Decadal Summer Drought Characteristics Over Yangtze-Huaihe River Basin Associated With Eurasian Land Thermal Anomalies

As the main planetary system affecting the precipitation over Yangtze-Huaihe river basin in summer changed in mid-July,this paper investigates the temporal and spatial characteristics of arid in Yangtze-Huaihe river basin and the characteristics of atmospheric abnormal circulation in interdecadal extreme drought years and their possible causes from June 1st to July 15th(Meiyu period)and July 16th to August 31st(drought period).This research based on daily precipitation data from 501 stations in northern China,the reanalyze data from European Centre for Medium-Range Weather Forecasts(ECMWF)and the sea temperature data from national oceanic and atmospheric administration(NOAA);Then this paper analyzes the time characteristics of the dynamic variability and kinetic energy budget of the European key area that lead to extreme drought in the Yangtze-Huaihe river basin during the Meiyu period by using the multiscale energy and vorticity analysis(MS-EVA).The results are as follows:(1)On the spatial scale,the number of rainless day in the northwest of the Yangtze-Huaihe river basin in the Meiyu period is more than that in the southeast,while there is no significant difference in the number of rainless day in the entire Yangtze-Huaihe river basin during the drought period.On the time scale,the drought in the Jianghuai basin was more severe than in other periods at the end of the 20th century,and there are dry trends during two periods.The drought in the Yangtze-Huaihe river basin is related to the Eurasian teleconnection in the middle and high latitudes.During the Meiyu period,there is a northeast-southwest anticyclone anomaly in Mongolia with a strong intensity range.Therefore,the Yangtze-Huaihe river basin is affected by the northerly wind and the subsidence movement is maintained,providing a circulation background that is easy to form a drought in this region.During the drought period,there is a positive anomaly at 500-hPa over the eastern Mongolia,which weakens the East Asian trough and causes the west Pacific subtropical high to move northward.This area is controlled by descending airstream,resulting in aridity.In the Meiyu period the 200-hPa wave energy from Eastern Europe propagates eastward and converges in the north of China,which intensifies abnormal anticyclone.Such phenomenon is conducive to the maintenance and development of drought in the Yangtze-Huaihe river basin.The Pacific decadal oscillation(PDO)during the Meiyu period has a significant turning point in the 1990s.The negative PDO phase can directly affect the precipitation by forcing Eurasian teleconnection pattern(EU).Besides,negative PDO can also indirect result in drought through making key area energy divergence.(2)During drought period,the 200hPa origins of wave energy are the North Atlantic and the north of China.Wave energy from North Atlantic propagates eastward,with a small amount transported to East Asia.In the northern China the energy diverges after combining with energy wave forced by surface.The weakened trough in Mongolia leads to the formation of anti-cyclonic abnormal center,which is conducive to the maintenance of drought.However,PDO has non-direct effect on the drought in the Yangtze River during the drought period,but it can lead to drought indirectly by transporting the energy from the origin.(3)Since the summer rainfall belt in the Yangtze-Huaihe river basin is mainly concentrated in the Meiyu period,the next study focused on drought events during the Meiyu period.It is found that in the Meiyu period when the kinetic energy of high-level in European key area had a tendency to increase,extreme drought events easily occurred in the Yangtze-Huaihe river basin.The kinetic energy here mainly comes from the energy conversion from synoptic-scale eddy,followed by the pressure work and vertical transport of kinetic energy;Buoyancy cornversion and energy conversion from the mean flow are outputs of high-level kinetic energy.Some of the increasing upper-strata kinetic energy comes from eastward kinetic energy of the east coast of the North Atlantic and the southwestern continent of Europe,and these energy is transported upwards after convergence in Europe to transfer energy to the upper strata which can transfer energy to higher levels.At the same time,due to the increase of ground heating in the key area,the vertical wind shear increases,the baroclinic stability decreases and the pressure work increases,which makes the upper kinetic energy supplemented.During this period,the energy conversion from synoptic-scale eddy also increased due to ground heating.The net energy added in the key area converges in the Yangtze-Huaihe river basin by the westerly circulation,which helps to enhance the ridge above the ground and promotes the occurrence of extreme drought events.The results explores the disturbance energy of Rossby wave associated with drought from the perspective of energy conversion,which present the interdecadal characteristics and factors of drought in the Yangtze-Huaihe river basin to some extent,and provide the basis for the study of multi-scale drought mechanism and arid climate prediction.