East Asian Jet Stream Research Articles

Compound spatial extremes of heatwaves and downstream air pollution events in East Asia

In light of increasing climate hazards globally that pose risk to public health, the compounded effects of two major hazards, heatwaves and air pollution, have become a focal point for environmental and health research. This study explores the intricate relationship between extreme temperature events in North China (NC) and South China (SC) – two prominent areas of aerosol exposure in East Asia – and the associated changes in aerosol optical depth (AOD) across the region. The heatwave events and regional AOD distribution revealed distinct patterns in their respective regions from June to September. Both NC and SC showed reduced AOD during heatwave events, while downstream regions experienced increased AOD levels. From the perspective of heatwaves in NC and SC, we present a more holistic picture of how large-scale modulators contribute to inducing air pollution hazards across East Asia. The analysis revealed a link between blocking high-pressure system and heatwave occurrences in NC, while a dominant Rossby wave train, influenced by the South China Sea, was identified as a major modulator in SC. Additionally, other large-scale circulatory systems, such as the Western Pacific Subtropical High, the East Asian jet stream, and the South Asian High, also play crucial roles in shaping these events. This suggests the potential for predicting downstream AOD events. The study underscores the importance of understanding the interconnectedness of meteorological and air quality phenomena to mitigate the adverse environmental impacts in East Asia.

Potential impacts of reduced winter Kara Sea ice on the dipole pattern of cold surge frequency over the tropical western Pacific

The impact of Arctic Sea ice melting on weather and climate extremes in the Northern Hemisphere has garnered widespread attention. Existing research has convincingly demonstrated the importance of this impact in mid-high latitudes, while its influence in areas beyond remains controversial. This study reveals the indirect influence of Kara Sea ice reduction on cold surge (CS) over the tropical western Pacific (TWP), with the East Asian jet stream serving as the connecting link. The leading mode of CSs over the TWP exhibits a zonal dipole characteristic, which is associated with cyclonic anomaly over the Philippine Sea. The enhanced cyclonic anomaly is caused by strengthened and northward-moved subtropical East Asian jet stream and weakened polar jet stream, which can lead to more CSs over the South China Sea and fewer CSs over the Philippine Sea. Such variations in the jet stream are contributed by the facilitated atmospheric blockings west of the Ural Mountains, which suppressed the circumpolar westerly winds and increased meridional temperature gradient in Northeast Asia. The connection between atmospheric blockings and Kara Sea ice can be confirmed through local vertical energy exchange. Simulations of the atmospheric response to the forcing of decreased Kara Sea ice support the proposed connection. Although there is no statistically significant correlation between tropical CSs and Kara Sea ice, this study highlights the potential impacts of Arctic climate change signal on weather and climate extremes over tropical regions.

Impacts of internal variability on winter temperature fluctuations over the Tibetan Plateau

The climate comprises an externally forced component and internal variability within the climate system. This variability arises from interactions among climate subsystems, significantly impacting climate change, particularly over the Tibetan Plateau (TP). Therefore, understanding the mechanisms governing TP temperature responses to internal variability is crucial. In this study, winter temperature components over the TP in response to internal variability and external forcing are distinguished using reconstructed monthly temperature data and large ensemble simulations from 11 CMIP6 coupled models spanning 1901 to 2014. Analysis reveals two cold periods (1901–1931, 1956–1999) and two warm periods (1932–1955, 2000–2014) within the internal-variability component from 1901 to 2014. These periods are significantly associated with the Atlantic Multi-decadal Oscillation (AMO) and Interdecadal Pacific Oscillation (IPO), explaining 38.9% of the winter temperature's internal-variability component over the TP. Manipulating AMO+ (AMO-) without IPO influence or IPO- (IPO+) without AMO impact intensifies the winter warm (cold) anomaly across the entire TP. This suggests that AMO and IPO contribute to the temperature's internal-variability component over the TP. Specifically, in winter, both AMO+ (AMO-) and IPO- (IPO+) induce a “− + −+” (+ − +−) Rossby wave train over Eurasia, influencing the East Asian trough, East Asian jet stream, Ural Blocking high, and the Siberian high, resulting in a warm (cold) anomaly over the TP. Moreover, our findings indicate that AMO and IPO collaboratively determine the cold or warm anomaly and its intensity over the TP in winter.

