-term climate prediction modeling.

Summer marine heatwaves in the tropical Indian Ocean associated with an unseasonable positive Indian Ocean Dipole event 2012

Marine heatwaves (MHWs) are anomalously warm events that profoundly affect climate change and local ecosystem. During the summer of 2012 (June–September), intense MHWs occurred in the tropical Indian Ocean (TIO) concurrently with an unseasonable positive Indian Ocean Dipole (pIOD) event. The MHW metrics (duration, frequency, cumulative intensity and maximum intensity) were characterized by northwestward–slanted patterns from west Australia to the Somalia coast. The analysis confirmed that these MHWs were closely associated with the unseasonable pIOD 2012. The weakening of Western North Pacific Subtropical High and strengthening of Australian High in spring induced an interhemispheric pressure gradient that drove two anticyclonic circulation patterns over the eastern TIO. The first anticyclonic circulation featured cross–equatorial wind anomalies from south of Java to the South China Sea/Philippine Sea, which led to strong upwelling off Sumatra–Java during the subsequent summer. The second anticyclonic circulation excited downwelling Rossby waves that propagated from the southeastern TIO to the western TIO. Thus, downwelling in the western pole and upwelling in the eastern pole led to a strong pIOD event peaking in summer, namely, the unseasonable pIOD 2012. These downwelling Rossby waves deepened the thermocline by more than 60 m and caused anomalous surface warming, thereby contributing to the occurrences of MHWs. With the development and peak of the unseasonable pIOD 2012, anomalous atmospheric circulation transported moisture from the TIO to the subtropical Western North Pacific (WNP), favoring a strong cyclonic anomaly that profoundly affected the summer monsoon rainfall over the subtropical WNP. This study provides some perspectives on the role of pIOD events in summer climate over the Indo–Northwest Pacific region.

Impact of the East Asian subtropical jet on summer precipitation in China and its response to Atlantic sea surface temperature

AbstractERA5 reanalysis data, precipitation data from China, and National Oceanic and Atmospheric Administration (NOAA) monthly sea surface temperature (SST) data are used to analyse the impact of the meridional position of the East Asian subtropical jet (EASJ) on summer precipitation in China and its correlation with Atlantic SST. The results indicate that when the EASJ significantly shifts northward, the western North Pacific subtropical high (WNPSH) weakens with an eastward displacement. Upper‐level convergence and moisture divergence, corresponding to descending motion, lead to decreased precipitation in the Yangtze River Valley (YRV). Meanwhile, upper‐level divergence occurs over South China (SC), the Hexi Corridor (HC), and Northeast China (NEC), where moisture converges and ascends, favouring an increase in precipitation. Conversely, when the EASJ undergoes a significant southward shift, the WNPSH strengthens and expands westward. Opposing atmospheric circulation patterns in these four regions result in reversed precipitation anomalies compared with those observed when the jet shifts northward. The meridional position of the EASJ is closely related to the summer subtropical Atlantic SST. The positive (negative) SST anomaly (SSTA) over the subtropical Atlantic induces negative (positive) geopotential height anomaly in the upper troposphere over the North Atlantic by modulating the atmospheric meridional circulation. Geopotential height anomalies trigger eastward‐propagating Rossby waves, generating anomalous cyclones and anticyclones over East Asia. These anomalous cyclones and anticyclones lead to zonal wind anomalies, which alter the strength of the westerlies on both sides of the climatological jet axis, thereby changing the jet's meridional position. Additionally, the difference in the propagation direction of wave activity flux between positive and negative SSTA alters the distribution of wave energy convergence and divergence in the EASJ region, further affecting the intensity of the average westerly winds on both sides of the climatological jet axis, ultimately producing the changes in the meridional position of the EASJ.

Record‐breaking heatwave in North China during the midsummer of 2023

AbstractIn midsummer 2023, a record‐breaking extreme heatwave hit North China, causing significant impacts on people's daily lives and production. This study comprehensively analysed the unique features of this extreme event based on five indices describing the intensity, frequency and impact range. The results show that four of these indices broke the historical record in 2023, and the spatial extent (Tmax >40°C) is the second highest in history. The number of hot days with Tmax >40°C (Tmax >35°C) even reached 23 times (4.2 times) the normal. The combined results of these five indices undoubtedly indicate that this heatwave event is featured by high intensity, long duration, numerous extremely hot days and wide impact range. Furthermore, physical mechanism study revealed that the abnormally warm high‐pressure system persisted in dominating North China, resulting in descending airflow and temperature increases. The enhanced and unprecedented westward extending Western North Pacific Subtropical High, the enhanced and eastward extending Iran Subtropical High and the abnormal high‐pressure over southwestern China form an east–west connected anomalous high‐pressure zone, blocking the water vapour transport from lower latitude oceans to North China. Noticeably, North China has experienced a reduction in precipitation since about 2 months preceding this extreme heatwave, leading to severe soil moisture deficiency and reduced evaporation. Consequently, it increased surface sensible heat flux that led to a rise in temperature. This local land‐atmosphere positive feedback mechanism plays a crucial role in the intensification and maintenance of this extreme heatwave event.

