The Anomalous Mei-yu Rainfall of Summer 2020 from a Circulation Clustering Perspective:Current and Possible Future Prevalence

Highly unusual amounts of rainfall were seen in the 2020 summer in many parts of China,Japan,and South Korea.At the intercontinental scale,case studies have attributed this exceptional event to a displacement of the climatological western North Pacific subtropical anticyclone,potentially associated Indian Ocean sea surface temperature patterns and a mid-latitude wave train emanating from the North Atlantic.Using clusters of spatial patterns of sea level pressure,we show that an unprecedented 80%of the 2020 summer days in East Asia were dominated by clusters of surface pressure greater than normal over the South China Sea.By examining the rainfall and water vapor fluxes in other years when these clusters were also prevalent,we find that the frequency of these types of clusters was likely to have been largely responsible for the unusual rainfall of 2020.From two ensembles of future climate projections,we show that summers like 2020 in East Asia may become more frequent and considerably wetter in a warmer world with an enhanced moisture supply.

The Combined Effects of the Tropical and Extratropical Quasi-biweekly Oscillations on the Record-setting Mei-yu Rainfall in the Summer of 2020

During June-July 2020,the strongest recorded mei-yu rainfall occurred in the middle and lower reaches of the Yangtze River.The rainfall processes exhibited an obvious quasi-biweekly(biweekly in brief)variability,and there are altogether five cycles.It is found that the biweekly rainfall cycle mainly arises from the collaborative effects of biweekly variabilities from both the tropics and extratropics.As for the tropics,the biweekly meridional march and retreat of the western Pacific subtropical high(WPSH)is particularly evident.As for the extratropics,geopotential height anomalies near Lake Baikal are active.The former is attributed to the intensified biweekly activity of the southwest-northeast oriented EastAsian Pacific wave train(EAP)originating from the tropical western Pacific,while the latter is associated with the biweekly activities of the eastward propagating Eurasia mid-high latitudinal wave train and the westward propagating North Pacific wave train.Why the biweekly activities of these wave trains intensified is further diagnosed from the perspective of thermodynamical forcing and also from the modulation of interannual background on intraseasonal variability.It is found that the strongest recorded convection anchoring over the tropical western Indian Ocean(IO)triggers anomalous descent over the tropical western Pacific,which modulates the biweekly activity of the EAP.Meanwhile,the anomalous diabatic heating over the IO causes changes of the meridional thermodynamic contrast across the IO to the high latitudes,which modulates the extratropical wave trains.A further diagnosis of barotropic kinetic energy conversion suggests that the active occurrence of two extratropical biweekly wave trains is attributed to the increased efficiency of energy conversion from basic flow.The westward propagation of the extratropical North Pacific wave train is attributed to the weakened and northshifted upper-level westerly,which is caused by the SST warmth near the Kuroshio extension.

2020年夏季重庆降水异常偏多成因分析

2020年夏季,重庆降水量为1961年以来同期第三多,6—7月为同期最多,极为异常。利用1961—2020年夏季重庆34个气象台站逐日降水和NCEP/NCAR逐日高度场、风场、相对湿度场等再分析资料及NOAA逐月海温场资料,采用相关、合成等现代统计诊断方法,分析了2020年重庆地区夏季降水出现异常偏多的主要原因。结果表明:2020年夏季,尤其是夏季6—7月,欧亚地区大气环流高低纬度呈“+-+”环流型分布,环流的经向度明显,出现重庆夏季典型多雨的第一型环流配置,从而造成夏季降水的异常偏多。外强迫信号分析表明:2019年秋冬季至2020春季ENSO暖事件使重庆夏季降水偏多的确定性概率增加,同时2019年秋季赤道印度洋偶极子正异常助推了2020年重庆夏季降水的异常偏多,两事件同时发生时其作用相互叠加,造成了重庆夏季降水的极端异常偏多。影响重庆夏季降水异常的主要外强迫信号包括ENSO和印度洋偶极子,提前监测和关注它们对重庆夏季降水预测意义重大。

The Extraordinary Rainfall over the Eastern Periphery of the Tibetan Plateau in August 2020

A large amount of accumulated precipitation was recorded over the Eastern Periphery of the Tibetan Plateau(EPTP)in August 2020.Using hourly rain gauge records and the ERA5 reanalysis dataset,we analyzed the unique characteristics of rainfall in August and the accompanying circulation conditions and conducted a comparison with previous data.This record-breaking amount of accumulated rainfall was centered on the northern slope of the EPTP.This location was in contrast with the historical records of the concentration of rainfall over the middle and southern slopes.The hourly rainfall in August 2020 was both more frequent and more intense than the climatological mean rainfall.An amplification effect of the topography was observed,with the precipitation over the EPTP showing a more significant change with terrain height in August 2020.A circulation analysis showed that cold(warm)anomalies existed over the north(south)of approximately 35°N compared with those in the years when the southern EPTP received more rain.The western Pacific subtropical high was more intense and extended to the west,and the low-level cold air from the north was more active.The enhanced low-level southerly winds on the periphery of the subtropical high injected warm,moist air further north than the climatological mean.These winds became easterly near the northern EPTP and were forced to ascend by the steep terrain.

Two Types of Diurnal Variations in Heavy Rainfall during July over Korea

This study examined the characteristics of the diurnal variations of heavy rainfall(≥110 mm in 12 hours)in Korea and the related atmospheric circulation for July from 1980−2020.During the analysis period,two dominant pattens of diurnal variation of the heavy rainfall emerged:all-day heavy rainfall(AD)and morning only heavy rainfall(MO)types.For the AD-type,the heavy rainfall is caused by abundant moisture content in conjunction with active convection in the morning(0000−1200,LST;LST=UTC+9)and the afternoon hours(1200−2400 LST).These systems are related to the enhanced moisture inflow and upward motion induced by the strengthening of the western North Pacific subtropical high and upper-tropospheric jet.For the MO-type,heavy rainfall occurs mostly in the morning hours;the associated atmospheric patterns are similar to the climatology.We find that the atmospheric pattern related to severe heavy rainfalls in 2020 corresponds to a typical AD-type and resembles the 1991 heavy-rainfall system in its overall synoptic/mesoscale circulations.The present results imply that extremely heavy rainfall episodes in Korea during the 2020 summer may occur again in the future associated with the recurring atmospheric phenomenon related to the heavy rainfall.