Climatic Features And Interannual Variations Of The Early Spring Rains Over Southeastern Tibetan Plateau

Based on TRMM, CMAP and GHCN-Daily precipitation observations, HadlSST and ERA-Interim reanalysis dataset, the climatic features and interannual variations of the early spring rains (ESR) over southeastern Tibetan Plateau (TP) are analyzed statistically. We also investigate its climatic causes and influencing factors preliminarily. The main results are as follows:(1) Climatic features of ESR over southeastern TP have been studied. The rain-belt is distributed along the Brahmaputra Valley, covering southeastern Tibet, northwestern Yunnan, northeastern India and northern Burma. It’s found by moving-t test that the rainfall experiences significant increasing at 17th pentad, a date that can be regarded as the onset of the ESR. The turning from northwesterly to southwesterly wind in the lower level over upper area is a symbol for the establishment of the ESR. The rain-belt starts to link to the surrounding area of precipitation at 24th pentad and it means the end of the ESR which as an independent rain-belt.(2) Such a southwesterly wind maintains robust during the ESR season, leading to convergence in wind speed and water vapor and the formation of the ESR. Meanwhile, the southwesterly wind will be forced to lift when it blows into the bell terrain over southeastern TP, thus forming maximum precipitation centers in the upwind slope. This is the immediate climatic cause of the ESR formation. Further study reveals that the southwesterly wind is associated both with the mechanical forcing of the TP and sensible heating over central southern the TP, suggesting that the presence of TP plays a fundamental role in the climatic formation of ESR.(3) There are obvious oscillations of the interannual precipitation in ESR (17th-24th pentads). The results of composited analysis show that the Western Pacific subtropical high and the anticyclone circulation over the Bay of Bengal (BOB) strengthens in flood years, so the southwesterly over upper area strengthens and it lead to the increase of rain. In early winter, the increase (decrease) of SST anomalies of the Indian Ocean key area or the decrease (increase) of SST anomalies of the Western Pacific key area will result in the significantly increase (decrease) of precipitation in ESR. The higher sensible heating over central southern TP in spring are favorable for stronger cyclonic circulation around southeastern TP, it benefits the rainfall, and vice versa.(4) The precipitation and upstream wind have been employed to obtain the annual onset dates of the ESR from 1979 to 2013. It is found that two-year-period change is significant. As a result of that the strong or weak southwesterly wind in the lower level over upper area, and the more or less water vapor which is provided by the BOB in March, ESR will starts early or late. Previous November-December sea surface temperature anomaly (SSTA) over northwest BOB will affect the supply of water vapor. The water vapor is more (less) than normal when the SSTA is higher(lower), so the ESR will starts earlier (late).