Regulation of Southwestern United States Precipitation by non-ENSO Teleconnections and the Impact of the Background Flow

Abstract In this study, we analyze drivers of non–El Niño–Southern Oscillation (ENSO) precipitation variability in the Southwest United States (SWUS) and the influence of the atmospheric basic state, using atmosphere-only and ocean–atmosphere coupled simulations from the Community Earth System Model version 2 (CESM2) large ensemble. A cluster analysis identifies three main wave trains associated with non-ENSO SWUS precipitation in the experiments: a meridional ENSO-type wave train, an arching Pacific–North American-type (PNA) wave train, and a circumglobal zonal wave train. The zonal wave train cluster frequency differs between models and ENSO phase, with decreased frequency during El Niño and the coupled runs, and increased frequency during La Niña and the atmosphere-only runs. This is consistent with an El Niño–like bias of the atmospheric circulation in the coupled model, with strengthened subtropical westerlies in the central and eastern North Pacific that cause a retraction of the waveguide in the midlatitude eastern North Pacific. As such, zonal wave trains from the East Asian jet stream (EAJS) are more likely to be diverted southward in the east Pacific in the coupled large ensemble, with a consequently smaller role in driving SWUS precipitation variability. This study illustrates the need to reduce model biases in the background flow, particularly relating to the jet stream, in order to accurately capture the role of large-scale teleconnections in driving SWUS precipitation variability and improve future forecasting capabilities.

Temperature change in the Tianshan Mountains and its external drivers

The Tianshan Mountains area (TMA), located in east Central Asia, has experienced faster temperature rise than the global average rate from the 1960s to the present. This initial estimate is carefully examined using a new dataset of high spatial resolution dynamically downscaled by an updated Weather Research and Forecasting (WRF) regional model. This dataset and other available climate data products show that the WRF model outputs describe temperature changes in TMA consistent with limited available observations (R2 = 0.95 and RMSE = 3.25 °C). The model data confirm that the temperature at TMA has risen at a rate of 0.18 °C/decade, faster than the global mean rate of 0.15 °C/decade. Strong warming rates are also found in spring and summer while the winter is cooling. In addition to revealing these variations of TAM temperatures the model outputs provide high resolution meteorological and climatic data and open the opportunity for studying surface hydrology and ecosystem responses in the region. Possible forcings of the changes of TMA temperature are explored from a global perspective using a set of ten climate indices. Results show that the most impactful processes on TMA temperatures are those associated with the contraction of the arctic sea-ice in recent decades, SST anomalies in the tropical Pacific, e.g., ENSO, SST variations in both the tropical Atlantic and North Atlantic Ocean. The east Asian jet stream also shows a significant impact on TMA temperatures likely by its impact on placing its entrance region over the Central Asia and thus weather disturbances in TMA.

CMIP6 projected response of the East Asian winter climate to the sea ice‐free Arctic

AbstractThis article evaluated the performance of 19 Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating the Arctic sea ice from the perspective of annual cycle, spatial pattern and temporal variation. Based on the evaluation, five models with better comprehensive performance capacity were optimized as the ensemble to project the response of the East Asian winter climate to the sea ice‐free Arctic occurring under the SSP2‐4.5 and SSP5‐8.5 scenarios, respectively. The ensemble projections indicate that the sea ice‐free Arctic is followed by a weakening of the East Asian winter monsoon, which is characterized with the shallower East Asian trough and weaker East Asian jet stream. Concurrently, the winter surface net radiation flux is projected to increase in the East Asian‐western North Pacific region. These changes favour large‐scale warming in the East Asian‐western North Pacific region. Moreover, the warming is more pronounced under the sea ice‐free Arctic of SSP2‐4.5 than under that of SSP5‐8.5. The winter precipitation tends to increase along the East Asian coast from South China to the Sea of Okhotsk. Such an increase is closely associated with the enhancement of low‐level moisture. Due to larger enhancement of moisture, there appears greater increase of precipitation in the monsoon region from South China to Japan under the sea ice‐free Arctic of SSP5‐8.5 compare to that under SSP2‐4.5. Substantial changes are also projected for the temperature and precipitation extremes, such as a general increase in warm days and warm nights and an overall intensification of extreme precipitation in the East Asian‐western North Pacific region.