Attribution of the unprecedented summer 2022 compound marine and terrestrial heatwave in the Northwest Pacific

In boreal summer (July–August) 2022, an unprecedented marine heatwave (MHW) occurred in the northwest Pacific Ocean (NWP), while a record-breaking terrestrial heatwave (THW) hit the Yangtze River Basin (YRB). The temperature anomalies caused by this compound MHW-THW event exceeded climatology by 2.5 standard deviations (SDs), breaking the historical record for nearly 100 years, with severe impacts on the ecosystems and social economy. To investigate the underlying causes, we explored the potential roles of anthropogenic forcing, atmospheric circulation, and ‘triple-dip’ La Niña on this compound event using the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model simulations. Results indicate that the 2022-like compound MHW-THW event was extremely unlikely to happen without anthropogenic warming, and that such extreme heatwaves were governed by the climatic mean temperature rather than changes in temperature variability. Notably, the anticyclone circulation patterns and associated high-pressure systems (i.e. western North Pacific subtropical high (WNPSH) and South Asian high (SAH)) increase the probability of a 2022-like MHW-THW event by 3.7 times. However, the La Niña phase has no significant effect on the occurrence probability of such events. We further estimate that the 2022-like MHW-THW event will become 7.5 and 11.4 times more likely under the SSP3-7.0 scenario by the middle and end of the 21st century, respectively. This study demonstrates the contribution of anthropogenic climate change and natural variability to the occurrence of compound MHW-THW events and highlights the urgent need to build mitigation strategies for compound MHW-THW events.

The relationship between the South China Sea summer monsoon onset and North Pacific meridional sea surface temperature anomalies

AbstractThis study investigates the connection between significant sea surface temperature (SST) anomalies in the North Pacific during boreal spring (February–April, FMA) and the subsequent South China Sea (SCS) summer monsoon (SCSSM) onset. The SST anomalies, similar to the Pacific meridional mode (PMM), referred to as the PMM+ mode, are defined to examine the new influencing factor on the SCSSM onset. Our findings reveal that the (February–March–April, FMA) PMM+ has a noteworthy positive correlation with the subsequent May SCSSM onset date, with this correlation being minimally affected by the El Niño–Southern Oscillation (ENSO) during preceding winter. A robust positive PMM+ in boreal spring can be persist until May via atmosphere–ocean interaction. The cooling area over Western North Pacific would reduce precipitation heating, thereby generating Rossby waves that reinforce the formation of the anomalous anticyclone over the SCS. As a result, easterly winds and suppressed convection prevail over the SCS, making the SCSSM break out later than normal. Furthermore, the amplification of anticyclonic vorticity anomalies also strengthens the western North Pacific subtropical high (WNPSH) stronger and shifts its position further westward compared to normal years, thereby blocking active convection to the west of the SCS. Given the weakened relationship between El Niño–Southern Oscillation (ENSO) and the SCSSM onset in recent years, the PMM+ could be considered as a promising preceding signal for the SCSSM onset, thus holding significant implications for the SCSSM prediction efforts.

Seasonal Cycle Delay of the Western North Pacific Tropical Cyclone Genesis Frequency in CMIP6 Simulations

AbstractObvious biases in simulating tropical cyclone (TC) genesis of the current climate models hamper our understanding of TC changes. In this study, we found a delay of the seasonal cycle of TC genesis frequency over the western North Pacific (WNP) in most Coupled Model Intercomparison Project Phase 6 models. During the active TC season, the simulated south‐warming and north‐cooling surface temperature bias amplifies the meridional gradient and excites thermal winds. This weakens the western North Pacific Subtropical High and easterly monsoon trough, which further reduces TC genesis frequency over the western WNP in summer. But in autumn, positive TC genesis biases were only observed in coupled models over the eastern WNP. Both seasons contribute to the delayed seasonal cycle of TC frequency in models. Our findings highlight the importance of accurate simulation of surface temperature by climate models to TC simulations and aid in future model improvements.

New characteristics of Meiyu precipitation changes in the middle and lower reaches of the Yangtze River since 2000

This study comprehensively analyzed the long-term changes of Meiyu in the middle and lower reaches of the Yangtze River with regard to precipitation, precipitation threshold, and interdecadal changes in atmospheric circulation. Results revealed obvious new features of Meiyu precipitation since 2000. (1) Meiyu precipitation shows a significant linear increase trend, with an average increment of 73.5 mm every 10 years. The increase in torrential rain is the most significant, accounting for 61% of the total precipitation increase, and its proportion has been increasing, while the proportions of light, moderate, and heavy rain in the total precipitation have been decreasing. (2) At the interdecadal scale, Meiyu precipitation and the local surface average temperature show opposite changes. Meiyu precipitation decreases by approximately 150 mm for every 1°C increase in the average temperature. This inverse correlation was not evident before 2000. (3) The saturated specific humidity in this area shows a significant increasing trend, indicating that precipitation in this area is caused by a higher threshold of atmospheric saturation and condensation, which may be a reason for the low Meiyu precipitation during high-temperature years. (4) The main atmospheric circulation in East Asia associated with Meiyu shows clear interdecadal changes, including the Western North Pacific Subtropical High and South Asian High having become significantly stronger and having extended westward and eastward, respectively, thereby facilitating the occurrence and persistence of Meiyu precipitation. The atmospheric circulation patterns associated with Meiyu, such as the Western North Pacific Subtropical High and South Asian High, have more significant impacts on Meiyu precipitation.

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

This 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.