The Influence Of Winter Southern Branch Of Westerly On Recipitation In China And Its Variability Mechanism

In the winter and spring, obstructed by the Tibetan Plateau, the low levelwesterly is split into two branches, which flow around the plateau from northside and south side respectively, thus exerts a significant influence on EastAsia circulation and its downstream weather. Utilizing the NCEP/NCARreanalysis data, we first analyzed the seasonal changes of the circumferentialwesterlies. Then based on the daily surface climatic data of China, the annualvariablity of the southern branch of western flow (SBWF) and its effect on thewinter precipitation in southern China are discussed. Finally, we try to find outthe mechanism causing the strong and weak SBWF.We find out the shift of the Divergence Line (DL, v=0contour) in the westof the Tibetan Plateau can indicate the seasonal change of circumferentialwesterlies. During the season transition in winter to summer, the DL has twosignificant northward jumps at17th and22nd pentad and disappears at28thpentad, then the DL rebuilds at59th pentad. The northward jump of DL in lowtroposphere is rapid, which is similar to the change of westerly jet in up level.During the transition in winter to summer, the revolution of DL precedes thenorthward jump of westerly jet, but lags behind the southward jump of westerlyjet during the transition in summer to winter. The thermal state of TibetanPlateau plays a key role during the revolution of circumferential westerlies.The change of circumferential flow around the Tibetan Plateau is mainlycaused by the strength variability of the westerly in the upstream. With thenorthwest winds in west of Tibetan Plateau, we define an intensity index ofSBWF, which can reflect the large-scale circulation features well. The indexshows an obvious inter-annual and inter-decadal variability, the period is2to4years and8to10years. Its inter-annual variability plays important impact on precipitation in winter over south China. Strong SBWF increases winterprecipitation over the region in the south of Yangtze River and weak SBWFdecreased the precipitation in the east of China. These effects are associatedto the anomaly of the related circulation. When SBWF strengthens anomaly,the anomalous Anti-Hadley circulation between the equator and the south ofYangtze River develops. So the sink motion near the equator and the risingmotion over the East China are enchanced. Then more water vapor istransported by the southerlies in the low level. Under the favorite convergencecondition, more precipitation occurred.The main factors that affect the strength of SBWF include AO, thermal andsnow cover condition of Tibetan Plateau and East Asian Winter Monsoon. AndAO is the key signal affecting the strength of westerly in the upstream ofTibetan Plateau. With the positive AO, a northwest-southeast direction wavetrain appears from north Atlantic to Arab Sea, which strengthens the westerlyin West Asia and is favor of the enhancing of SBWF.However, the strong prevail SBWF condition causing strong westerlydownstream is the weakening of pressure over middle and low latitude of Asiaand the strengthening of subtropical high pressure over the East Indian Oceanand South China Sea. The easterly over the tropical ocean associated withpositive AO benefits the formation of the above anticyclone. In winter, theweakening of cold source over Tibetan Plateau can cause the strengthening ofwesterly in the upstream. This leads to more precipitation and latent heatrelease increasing, which can weaken the cold source of Tibetan Plateau more.Thus a positive feedback is formed. Meanwhile, the weakening of cold sourceof plateau can descend the pressure over west Asia and plateau, which isconducive to the appearance of such pressure field with low in north and highin south. More snow in plateau can cause downward motion over East IndianOcean, which can strengthen the subtropical high. SBWF shows a negativerelationship with East Asian Winter Monsoon. The meridional circulationstrengthens during strong winter monsoon and the zonal circulation enhancesin company with the strong SBWF, and vice versa. Therefore, we can concludethat AO, thermal and snow condition of the Tibetan Plateau and East Asianwinter monsoon cause the change of SBWF.