Dynamical Analysis of the Winter Middle East Jet Stream and Comparison with the East Asian and North American Jet Streams

Abstract The wintertime Middle East jet stream (MEJS) is an important upstream signal for the East Asian winter monsoon and the subsequent Asian summer monsoon. Thus, the maintenance and interannual variations of the MEJS as well as its similarities and differences with the East Asian jet stream (EAJS) and the North American jet stream (NAJS) are studied dynamically using the geopotential tendency equation and empirical orthogonal function analysis. Analysis reveals that the MEJS is mainly maintained by tropical diabatic heating and the low-frequency transient eddy (TE) vorticity forcing. It is different from the EAJS, which is maintained by both tropical diabatic heating and high-frequency TE vorticity forcing, and the NAJS, which is mainly sustained by high-frequency TE vorticity forcing. Furthermore, while temperature advection plays a considerable role in the maintenance of EAJS and NAJS, it is less important for the MEJS. On interannual time scales, the meridional shift of the MEJS is related to low-frequency TE heating, while the variation of the jet’s intensity is associated with temperature advection. For both EAJS and NAJS, the interannual variations are mainly contributed by high-frequency TE vorticity forcing, although temperature advection also promotes their meridional shifts. These results suggest that whether or not the maintenance of the jet streams is related to tropical diabatic heating, their interannual variations are not directly induced by this forcing. Significance Statement The wintertime Middle East jet stream (MEJS) is a narrow and strong westerly wind belt over the Middle East whose variations in intensity and location can affect the Asian monsoon significantly. However, little effort has been devoted to investigating the MEJS. Thus, dynamical diagnosis and statistical analysis are applied in this study to understand the MEJS and its variability comprehensively. Analysis reveals that low-frequency transient eddies, which are the mobile atmospheric systems with a lifespan longer than 10 days, are important for both the maintenance and the interannual variability of the MEJS. This phenomenon is apparently different from the East Asian and North American jet streams, in which synoptic transient eddies (lifetime shorter than 10 days) play an essential role.

Recent Decadal Weakening of the Summertime Rainfall Interannual Variability Over Yellow-Huaihe River Valley Attributable to the Western Pacific Cooling

This study focuses on the interdecadal transition of summer rainfall interannual variability over the Yellow-Huaihe River valley (YHRV). It is found that the interannual variability of summer rainfall over the YHRV becomes significantly weakened after the late 2000s. In the decade before the late 2000s (hereafter P1), the variance is 1.28 mm d−1 and the mean value of interannual variability is 1.49 mm d−1. In the decade after the late 2000s (P2), the variance is 0.35 mm d−1 and the mean value of interannual variability is 0.62 mm d−1. The variance and mean value of summer rainfall interannual variability have considerably decreased by 72.7% and 58.4% since the late 2000s, respectively. The reasons for the aforementioned interdecadal transition are explored. The results show that the interdecadal shift of the East Asian jet stream axis and the western Pacific subtropical high (WPSH) are two major factors leading to the interdecadal transition of summer rainfall interannual variability over the YHRV. The northward shift of the jet stream axis, which resembles the Pacific-Japan (PJ) pattern, suppresses the development of summer rainfall over the YHRV. The interannual oscillation of WPSH in P1 is more significant than that in P2, which is consistent with the weakened interannual variability of summer rainfall in P2. Further analysis reveals that the modulation factors responsible for the above changes are El Niño in its decaying phase in the equatorial central eastern Pacific and the warming in the Maritime continent and the western Pacific (WP) in P1. While in P2, the primary modulation factor is La Niña in its developing phase in the equatorial central eastern Pacific. As for the mechanism of the growth of summer rainfall in P1 positive years, the WP warm pool and El Niño in its decaying phase strengthen the Walker circulation in the tropical Pacific and induce Hadley circulation, whose ascending branch over the YHRV provides a favorable condition for the development of rainfall. At the same time, the northward propagation of Rossby waves in response to the above SSTA pattern suppresses convective activities over the Northwestern Pacific (NWP), which indirectly intensifies the WPSH. As a result, the PJ pattern is triggered and provides a favorable large-scale circulation condition for the growth of rainfall.

