North Pacific Subtropical High Research Articles

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.

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.

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.

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.

Origins of Uncertainty in the Response of the Summer North Pacific Subtropical High to CO2 Forcing

AbstractThe variability of the summer North Pacific Subtropical High (NPSH) has substantial socioeconomic impacts. However, state‐of‐the‐art climate models significantly disagree on the response of the NPSH to anthropogenic warming. Inter‐model spread in NPSH projections originates from models' inconsistency in simulating tropical precipitation changes. This inconsistency in precipitation changes is partly due to inter‐model spread in tropical sea surface temperature (SST) changes, but it can also occur independently of uncertainty in SST changes. Here, we show that both types of precipitation uncertainty influence the NPSH via the Matsuno‐Gill wave response, but their relative impact varies by region. Through the modulation of low cloud fraction, inter‐model spread of the NPSH can have a further impact on extra‐tropical land surface temperature. The teleconnection between tropical precipitation and the NPSH is examined through a series of numerical experiments.

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.

The influence of ENSO during spring over northwestern Mexico

AbstractThis study focuses on better understanding how El Niño‐Southern Oscillation (ENSO) influences the occurrence of particularly dry conditions (warm temperature and dry precipitation anomalies) in northwestern Mexico during boreal spring (March–May), a season that is characterized by the driest regional conditions, and by a high socioeconomic sensitivity to the interannual variations in them. For that, surface observations (CRU TS4.05 and ERSSTv5) and low‐tropospheric circulation reanalysis (ERA5) were analysed using linear regressions, contingency tables, and composites over two periods, 1901–2020 and 1959–2020. The results confirm that the positive phase of ENSO (El Niño) tends to reduce the probabilities of a drier than normal spring and that its negative phase (La Niña) increases them, but suggest that such signal is relatively weak. In particular, the drying effects associated to La Niña are generally small, as they explain less than 10% of the interannual variability in precipitation, and most of the driest springs occurred during neutral years. The results also shed light into the causes of the driest springs: internal atmospheric variability. On average, during the driest springs, sea surface temperature anomalies are statistically non existent over the eastern Pacific, but these events are characterized by an expansion of the southeastern (SE) side of North Pacific Subtropical High (NPSH). It is found that during spring, a strengthened SE side of the NPSH weakens the moisture flux from the Pacific Ocean into northwestern Mexico. These results suggest that the seasonal predictability of spring for northwestern Mexico might not only depend on the strength and phase of ENSO but also on the processes controlling the variability of ridges over the extratropical eastern Pacific.

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.

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.

Remote dynamic and thermodynamic effects of typhoons on Meiyu–Baiu precipitation in Japan assessed with bogus typhoon experiments

To advance comprehensive knowledge of typhoon-induced remote precipitation and anomalous atmospheric circulations during the early summer rainy season (Meiyu-Baiu), we performed numerical simulations in which different bogus typhoons are imbedded in the July climatological mean field as ideal initial and lateral boundary conditions using a regional atmospheric model, and we compared experiments with and without the bogus typhoon. The dominance of an anomalous local high (ALH) remotely induced by a typhoon leads to a poleward shift of the primary Meiyu–Baiu frontal zone in the form of the enhancement of the North Pacific subtropical high (NPSH). Typhoon-induced negative potential vorticity advection plays a dominant role in forming the ALH around Japan. Despite the shift of the frontal zone, heavy precipitation in excess of 100 mm per day occurs along the Pacific coast of western Japan, which is triggered by two primary moisture inflows. One is the low-level moisture inflow along the moisture conveyor belt (MCB), which is an appreciable low-level moisture flux zone stretching from the Indian Ocean to a typhoon over the western North Pacific, due to the intensified horizontal pressure gradient between the typhoon and the NPSH. The MCB is more robust in the experiment in which the typhoon is close to the South China Sea where the monsoon westerlies prevail. The parcels along the MCB bring moisture into western Japan even though moisture losses due to condensation occur on the way, to some degree. The other is the moisture inflow through the atmospheric boundary layer along the southwestern periphery of the NPSH. The parcels along the NPSH periphery easily receive moisture from the underlying ocean as they approach western Japan. Both inflows are enhanced by the ALH formation over Japan. The combined effect of the typhoon and its induced ALH is responsible for the prominence of the two major moisture inflows into western Japan, leading to local heavy precipitation in that region.

