Research Of The Mechanisms For The Spring-Winter Precipitation Change And The Characteristics Of Precipitation Recycling Over The Three-River Headwaters Region

Global warming changed the global hydrological cycle significantly.The Three-River Headwaters region（TRH）,located in the hinterland of the Tibetan Plateau（TP）,is highly vulnerable to climate change.It is the headwater region of three major rivers in East Asia（i.e.,the Yangtze River,the Yellow River,and the Lancang River/Mekong River）,which is called the“Asian Water Tower”.Thus,the research of precipitation changes and associated mechanisms is a hot topic in climatology and hydrology and critical for ensuring regional ecological security and economic development in downstream regions.Precipitation over the TRH region exerts important impacts on the distribution of glaciers,snow cover,and spring runoff in this region.However,the main focus of the research was on the physical mechanisms of the summer precipitation over the TRH region and the TP,few studies shed light on the responsible mechanisms of the changed precipitation for spring and winter.To investigate the variation of the spring-winter precipitation and associated mechanisms over the TRH region,this study analyzed the seasonal precipitation change and the associated mechanisms for the spring-winter precipitation from 1961 to 2014using multiple precipitation datasets and NCEP/NCAR reanalysis.Moreover,it aims to analyze the water budget for the spring precipitation change and understand the added value of the Weather Research Forecast Model（WRF）compared to the ERA5reanalysis in the atmospheric water budget description over the high-altitude.In the end,the characteristics of the spring precipitation recycling based on the ET-Tagging extended WRF model are further investigated.The main results are as follows:（1）The spring-winter precipitation over the TRH region increases significantly in the recent five decades and shows a“dry-to-wet”shift at the end of the 1980s.The spring-winter precipitation over the TRH region is affected mainly by the moisture transport from the Bay of Bengal,the tropical ocean,the South China Sea,and the westerly moisture transport in the middle troposphere.Based on the NCEP/NCAR reanalysis and the global sea surface temperature（SST）dataset,the results show that the increase of the spring-winter precipitation is possibly related to the enhancement of the Walker circulation and the local Hadley circulation because of the La Ni?a-like SST anomaly pattern（abnormal warming in the tropical Indo-western Pacific and anomalous cooling in the equatorial central-eastern Pacific）.The enhanced Walker circulation influences the mid-latitude atmosphere circulation via teleconnections and results in increased spring-winter precipitation over the TRH region.Corresponding to the anomalous vertical motions over the tropical oceans during the wet period,negative anomalies of 500 h Pa geopotential height in the TRH region and the strengthened East Asian subtropical westerly jet（EASJ）are conducive to the convergence in the low level and divergence in the high level,and therefore leads to the increase of the precipitation in the TRH region.During the dry period,a zonal teleconnection pattern of 500 h Pa geopotential height from upstream to downstream around 35?N-60?N depicts positive anomalies around the Ural Mountains and the Okhotsk Sea,and negative anomalies in Mongolia,which should suppress the easterly anomalies in the TP and conducive to the water export from the eastern boundary of the TRH region.Meanwhile,a negative phase of the North Pacific Oscillation（NPO）pattern is found in the North Pacific and the Aleutian Islands,which can suppress the westward expansion of the anticyclone in the western North Pacific and weaken the moisture transport from the tropical oceans and the South China Sea leading to the decrease of the spring-winter precipitation over the TRH region.（2）The high-resolution regional climate modeling（WRF）can represent the influence of the orographic barrier in the TP better and shows the advantage for the analysis of the regional water budget in complex terrain.The results of the ERA5reanalysis and the WRF model are consistent with the description of the trend of the spring precipitation and the water budget over the TRH region.For example,both of them indicate that the atmospheric moisture convergence dominates the change of the spring precipitation over the TRH region.Especially the moisture convergence enhanced significantly in May during the wet period because of the onset of the South Asia summer monsoon,which leads to the largest increase of precipitation in May.They both indicate that the water input for the TRH region in spring comes from the western and southern boundaries and is exported from the eastern boundary.The increase in spring precipitation is mainly due to the increase of southerly moisture transport.Both the results of the decomposition for the moisture convergence based on the ERA5 and WRF indicate that the dynamic convergence（wind anomalies）dominates the change of the mean moisture convergence,while the increased specific humidity provides more moisture for precipitation in the study region.But the ERA5 significantly overestimates the spring precipitation by about 0.6 mm day^-1due to the insufficient description of the terrain.The high-resolution regional climate modeling only overestimates about 0.14mm day^-1compared to observations due to the reduction of the simulated southerly moisture transport which results in weakened moisture convergence.（3）The main reason for the increased spring precipitation over the TRH region is the enhanced external moisture input,while the local evapotranspiration plays a minor role.The mean contribution of the evapotranspiration to precipitation（precipitation recycling ratio）in spring is 12%and shows a decreasing trend.In spatial,the precipitation recycling ratio is the largest in the northern part.The peak time of the tagged precipitation（BJT 17:00）is earlier than the total precipitation（BJT 20:00）.The tagged precipitation is mainly distributed in the northeast of the TRH region because of the prevailing westerlies and the topography of the TP.Most of the tagged water is exported from the study area,therefore,the contribution of the local evapotranspiration for precipitation change is small.