Interannual relationship between displacement and intensity of East Asian jet stream and haze over eastern China in winter

A recent case study indicated that the weakening of the East Asian subtropical jet was an important cause of the severe haze in North China in the winter of 2015. However, the interannual relationship between two key features, the displacement and the intensity of the East Asian jet stream (EAJS) and the haze days over eastern China (HDEC), remains unclear. Observed data, ERA-Interim reanalysis, and Community Earth System Model Large Ensemble Numerical Simulation(CESM-LENS) were used to investigate the interannual relationship between the EAJS and HDEC during the winter season from 1980 to 2017 and its possible associated atmospheric mechanisms. The results show that the northward movement of the EAJS is conducive to more HDEC by weakening synoptic-scale transient eddy activities (STEA) and baroclinicity, forming an upper-level anticyclonic anomaly over eastern China (EC). The local meteorological conditions (e.g., stronger temperature inversion potential, higher relative humidity, descending motion) are favorable for the accumulation of HDEC, showing consistent variations in more haze in the entire region of EC. The southward movement of the EAJS has the opposite effect. The strong East Asian subtropical jet (weak polar-front jet) could result in the distribution of the meridional dipole with less haze in north EC and more haze in south EC. The mean flow loses energy to the STEA over north EC and increases the baroclinicity, which is favorable for dispersing HDEC. However, the configuration of upper-level cyclonic and low-level southwest wind anomalies that appeared in south EC weakened the STEA, which favored the accumulation of HDEC. The observed results were further verified by CESM-LENS.

Winter Warming in North America Induced by Urbanization in China

AbstractRapid urbanization in China in the past several decades has been shown to affect surface air temperature (SAT) locally, but its global impact is unclear. Using a global climate model, we have investigated the potential impact of urbanization in China on SAT at the global scale. The expansion of urban areas in China from 1985 to 2017 leads to winter surface air warming by as much as 0.8°C in North America (NA), largely caused by warm temperature advection due to global‐scale atmospheric circulation changes. The East Asian jet stream moves northward in eastern and northeastern China where the expansion of urban areas has increased SAT and sensible heat flux, forcing change of the atmospheric stationary waves and so the structure of the jet streams.

Joint effects of three oceans on the 2020 super mei‐yu

An unexpected super mei-yu struck in 2020 in the Yangtze–Huaihe River basin, southern Korea, and southern Japan (hereafter referred to as the mei-yu regions), causing many casualties and huge economic losses. The super mei-yu was characterized by a remarkably early onset (around 1 June), late withdrawal (around 1 August), and intense rainfall during the mei-yu season. The precipitation in the early onset and late withdrawal stages contributed more than half of the total mei-yu-period precipitation over the mei-yu regions in 2020. In this study, the authors explored the dominant remote forcing of the mei-yu early onset and late withdrawal to understand the mechanisms of this super mei-yu. The early onset can mainly be attributed to an early northward-shifted East Asian jet stream (EAJS). The late withdrawal mainly resulted from the stagnant EAJS and the western North Pacific subtropical high (WPSH) during 10 July to 1 August. Specifically, North Atlantic sea surface temperature anomalies (SSTAs) excited a Rossby wave, which was steered by atmospheric anomalies related to the western North Pacific SSTAs, causing the early northward-shifted EAJS and generating an early onset. The record-breaking warm SSTAs over the North Indian Ocean to South China Sea and the reduced sea-ice concentration (SIC) over the Laptev–East Siberian Sea played important roles in causing the stagnant WPSH and EAJS during July, which led to the late withdrawal. Meanwhile, the SIC anomalies may have caused the inhomogeneous rainfall distribution in the mei-yu regions. Furthermore, projection results suggest that the probability of a late mei-yu withdrawal similar to the 2020 case will increase in the future. Finally, potential predictors of an extreme mei-yu are discussed.摘要2020年发生在江淮流域, 朝鲜半岛和日本南部 (简称梅雨区) 的暴力梅造成了巨大的人员伤亡和经济损失. 此次暴力梅的主要特征为: 入梅早 (6月1日) , 出梅晚 (8月1日) 以及较强的梅雨期降水. 2020年异常早入梅和晚出梅时期的降水占梅雨期总降水的一半以上. 因此, 为了深入解析2020暴力梅的机制, 本文将分析2020异常早入梅和晚出梅的主导遥强迫机制. 研究表明, 东亚急流提前北跳是造成入梅偏早的主要原因, 而西北太平洋副热带高压 (西太副高) 的作用并不显著. 具体来说, 5月北大西洋海表温度 (SST) 异常激发出东传的罗斯贝波, 同时西北太平洋的暖SST异常改变该罗斯贝波的传播. 进而在两者共同的作用下造成东亚急流的提前北跳, 并导致入梅偏早. 2020年7月破纪录的北印度洋-南海SST暖异常和拉普特夫-东西伯利亚海的海冰密集度 (SIC) 的偏少阻止西太副高和东亚急流的北跳.推迟北跳的西太副高和东亚急流有利于梅雨降水增多, 最终导致出梅偏晚. 同时SIC异常可能是梅雨雨带偏北的原因. 此外, 预估结果表明, 类似于2020出梅偏晚的概率在未来将会增加. 最后, 本文也探讨了暴力梅的潜在预测因子.

