A Study On Mechanisms Of Interdecadal Anomaly Of Rainfall During The Pre-Rainy Season In South China

The temporal and spatial distributions of the interdecadal anomaly of precipitation over South China (SC) during the pre-rainy season are investigated from the perspective of mean and variability, by using the monthly mean rainfall data at 160 China stations for 1981-2013. Combined with NCEP/NCAR reanalysis data and NOAA sea surface temperature data, the relationship between interdecadal anomaly of rainfall in the first rainy in SC and atmospheric circulation and sea surface temperature (SST) are discussed by using a variety of statistical analysis methods. Finally, the possible mechanism of precipitation anomaly during the pre-rainy season over SC is proposed. The main conclusions are as follows:(1) The precipitation over SC witnessed a marked decadal mutation, and turned from negative to positive anomaly. The significant interdecadal changes in 1990s when the south was colder than north in the upper troposphere (40°N bounded) and the lower was warmer than the upper in the troposphere, made high-low level circulation field presenting a favorable situation, which is in favor of the cold air in the north meeting with the warm and wet water vapor from Bay of Bengal and west Pacific and making a convergence and rising in SC; During the two periods before and after the interdecadal turning, there are significant differences of the Bay of Bengal trough over the tropical, EAP teleconnection wave train, and the ridge to the south and the west of Lake Baikal over the mid latitude, so the interdecadal anomalies of these systems are the important reasons of the precipitation anomalies during the first rainy season in SC.(2) Interdecadal anomaly of rainfall in the pre-flood season in SC and SST in key areas of the South Pacific Ocean and the North Atlantic SST triple shows a significant interdecadal positive correlation. When SST in key areas of the South Pacific Ocean is warmer than normal and the North Atlantic SST shows "+-+" triple mode, they would excited the "+-" alternating decadal teleconnection pattern from the SST key region to SC respectively. The teleconnection wave train comed from South Pacific and Atlantic make the East Asian region atmospheric circulation anomalies, which are conductive to rainfall in SC above normal, leading to interdecadal rainfall anomalies above normal in the first rainy season over SC.(3) The interannual variability (standard deviation) of rainfall in the first rainy occurred interdecadal abrupt change from weak to strong in around 2003, meanwhile the frequency of extreme droughts and floods events over SC significantly enhanced after 2003. Interdecadal transition of interannual variability of rainfall in the pre-rainy season over SC is closely related to the SST in the eastern equatorial Pacific. From early 1990s to early twenty-first Century, the correlation between the annual variation of rainfall in the pre-flood in SC and the SST in the equatorial eastern Pacific is significant positive, while the correlation relationship between them changed weak after 2003.(4) During the period of 1994-2003, the possible physical mechanisms of influences of the SST in the equatorial eastern Pacific on interannual rainfall anomalies in the pre-rainy season over SC is:When the eastern equatorial Pacific SST is anomaly warmer, which would stimulate the formation of an abnormal anti-Walker circulation between East Pacific and West pacific in equator, and the eastern equatorial Pacific for anomalous ascending motion, while the tropical western Pacific for anomalous descending motion with the anomalous low-level anticyclonic circulation. The divergence airflow from the periphery of anticyclonic transport much more the western Pacific warm moist vapor to SC, and the warm water vapor convergence and ascending in SC, eventually more rainfall in the first rainy season over SC(5) After 2003, the abnormal change of Northwest Pacific SST is an important external forcing factor resulting in rainfall anomalies in the pre-flood season in SC. The possible mechanisms of the effects of SSTA over Northwest pacific on precipitation anomalies are that: Northwest Pacific SST negative (positive) anomalies, which excite the formation of descending (ascending), associated with low-level divergence (convergence) airflow and low-level anticyclonic (cyclonic) circulation near the Philippines, meanwhile the South China region for the abnormal low-level convergence (divergence) with ascending (descending). Northwest Pacific SST anomalies make the relationship between the Philippines area and the Southern China area are connected by an anomalous vertical circulation, thereby affecting precipitation anomalies in the pre-rainy season in SC.