Moisture sources and isotopic composition of a record-breaking heavy Meiyu-Baiu rainfall in southwestern Japan in early July 2020

Torrential Meiyu-Baiu rainfall events have become more frequent in East Asia this decade. The moisture source and isotopic composition of precipitation are crucial for understanding the underlying dynamical and thermodynamic processes of extreme heavy Meiyu-Baiu rainfall. To clarify the moisture source and isotopic composition, we simulated a record-breaking heavy Meiyu-Baiu rainfall over Kyushu in southwestern Japan in early July 2020 using an isotopic regional spectral model. In addition, we observed the isotopic composition of the rainfall in Kyushu to validate the simulations. Moisture sources were classified into three groups: Asian monsoon (AM) moisture from the Indian Ocean, the South China Sea, and the East Asian Continent; North Pacific subtropical high (NPSH) moisture from the Pacific Ocean and the Philippine Sea; and local moisture from the East China Sea, the Kuroshio region, and the Sea of Japan. The remote AM and NPSH moisture was dominant. The NPSH moisture, transported by the NPSH at low levels, gradually increased in the early period. While AM moisture, transported by the monsoonal southwesterly flow at the mid-level, rapidly increased in association with a Baiu frontal depression and merged with the NPSH moisture in the middle period. Meanwhile, a quasi-stationary convective band (QSCB) formed and then triggered heavy rainfall. As for the isotopic composition, condensation exhibited lower δ2H and higher d-excess with more AM moisture in the beginning and on the north side of the QSCB. Regarding the effects of hydrometeorological processes on the isotopic composition, the lower δ2H and higher d-excess of AM moisture were principally attributable to more rainout and below-cloud evaporation during the transport process, rather than surface evaporation in moisture source regions. This study systematically clarified the major moisture sources and their corresponding isotopic dynamics with respect to the hydrometeorological processes of surface evaporation, moisture transport, rainout, and below-cloud evaporation for torrential Meiyu-Baiu rainfall. The findings may be useful in weather forecasting and disaster prevention for heavy Meiyu-Baiu rainfall in East Asia.

What Are the Dominant Synoptic Patterns Leading to the Summer Regional Hourly Extreme Precipitation Events Over Central‐Eastern Tibetan Plateau and Sichuan Basin?

AbstractRevealing the synoptic patterns related to hourly extreme precipitation (EP) is very important to deepen our recognition and understanding of EP formation. The predominant synoptic patterns associated with the summer regional hourly EP events (RHEPE) over the central‐eastern Tibetan Plateau (CETP) and Sichuan Basin (SCB) have been systematically identified. Results show the summer RHEPE over CETP and SCB are dominated by the background large‐scale circulations featured by the configuration of eastward‐extended South Asia high (SAH) and westward‐extended Western North Pacific Subtropical High (WNPSH) and their northward advance, except for that an obvious low‐level vortex imbedded in the background large‐scale circulations is mainly responsible for the summer RHEPE over SCB. The frequency and intensity of the total summer RHEPE over CETP and SCB all show an obvious increasing trend during 2000–2020, which is largely contributed by the synoptic pattern characterized by the configuration of eastward extended SAH and westward extended WNPSH.

Mechanism for the Abnormal Extension of North Pacific Subtropical High toward Japan in Late June 2022

Mechanism for the Abnormal Extension of North Pacific Subtropical High toward Japan in Late June 2022

Interdecadal Change in the Interannual Variation of the Western Edge of the Western North Pacific Subtropical High During Early Summer and the Influenceof Tropical Sea Surface Temperature