Climatic factors contributing to interannual and interdecadal variations in the meridional displacement of the East Asian jet stream in boreal winter

We employ the fifth generation of the European Centre for Medium-Range Weather Forecasts Reanalysis to investigate the leading mode of the East Asian zonal wind and the contribution of associated climatic factors in boreal winter during 1979–2020. The dominant mode, accounting for 46% of the explained variance, indicates that the East Asian subtropical jet (EASJ) and East Asian polar frontal jet (EAPJ) are meridionally displaced in opposite directions. The corresponding first principal component (PC1) shows clear interannual and interdecadal variations, with a regime shift in the year 1997. The dominant mode is significantly related to the Niño3.4 sea surface temperature (SST), the Barents–Kara Seas (BKS) sea ice concentration (SIC), the Arctic Oscillation (AO) and the Tibetan Plateau (TP) surface air temperature (SAT) on interannual and interdecadal timescales. Multiple linear regression with these variables reproduces PC1 well. The interdecadal variation of PC1 is dominated by the BKS SIC and Niño3.4 SST. The Niño3.4 SST explains more than half of the variance of PC1 on an interannual time scale, which is more than all of the other factors combined. The El Niño phase and BKS SIC are associated with the distance between the EASJ axis (EASJA) and the EAPJ axis (EAPJA). The AO and TP SAT may influence the meridional migration of the EASJA and EAPJA, respectively. The distance between the EASJA and EAPJA also depends on the configuration of the southern Asian cyclone and Siberian anticyclone.

Multidecadal Variations in the East Asian Winter Monsoon and Their Relationship with the Atlantic Multidecadal Oscillation since 1850

AbstractThis study investigates the characteristics and physical mechanisms of the multidecadal variations in the East Asian winter (December–February) monsoon (EAWM) since 1850 based on multiple observational and reanalysis datasets. The results indicate that the EAWM undergoes multidecadal weakening during the periods of 1869–1919 and 1986–2004 but strengthening during the period of 1920–85. Similar evolutions can be observed in the time series of the area-averaged winter surface air temperature over East Asia. Associated with the EAWM multidecadal variations, a quasi-barotropic Rossby wave train originating from the subtropical North Atlantic propagating across the Eurasian continent to Northeast Asia also experiences phase shifting at the same time. In its positive phase, the low-level anticyclonic anomaly over the northern Eurasian continent causes a stronger Siberian high; the mid- and high-level cyclonic anomalies over Northeast Asia deepen the East Asian trough and strengthen the East Asian jet stream, respectively. Thus, the positive phase of the wave train is conducive to stronger EAWMs and vice versa. The diagnostic analysis of the Rossby wave source indicates that the upper-tropospheric divergence anomalies over the North Atlantic can favor the excitation of this wave train, and the feedback forcing of high-frequency eddies plays important roles in its maintenance. In addition, the phase shifting of the Atlantic multidecadal oscillation (AMO) can induce a similar Rossby wave train across the Eurasian continent, through which it further modulates the multidecadal variations in the EAWM. Warm phases of the AMO are favorable for a stronger EAWM and colder midlatitude Eurasian continent and vice versa.

