Spatio-temporal Variabilities Of The Precipitation During The First Rainy Season In South China

The First Rainy Season (FRS) in South China is the first typical stage of the rain belt in East Asian subtropical area which progresses stage by stage on China mainland. The definitions on the onset and end dates of FRS, its spatio-temporal precipitation varivbilities, the anomalies of circulation features and its possible mechanisms influenced by the atmosphere and the oceans are always hot issues in this field. By using NCEP/NCAR reanalysis data, and760stations daily precipitation data in china, composite analysis, dynamic and synoptic diagnosis analysis are performed generally to investigate the above scientific issues on FRS via multiplicate statistical methods. The major conclusions are as follows:(1) Onset and end of FRS in China are examined for the period1957-2004. Two aspects, one is the circulation background which represents the transformation of the general circulations and the setup of the rain belt and the other is the regional concentration of precipitation which has been treated with5-day averages (pentads), are the primary factors to mark off the dates. After choosing the appropriate research area and stations in South China, the onset and end dates are determined objectively by a set of thresholds. Results show that the FRS begins in the19th pentad (1st pentad in April) and ends in the34th pentad (4th pentad in June) in which the rainfall has distinctive staggered features. FRS is composed of the frontal precipitation period and the summer monsoon precipitation period. It is found that the general circulation favors the increase in temperature and humidity around the beginning so as to consequently cause the remarkable rainfall while the ending of the FRS is due to the seasonal transition of the general circulation in East Asia, especially with the first northward shift of the Western Pacific Subtropical High (WPSH).(2) An index of FRS is defined which can represent the features of precipitation during FRS. Results show that the precipitation have obvious interannual variability with2-4years and quasi-8years periods. Its interannual trend descends substantively in recent48years which might be the main reason for the decrease in annually total rainfall in South China. The spatial distribution of FRS precipitation has four different patterns in which the whole region rains identically more or less is the main mode. Being early or late of the beginning or ending dates of FRS has remarkable influence on its precipitation and there has a weak positive relationship between the onset and the end.(3) When it rains more during FRS, there exits a kind of dipoles circulation which distribute symmetrically around the northern Pacific storm track at30°N over East Asia. The favorite conditions for positive anomalies in precipitation include a stronger WPSH shifting westward and migrating southward, the strengthened Asian polar vortex, a high pressure dominating on the Siberian Plain, activities of the cold air in mid-latitude with high frequency, a deeper Asian trough moving southward, and an active southern branch trough in Indo-China Peninsula. These anomaly patterns also correspond to the strong Northern Pacific Oscillation (NPO), the strong northern Atlantic oscillation and the positive index of Southern Oscillation (SO). The tropic circulation anomaly in Western Pacific will have an indirect influence on the FRS precipitation via a kind of teleconnection similar to the East Asia-Pacific teleconnection wave train.(4) The lag and simultaneous correlations between precipitation in South China and the sea surface temperature (SST) in some key regions, such as the western and eastern Pacific Ocean, experiences a shift of inversion which changes from positive to negative in2～3years (that is about30months). The course of these correlations and the sea surface temperature anomaly (SSTA) pattern is similar to the El Nino-Southern Oscillation (ENSO). The area of120°E～180,20°S～20°N is a key region in western Pacific warm pool whose SSTA is negatively sensitive to FRS rainfall. The leading mode that the SSTA in tropic Pacific Ocean exerts its influence on FRS rainfall is ENSO mode. When it is in positive phase of ENSO cycle with cold SSTA in the Western Pacific Warm Pool and warm SSTA in the equatorial Eastern Pacific, it rains more than usual during the FRS. While in its negative phase of ENSO cycle, the rainfall is relative less. The abnormal signal in tropical Western Pacific appears4months earlier than that in the equatorial Eastern Pacific Ocean.(5) The possible mechanism that the key regional SSTA in pre-winter influences the FRS precipitation might be:when the tropical Western Pacific SST is warmer, the convective activities over the sea near Philippines will be strengthened. This will generate an anomalous planet-scale disturbance like Nitta waveband in Northern hemisphere. The anomalous convective activities will also bring on the anomalies of Walker circulation and regional Hadley circulation in mid-low latitude over Eastern Asian Pacific. The latter, via the East Asia-Pacific teleconnection wave train, will play its role on the abnormal of500hPa geopotential height in Northern hemisphere. This type of distribution will be benefit to the east (west) shifting of the WPSH, the weakening of the mid-latitude trough in East Asia and the dropping of the Okhotsk high. Consequently, these will induce it rains more than usual during FRS.(6) The beginning of FRS has a delaying trend while the ending shows a trend of ahead over the past50years. So the total length of FRS days tends to be shorter. The dates of beginning and ending have significant correlations with the pre-winter circulations. When the Northerri’hemisphere Westerlies Jet is stronger in mid-high latitude, with the strengthened abnormal cyclonic circulation on the north side of the Jet (showing a deeper East Asia trough) and the strengthened anti-cyclonic circulation on the south side (displaying a stronger subtropical high), indicating that the meridinal circulation is dominant and the winter monsoon is powerful. This is in favor of the southward invasion of the cold air and results in an earlier onset of FRS, and vice versa. As for the earlier end years, it shows a stronger and more westward WPSH in the previous winter, with a southward migrating and stronger East Asia trough in mid-high latitude. The meridinal pressure gradient is much greater indicating that the winter monsoon is abnormally intensified.(7) The SSTA displays significant differences between the early and late onset years. For the early years, the pre-winter SST in tropic Western Pacific warm pool is colder while the SST from the coastal of East Asia to the west part of North Pacific is warmer with colder SSTA in equatorial East Pacific. As for the late years, the SSTA has opposite features. Results also show that the two regions, on lies from the middle of Northern Pacific to the coastal of East Asia and the other is in the warm pool of Western Pacific, are the significantly positive or negative sea areas correlated with the end of FRS in pre-winter, respectively. When FRS ends earlier, it has positive SST anomalies in warm pool and negative anomalies in mid-west Northern Pacific, and vice versa.