This study reveals that the interannual variability of the western edge of the western North Pacific (WNP) subtropical high (WNPSH) in early summer experienced an interdecadal decrease around 1990. Correspondingly, the zonal movement of the WNPSH and the zonal extension of the high-pressure anomaly over the WNP (WNPHA) in abnormal years possess smaller ranges after 1990. The different influences of the tropical SSTAs are important for this interdecadal change, which exhibit slow El Niño decaying pattern before 1990 while rapid transformation from El Niño to La Niña after 1990. The early summer tropical SSTAs and the relevant atmospheric circulation anomalies present obvious interdecadal differences. Before 1990, the warm SSTAs over the northern Indian Ocean and southern South China Sea favor the WNPHA through eastward-propagating Kelvin wave and meridional-vertical circulation, respectively. Meanwhile, the warm SSTA over the tropical central Pacific induces anomalous ascent to its northwest through the Gill response, which could strengthen the anomalous descent over the WNP through meridional-vertical circulation and further favor the eastward extension of the WNPHA to central Pacific. After 1990, the warm SSTAs over the Maritime Continent and northern Indian Ocean cause the WNPHA through meridional-vertical and zonal-vertical circulation, respectively. Overall, the anomalous warm SSTs and ascent and the resultant anomalous descent over the WNP are located more westward and southward after 1990 than before 1990. Consequently, the WNPHA features narrower zonal range and less eastward extension after 1990, corresponding to the interdecadal decease in the interannual variability of the western edge of the WNPSH. On the other hand, the dominant oscillation period of ENSO experienced an interdecadal reduction around 1990, contributing to the change of the El Niño SSTA associated with the anomalous WNPSH from slow decaying type to rapid transformation type.

Long‐Lead Predictability of Western North Pacific Subtropical High

AbstractWe present a new assembly of three steppingstones in thorough inquiry of the complex chain reaction from the diverse equatorial sea surface temperature (SST) pattern two‐season ahead to the East Asia summer weather condition, by regarding the western North Pacific subtropical high (WNPSH) as an anchor. We identified three predictors that are discordant with conventional climatic indices and achieve a good skill in the summer WNPSH prediction (temporal correlation coefficient of 0.78) with a two‐season long lead in the leave‐1‐year‐out cross‐validation during 1981–2020, despite the diversity of El Niño‐Southern Oscillation (ENSO) and Indian ocean dipole (IOD) patterns. Specifically, the first predictor captures the initiation and development of both IOD and El Niño via the SST warming over central equatorial Pacific in autumn; the second one serves as a better delegate of IOD by capturing the easterly acceleration over tropical Indian ocean; the third one illustrates the unprecedented informative role of upper‐level high pressure over the entire tropics in the WNPSH prediction with 5months lead time. They mediate the discordance between conventional climate indices and the diverse WNPSH response and caution people to revisit and rethink the complex atmosphere‐ocean coupled response from the diverse tropical SST patterns two seasons ahead to EA summer weather condition.

Medium-Range Predictability of Boreal Summer Western North Pacific Subtropical High and Its ENSO Modulation

In boreal summer, variations of intensity and location of the western North Pacific subtropical high (WNPSH) have significant impacts on weather and climate in East Asia. In this study, the medium-range prediction of WNPSH is comprehensively evaluated with various scores based on reforecast data of the National Centers for Environmental Prediction - Global Ensemble Forecast System, and the predictability source of the WNPSH medium-range forecasting is further analyzed by examining how well the model can reproduce the modulation of El Niño-Southern Oscillation (ENSO) on WNPSH as observed. The results show that this system has a systematic bias in the WNPSH forecasts, mainly manifested by the weak strength and the southeastward shifted position, and such a bias further increases with lead time. Effective prediction skills of WNPSH are 10–11 days for its intensity and area, 7 days for its ridge line, but only 1–3 days for its western boundary ridge point in terms of different scores, respectively, which can be improved through developing a bias correction method of prediction. It is demonstrated that the medium-range predictability of WNPSH is mainly originated from ENSO and its significant lagged effects on WNPSH well revealed in observation can be realistically reproduced by this system within the effective prediction lengths. A strong ENSO modulation of the WNPSH prediction skills has been clearly found in terms of the different indices, which depends on the ENSO’s developing and decaying phases. The intensity and area of WNPSH are usually highly predictable due to the ENSO effect being reproduced by the system well while the location indices of WNPSH have relatively low predictability, which are mainly affected by internal variability and difficultly captured by the model. Predictability analysis of WNPSH as modulated by ENSO shows good potential for medium-range forecasting with high skills.