Implications of the Pacific meridional mode for summer precipitation extremes over China

This study examines the association between the Pacific meridional mode (PMM) and summer precipitation extremes over China using observational and reanalysis datasets. Results suggest that positive PMM phases are accompanied by suppressed extreme precipitation over the mid-lower reaches of the Yangtze River basin (MLYRB) and strengthened rainfall extremes over North China (NC). The teleconnections of PMM to regional precipitation extremes are mainly through a low-level westward and an upper-level eastward propagation of signals emanating from PMM. In the lower atmosphere, a prominent anomalous cyclone is generated as a Gill-Matsuno-type response to increased diabatic heating associated with the warming component of PMM. This cyclone excites a meridional circulation over eastern China with an anomalous anticyclone over the MLYRB and cyclone over NC, showing a Pacific-Japan-like pattern. The upper-tropospheric circulation changes are characterized by an eastward Rossby wave train and a poleward shift of the East Asian jet stream. Both the lower- and upper-atmospheric configurations are conducive to descending motion and decreases in summertime precipitation extremes over the MLYRB and ascent and increases in precipitation extremes over NC during the positive PMM phase. Our findings reveal that PMM plays a role in shaping the spatial pattern of summertime precipitation extremes, and understanding their association can be beneficial to the forecasting of regional precipitation extremes over China.

The observed influence of the Quasi‐Biennial Oscillation in the lower equatorial stratosphere on the East Asian winter monsoon during early boreal winter

AbstractThis study examines the relationship between the Quasi‐Biennial Oscillation (QBO) and the variability of the East Asian winter monsoon (EAWM) in the Northern Hemisphere (NH) winter. Here, we consider the effects of QBO winds in the lower stratosphere at 70 hPa. It is found that during the period of 1958–2019, the EAWM in the early winter months (November–December) is weaker in the easterly phase of the QBO (EQBO) than in the westerly phase of the QBO (WQBO). During EQBO, the northerly monsoon flow is weakened, and anomalous warm temperatures occur over East Asia. Moreover, components of the EAWM circulation system, including the Siberian High, Aleutian Low, East Asian trough, and East Asian jet stream, are all weakened. Our further analysis suggests that the QBO may influence the EAWM directly via a subtropical pathway. The EQBO tends to weaken the East Asian jet stream and shift it poleward, and the changes in the East Asian jet stream lead to a weakened EAWM. The activities of planetary waves are also changed in association with the QBO. Examinations of individual zonal wavenumbers (WNs) of planetary waves in the SLP field reveal that WN 1 is strengthened while WNs 2 and 3 are weakened during EQBO. Specifically, the anomalous WN 2 leads to weakening of the Siberian High, Aleutian Low, and East Asian trough, and the anomalous WN 3 reduces the pressure gradient between the East Asia landmass and the western Pacific. The changes in WNs 2 and 3 associated with the QBO play a dominant role in anomalous EAWM.

Surface temperature‐related variations in the East Asian summer monsoon during three warming stages

AbstractThe global mean surface temperature has experienced different warming stages, but there is less work on discussion of the regional climate changes under the background of different global warming rates. This study investigated the surface temperature (ST)‐related trends in the East Asian summer monsoon (EASM) and underlying mechanisms during three warming stages, with a view to develop a new understanding of the responses of the EASM to global warming. Based on the statistical method of piecewise linear regression (PLR) to ST anomaly time series, the ST variation process from 1958 to 2014 has been divided into three stages with different linear trends: 1958–1975 (hiatus), 1976–1997 (acceleration) and 1998–2014 (slowdown). During the first stage, accompanied by two anomalous lower‐level cyclones, a weakening trend of the southerly winds over eastern China is observed. The enhanced cyclone over the Yellow Sea, with local ST cooling, is associated with the CGT‐like (circumglobal teleconnection) pattern, while the other one over the Philippines can be attributed to the local sea surface temperature (SST) warming. The ST‐related anomalous EASM circulation exhibits an enhanced lower‐level anticyclone over the northern Philippine Sea during the second stage. The warming around Lake Baikal and the phase change of the East Atlantic/Western Russia (EAWR) pattern can weaken upper‐level East Asian jet stream (EAJS) and then provide a favourable condition for the descent related to the lower‐level anticyclonic anomaly. As for the third stage, the anomalous ST and related EASM circulation both display a dipole pattern. Through influencing the meridional thermal contrast and distribution of convergence over East Asia (EA), the interaction between anomalous ST and EASM circulation may be the major mechanism that results in the increased (decreased) ST and enhanced barotropic anticyclone (cyclone) over southern (northern) EA.

