Comparison of remote sensing data with in-situ wind observation during the development of the South China Sea monsoon

Wind measurements derived from QuikSCAT data were compared with those measured by anemometer on Yongxing Island in the South China Sea (SCS) for the period from April 2008 to November 2009. The comparison confirms that QuikSCAT estimates of wind speed and direction are generally accurate, except for the extremes of high wind speeds (>13.8m/s) and very low wind speeds (<1.5m/s) where direction is poorly predicted. In-situ observations show that the summer monsoon in the northern SCS starts between May 6 and June 1. From March 13, 2010 to August 31, 2010, comparisons of sea surface temperature (SST) and rainfall from AMSR-E with data from a buoy located at Xisha Islands, as well as wind measurements derived from ASCAT and observations from an automatic weather station show that QuikSCAT, ASCAT and AMSR-E data are good enough for research. It is feasible to optimize the usage of remote-sensing data if validated with in-situ measurements. Remarkable changes were observed in wind, barometric pressure, humidity, outgoing longwave radiation (OLR), air temperature, rainfall and SST during the monsoon onset. The eastward shift of western Pacific subtropical high and the southward movement of continental cold front preceded the monsoon onset in SCS. The starting dates of SCS summer monsoon indicated that the southwest monsoon starts in the Indochinese Peninsula and forms an eastward zonal belt, and then the belt bifurcates in the SCS, with one part moving northeastward into the tropical western North Pacific, and another southward into western Kalimantan. This largely determined the pattern of the SCS summer monsoon. Wavelet analysis of zonal wind and OLR at Xisha showed that intra-seasonal variability played an important role in the summer. This work improves the accuracy of the amplitude of intra-seasonal and synoptic variation obtained from remote-sensed data.

TYPICAL STRONG AND WEAK SOUTH ASIAN SUMMER MONSOON YEARS AND SEA SURFACE TEMPERATURE OF ARABIAN SEA

The South Asian Summer Monsoon （SASM） is an important member of the monsoon system for Asia. It is made up of low-level subsystems of the Mascarene high in the Southern Hemisphere, cross-equatorial Somali jet stream, 850-hPa westerly jet over the Arabian Sea, Indian monsoon trough north of the Bay of Bengal through west India and upper-level tropical easterly jet centered at 5°N and South Asia high centered at 30°N. During the summer monsoon, convection is intense in South Asia, with large scale and in association with abundant amount of latent heat release from condensation. Its anomalies affect not only the industrial and agricultural production and people＇s life in South Asia, but also the southwestern part of China. SASM is therefore drawing attention from quite a number of meteorologists from home and abroad. For instance, in their search for indicators of the summer monsoon in the region, Parthasarathy et al. Webster et al. and Goswami et al. defined a number of indexes based on precipitation and circulation. Wang et al.studied existing, widely-used indexes and came up with different regional indexes for the circulation and convection of SASM. Hahn et al.worked on the effect of topography on SASM. With wind field data, Wang et al. divided the years by the intensity of SASM and analyzed the characteristics of interannual variation and circulation for strong and weak years of monsoon. They found that the SASM intensified and weakened as a whole and there were four types of monsoon, being wholly strong and weak, stronger in the west than in the east and weaker in the west than in the east. Yan et al.discow,＇red sharp differences in individual members of the SASM at upper and lower levels over middle and lowe,r latitudes in both strong and weak years of the monsoon. Using, the dynamics method, Zhu et al. took the South Asia winter and summer monsoons as two stable equilibrium states and discussed the formation mechanism from the viewpoint of non-linear equilibrium theory. Their result further shows that in addition to thermal difference between land and sea, the topographic effect of South Asia also has significant restraints and influence on the formation and activity of the monsoon