Detecting Interdecadal Change in Western North Pacific Tropical Cyclone Genesis Based on Cluster Analysis Using pHash + Kmeans

Previous studies have noted an abrupt decrease in western North Pacific (WNP) tropical cyclone (TC) genesis frequency and a westward shift in genesis location since the late 1990s. The recent application of cluster analysis in TC research shows the effect of detecting the contribution of the Western North Pacific Subtropical High (WNPSH) and the interdecadal Pacific oscillation (IPO) on interdecadal change in WNP TCs. In this work, we also apply a clustering algorithm called pHash + Kmeans to group WNP TCs into three classes based on their genesis environmental conditions. The clustering results show that an abrupt decrease after 1998 is related primarily to a decrease in the dominant class (Class3, located mainly in the southern and eastern WNP), and an increase after 2010 occurs because of a new dominant class (Class1, located mainly in the northwestern WNP), which indicates that the WNP environment suppresses Class3 genesis after 1998 and enhances Class1 genesis after 2010. Three periods (P1: 1979–1997, P2: 1998–2010, and P3: 2011–2020) and three regions (SCS: 100°E-120°E, EQ-30°N; WNP1: 120°E-140°E, EQ-30°N; and WNP2: 140°E-160°W, EQ-30°N) are divided to further confirm the above findings. In P1, high (low) mid-level relative humidity (RH), intense (weak) low-level vorticity, and weak (strong) vertical wind shear (VWS) are distributed in WNP2 (SCS and WNP1), indicating suitable environmental conditions for TC genesis in WNP2 but unsuitable conditions in SCS and WNP1. This situation is the opposite in P2, leading to a decrease in genesis frequency and a westward shift in genesis location. In P3, strong low-pressure vorticity and thermodynamic conditions occur in SCS and WNP1, contributing to an increase in TC genesis frequency.

Linkage between anomalies of pre-summer thawing of frozen soil over the Tibetan Plateau and summer precipitation in East Asia

The Tibetan Plateau (TP) is sensitive to climate change in the land–atmosphere coupling mechanism due to its complex topographic features and unique geographic location. This study explored the teleconnection between pre-summer thawing of frozen soil over the TP and summer precipitation in East Asia in the Meiyu–Baiu rainy season (June, July) from 1981 to 2019 using maximum covariance analysis (MCA). The precipitation fields forced by thawing of frozen soil were calculated by the coupled manifold technique. The variations in East Asian precipitation are significantly impacted by thawing of frozen soil over the TP, with a variance explained ratio in the surrounding middle and lower reaches of Yangtze River (MLYR) of around 10%–20%. The MCA analysis also revealed that the thickness of pre-summer frozen soil thawing had a positive relationship with summer precipitation in the MLYR and southern Japan (fraction = 0.59, correlation ≈ 0.99). To find out the possible mechanism, composite analyses were conducted on atmospheric and surface components with reanalysis products. The analysis results suggested that more (less) frozen soil thawing would increase (decrease) the sensible heat and land surface temperature with enhanced (weakened) surface diabatic heating over the TP. Then, the positive (negative) surface diabatic heating would result in an enhanced (weakened) South Asia High extending eastward followed by stronger (weaker) upper troposphere (200 hPa) westerlies, as well as the West North Pacific Subtropical High extending westward. As a result, the northeastward movement of the integrated vapor transport intensified (weakened) with the westward extension (eastward retreat) of the Meiyu–Baiu rain belt, leading to more (less) precipitation in the MLYR and southern Japan. It is thus suggested that pre-summer thawing of frozen soil over the TP could play a vital role in regulating East Asian summer precipitation and movement of the Meiyu–Baiu rain belt.

Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021

In July–August 2021, intense marine heatwaves (MHWs) occurred at the sea surface over extensive areas of the northwestern Pacific Ocean, including the entire Sea of Japan and part of the Sea of Okhotsk. In extent and intensity, these MHWs were the largest since 1982, when satellite measurements of global sea surface temperatures started. The MHWs in summer 2021 were observed at the sea surface and occurred concomitantly with a stable shallow oceanic surface boundary layer. The distribution of the MHWs was strongly related to heat fluxes at the sea surface, indicating that the MHWs were generated mainly by atmospheric forcing. The MHWs started to develop after around 10 July, concurrent with an extreme northward shift of the atmospheric westerly jet. The MHWs developed rapidly under an atmospheric high-pressure system near the sea surface, associated with a northwestward expansion of the North Pacific Subtropical High. The MHWs exhibited peaks around 30 July to 1 August. Subsequently, following the southward displacement of the westerly jet, the MHWs weakened and then shrank abruptly, synchronously with rapid deepening of the oceanic surface boundary layer. By 18 August, the MHWs had disappeared.