The Causes Of Abruptly Drought-flooding Turn In The Mid-lower Reaches Of Yangtze River During Spring To Summer In2011

Precipitation in the mid-lower reaches of Yangtze River Valley (YRV) region is one ofimportant part in the East Asian monsoon system, its variability is closely related to the localeconomic development and people’s life. During January to May in2011, YRV region haswitnessed a negative precipitation anomaly, severe drought occurred in most areas. However,the precipitation increased sharply in June, with four concentrated heavy rainfall in thisregion, which resulted in a sharp turn from drought to flooding. On the basis of722stationsobserved precipitation and the NCEP/NCAR reanalyzed data sets during1980-2011, thepossible causes of abruptly drought-flooding turn in the mid-lower reaches of YRV regionduring spring to early summer in2011is discussed in this study. The cloud top Black BodyTemperature (TBB) observed by Fengyun satellite (FY-2D) is also analyzed to reveal thecauses of the precipitation in June. To explain the influence of earlier Sea SurfaceTemperature (SST) and homochronous Tibetan Plateau (TP) heating on precipitation, virtueof EOF, linear regression analysis and synoptic analysis methods are used, in addition, NCARCommunity Atmosphere Model3.0(NCAR CAM3) numerical simulation sensitivity test isutilized to verify the impact of SST anomaly. The main results are summarized as follows:(1) Abruptly drought-flooding turn occurred in the mid-lower reaches of YRV regionduring spring to early summer in2011. This event is found to be resulted by the joint impactof the equatorial Pacific SST and TP heating. It is found the persistent La Ni(n|~)a duringJanuary to May in2011maintain the125E eastward shift of western North Pacificsubtropical high (WNPSH), therefore, the southwesterly moisture flux failed to reach theYRV region, which possibly caused the persistent drought events during spring. However, thesensitive heating flux enhances in TP with the decaying of La Ni(n|~)a in June, which induces awestward shift of WNPSH to110E, and results in the southwesterly winds and convections in the west of WNPSH and the downward-propagated conventions over the TP meets in themid lower reaches of YRV region, and then results in the subsequent rainfall and flooding inearly summer.(2) CAM3numerical model is able to simulate the SST-forced atmospheric circulationand precipitation. The2011SST-sensitivity run show that, the SST-forced WNPSH is locatedin the east of125E, the circulation is not propitious to the precipitation in the mid-lowersreaches of YRV, the model-results implies that SST-forced persistent drought would last fromspring to early summer in this region. However, compared with the observation of abruptlydrought-flooding, the influence of SST is only responsible for the pre-seasonal drought, itcannot explain the dramatic increase of precipitation in June.(3) There are four rainfall-associated synoptic processes in June of2011, and eachprocess is accompanied with the positive vortices and convective cloud developing in themid-lowers reaches of YRV. The intensive baroclinic instability-related vertical shear ofhorizontal wind fields is found from the north side in TP region to the north of YRV region.Temporal evolution of vortices and TBB indicate that the vortex and convection originatesfrom TP region, and advance eastward; the effect of TP heating and baroclinic instabilitydevelop eastward as well. These synoptic system combines with the westward shift ofWNPSH, and results in the abruptly increase of the precipitation in June eventually.