Impact of the Thermal State of the Tropical Western Pacific on Onset Date and Process of the South China Sea Summer Monsoon

Since the early or late onset of the South China Sea summer monsoon （SCSM） has a large impact on summer monsoon rainfall in East Asia, the mechanism and process of early or late onset of the SCSM are an worthy issue to study. In this paper, the results analyzed by using the observed data show that the onset date and process of the SCSM are closely associated with the thermal state of the tropical western Pacific in spring. When the tropical western Pacific is in a warming state in spring, the western Pacific subtropical high shifts eastward, and twin cyclones are early caused over the Bay of Bengal and Sumatra before the SCSM onset. In this case, the cyclonic circulation located over the Bay of Bengal can be early intensified and become into a strong trough. Thus, the westerly flow and convective activity can be intensified over Sumatra, the Indo-China Peninsula and the South China Sea （SCS） in mid-May. This leads to early onset of the SCSM. In contrast, when the tropical western Pacific is in a cooling state, the western Pacific subtropical high anomalously shifts westward, the twin cyclones located over the equatorial eastern Indian Ocean and Sumatra are weakened, and the twin anomaly anticyclones appear over these regions from late April to mid-May. Thus, the westerly flow and convective activity cannot be early intensified over the Indo-China Peninsula and the SCS. Only when the western Pacific subtropical high moves eastward, the weak trough located over the Bay of Bengal can be intensified and become into a strong trough, the strong southwesterly wind and convective activity can be intensified over the Indo-China Peninsula and the SCS in late May. Thus, this leads to late onset of the SCSM. Moreover, in this paper, the influencing mechanism of the thermal state of the tropical western Pacific on the SCSM onset is discussed further from the Walker circulation anomalies in the different thermal states of the tropical western Pacific.

Diagnostic Study of Apparent Heat Sources and Moisture Sinks in the South China Sea and its Adjacent Areas during the Onset of 1998 SCS Monsoon

The apparent heat sources (?Q1 ?) and moisture sinks (?Q2 ?) are calculated based on the reanalyzed data of the South China Sea Monsoon Experiment (SCSMEX) from May 1 to August 31, 1998. It is found that the formation and distribution of the atmospheric heat sources are important for the monsoon onset. The earlier onset of the SCS monsoon is the result of enduring atmospheric heating in the Indo–China Peninsula and South China areas. The atmospheric heating firstly appears in the Indo–China Peninsula area and the sensible heat is the major one. The 30–50 day periodic oscillation of atmospheric heat sources between the SCS area and the western Pacific warm pool has a reverse phase distribution before the middle of July and the low frequency oscillation of heat sources in SCS area has an obvious longitudinal propagation. The 30–50 day low frequency oscillation has vital modificatory effects on the summer monsoon evolution during 1998. Key words Apparent heat sources - Apparent moisture sinks - The South China Sea monsoon - Diagnostic Study Sponsored by the National Key Project of Fundamental Research “ SCSMEX” and the Research Fund for the Doctoral Program of Higher Education: “ Study of the Air-sea Interaction in the SCS Monsoon Region”.

DRAFT PROPLSAL FOR THE SOUTH CHINA SEA MONSOON EXPERIMENT(SCSMEX)

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Observational and Modeling Studies of Impacts of the South China Sea Monsoon on the Monsoon Rainfall in the Middle-Lower Reaches of the Yangtze River During Summer

Based on the ERA-40 and NCEP/NCAR reanalysis data,the NOAA Climate Prediction Center＇s merged analysis of precipitation（CMAP）,and the fifth-generation PSU/NCAR Mesoscale Model version 3（MM5v3）,we defined a monsoon intensity index over the East Asian tropical region and analyzed the impacts of summer（June-July） South China Sea（SCS） monsoon anomaly on monsoon precipitation over the middle-lower reaches of the Yangtze River（MLRYR） using both observational data analysis and numerical simulation methods.The results from the data analysis show that the interannual variations of the tropical monsoon over the SCS are negatively correlated with the southwesterly winds and precipitation over the MLRYR during June-July.Corresponding to stronger（weaker） tropical monsoon and precipitation,the southwesterly winds are weaker（stronger） over the MLRYR,with less（more） local precipitation.The simulation results further exhibit that when changing the SCS monsoon intensity,there are significant variations of monsoon and precipitation over the MLRYR.The simulated anomalies generally consist with the observations,which verifies the impact of the tropical monsoon on the monsoon precipitation over the MLRYR.This impact might be supported by certain physical processes.Moreover,when the tropical summer monsoon is stronger,the tropical anomalous westerly winds and positive precipitation anomalies usually maintain in the tropics and do not move northward into the MLRYR,hence the transport of water vapor toward southern China is weakened and the southwest flow and precipitation over southern China are also attenuated.On the other hand,the strengthened tropical monsoon may result in the weakening and southward shift of the western Pacific subtropical high through self-adjustment of the atmospheric circulation,leading to the weakening of the monsoon flows and precipitation over the MLRYR.

Temporal and Spatial Distributions of the Spring Persistent Rains over Southeastern China

The spring persistent rains （SPR） over southeastern China （SEC） is a synoptic and climatic phenomenon that is unique in East Asia. Sufficient evidence proves that it results from the mechanical and thermal effects of the giant Tibetan Plateau （TP）, but its temporal span and spatial distribution are not clear at present. A climatological analysis of the NCEP/NCAR circulation and sensible heat data shows that at the 13th pentad of the solar year （lst pentad of March） there are remarkable increases in the sensible heating over the main and southeastern part of the TP, the southwesterly velocity over the southeastern flank of the TP and SEC, and rainfall over SEC, indicating the onset of the SPR. However, after the 27th pentad of the solar year （3rd pentad of May）, these variables, except for the sensible heating over the main part of the TP, decrease rapidly. The ridge line of the subtropical high in the mid-low troposphere over the South China Sea （SCS） slopes northward to the SCS and the SCS monsoon instead of southward as before breaks out, indicating the end The rain belt center over SEC shifts of the SPR. Hence, it is reasonable to define the SPR temporal span from the 13th to 27th pentad of the solar year. Data analysis and numerical sensitivity experiments show that, although the warm and cold airs converge at about 30°N in the SPR period, the distribution and intensity of the SPR rain belt are obviously influenced by the topography of the Nanling and Wuyi Mountains （NWM）. The mountains can block and lift cold and warm airs, strengthening frontogenesis and rainfall. As a result, the axis of the SPR rain belt is superposed over that of the mountain range. Accordingly, the spatial distribution of the SPR extends over most of the SEC, more specifically, to the south of the middle and lower reaches of the Yangtze River （30°N）, and to the east of 110°E.