Recent Intensified Influence of the Winter North Pacific Sea Surface Temperature on the Mei-Yu Withdrawal Date

AbstractThe mei-yu withdrawal date (MWD) is a crucial indicator of flood/drought conditions over East Asia. It is characterized by a strong interannual variability, but its underlying mechanism remains unknown. We investigated the possible effects of the winter sea surface temperature (SST) in the North Pacific Ocean on the MWD on interannual to interdecadal time scales. Both our observations and model results suggest that the winter SST anomalies associated with the MWD are mainly contributed to by a combination of the first two leading modes of the winter SST in the North Pacific, which have a horseshoe shape (the NPSST). The statistical results indicate that the intimate linkage between the NPSST and the MWD has intensified since the early 1990s. During the time period 1990–2016, the NPSST-related SST anomalies persisted from winter to the following seasons and affected the SST over the tropical Pacific in July. Subsequently, the SST anomalies throughout the North Pacific strengthened the southward migration of the East Asian jet stream (EAJS) and the southward and westward displacement of the western North Pacific subtropical high (WPSH), leading to an increase in mei-yu rainfall from 1 to 20 July. More convincingly, the anomalous EAJS and WPSH induced by the SST anomalies can be reproduced well by numerical simulations. By contrast, the influence of the NPSST on the EASJ and WPSH were not clear between 1961 and 1985. This study further illustrates that the enhanced interannual variability of the NPSST may be attributed to the more persistent SST anomalies during the time period 1990–2016.

Remote impacts from the tropical Indian Ocean on haze pollution in January over the Yangtze River Delta

Haze events in the Yangtze River Delta (YRD) region have recently been occurring more frequently and with dramatic damages inflicted on human and ecosystem health. In this study, observational analyses and numerical experiments are used to investigate the meteorological conditions associated with haze pollution, with the main emphasis on the impacts of the preceding sea surface temperature (SST) in the tropical Indian Ocean (TIO). The results show that the December SST in the TIO has a significant positive correlation with the number of haze days in January over the YRD, especially during 1999–2017. In December, the positive SST anomalies in the TIO heat the overlying air, and then in the following January provoke a Matsuno–Gill-like pattern and a series of Rossby wave–like trains in the upper troposphere, transmitting signals to the YRD and downstream through the Sea of Japan and Aleutian Islands. The cyclonic anomalies in the YRD seem to significantly weaken the East Asian jet stream by means of anomalous easterlies, and subsequently affect the climate in the region. Near the surface, the increased surface air temperature and southerly winds, along with the decreased surface wind speed, accompanied by influences from upstream areas, are conducive to the occurrence of haze. These observational results were also reproduced well in CESM-LE simulations.摘要近年来, 长三角霾事件频发, 对人类和生态系统健康造成了严重危害.结果表明:热带印度洋12月海温与长三角1月霾日数呈显著正相关, 特别是1999–2017年.12月, 海温正异常加热上层空气, 1月在对流层上层激发类似Matsuno-Gill模态和一系列Rossby波列, 将信号传输至长三角, 随后向下游日本海和阿留申群岛传播.长三角异常气旋中偏东风削弱东亚急流, 影响该地气候.在近地面, SAT增加, 偏南风盛行, 风速减小, 并伴有上游地区影响, 有利于霾污染发生.上述分析结果在CESM-LE数值实验中得到很好的再现.

Urban heat islands in Hong Kong: Bonding with atmospheric stability

AbstractA barrier to urban heat island (UHI) mitigation is the lack of quantitative attribution of the various contributions to UHI intensity. This study demonstrates the daily and seasonal dynamics of UHIs in Hong Kong, a subtropical high‐density city. The nocturnal UHIs of the city are grouped according to various dynamic stability conditions (neutral, weak stable, and strong stable) of the boundary layer. Results indicate that the stronger the atmospheric stability, the more intense the UHI. The atmospheric anomalies linked to these stability classifications are hence revealed. In summer, nights of neutral (strong stable) stratification are controlled by low (high) pressure with rising (sinking) motion, less (more) precipitation, and lower (higher) air temperature at the surface. In winter, the influence of the large‐scale circulation system of the East Asian winter monsoon is significant. In the upper layer, the East Asian jet stream retreats westward (is displaced northward) on nights with neutral (strong stable) atmospheric stratification. At the surface, southeast China is hot and humid on neutral nights, while on strong stable nights, the coastal regions of southeast China are dry, and East Asia is dominated by positive surface air temperature anomalies. Atmospheric anomalies are generally nonsignificant on nights with weak stable stratification in both summer and winter. These findings provide potential predictors for UHI intensity.

东亚地区急流与地表温度的关系

东亚地区急流与地表温度的关系