Analysis On The Climate Properties Of Meiyu Season And Its Anomalies Over Changjiang River And The Huaihe River Basins

Based on NCEP/NCAR reanalysis data, sea surface temperature(SST) data,out-going longwave radiation(OLR)data, daily precipitation data and GPCPprecipitation data, the climate properties of Meiyu season and its anomalies overChangjiang River and the Huaihe River Basins are analyzed statistically. The mainresults are as follows:(1)Climate properties of Meiyu season over Changjiang River and the HuaiheRiver Basins in the past 55 years have been studied. The results show that the onsetand withdrawal of Meiyu season are closely related with the end of stable Calcuttawesterly at 200 hPa and the seasonal northward shift of the western Pacificsubtropical high(WPSH), and they are the most important characteristics. The onsetand withdrawal of Meiyu season have significant interannual variation. DuringMeiyu season onset, the south Asian high(SAH) enhances and migrates eastward, theWPSH migrates westward and shifts northward, the summer monsoon becomesweaker, and the cold and warm air converges in Yangtse-Huaihe river valley. DuringMeiyu season withdrawal, the SAH intensifies, extends westward and jumpsnorthward, the east Asian subtropical westerly jet weakens and moves northward, theWPSH jumps northward twice, the summer monsoon becomes stronger, and the rainbelt propagates to the northern areas of Changjiang river and the Huaihe river basins.(2)In the same term of early Meiyu season onset(take the early years asexample), the SAH is more intense and lies more northward, the east and west Asiansubtropical westerly jet are weaker and lie more northward, and the deep trough ofeast Asia is weaker. The tropical convection is activated in a wide area including theIndo-China Peninsula, the western Pacific warm pool to the east of the Philippinesand the mid-east Pacific across the equator. Moreover, the ITCZ is fortified in theSouth China Sea(SCS), and the Indian southwest monsoon outbreaks early. With thepositive abnormal heating above the Tibetan plateau, the temperature gradientbetween ocean and land becomes conspicuous. The whole situations mentionedabove favor the early northward movement of the WPSH. The Meiyu season beginslate(early) with the warm(cool) phrase of ENSO, and the SSTA of Nino4 inwinter(January & February) and spring(March & April) can be regarded as a crediblesignal for forecasting the onset of Meiyu season. The weak(strong) convectiveactivities around the "land bridge", the Philippines, the western Pacific warm pooland the Indian peninsula correspond with the warm(cool) phase of ENSO. Thestrong(weak) convective activities around the above regions may(not) facilitate thenorthward jump of the WPSH and the outbreak of Indian summer monsoon. As a res- ult, the Meiyu season begins early(late).(3)In the same term of early Meiyu season withdrawal, the SAH is more intenseand lies more northward, the east and west Asian subtropical westerly jet are weakerand lie more northward, and the northeast cold vortex is weaker. The tropicalconvection is activated in the Indo-China Peninsula, the SCS, the Philippines, and thewestern Pacific warm pool. The ITCZ intensifies in the SCS. The whole situationsmentioned above favor the early northward migration of the WPSH. The short-termclimate prediction of the Meiyu season withdrawal is more difficult than the onset ofMeiyu season. The predictive signals of the Meiyu season withdrawal are mainlyconcentrated on the previous SST singularities(April to June) in the mid-west of thenorthern Pacific around the 35°N of the date line. The convection in the "landbridge", the Philippines and the western Pacific warm pool is the predictive signalsas well. The possible mechanism that the mid-west of the northern Pacifictemperature anomalies impacts on the withdrawal of Meiyu season is: the SSTA inthe mid-west of the northern Pacific leads to the abnormal intensity of the northeastcold vortex in the same term of the Meiyu season withdrawal, which affects theintensity and position of the WPSH, and thus influences the withdrawal of Meiyuseason.