Research On The Relations Between The Easterly Waves In South China And The SST In The South China Sea And The West Pacific Ocean

Easterly wave is a trough or an area with the largest cyclonic curvature in the easterlies situating in the south area of subtropical high, moving from east to west in the form of wave.The easterly waves that influence China mainly generate on the surface of the western Pacific or the offshore areas, then spread westward to affect the coastal regions of South China and Southeast China, even the middle and lower reaches of the Yangtze river and the Southwest China.Normally, pure easterly wave can only bring some convectional precipitation. However, after combining with the southwest monsoon or a typhoon, an easterly wave can cause heavy precipitation more persistent. The complicated structures and the changeable propagation paths of easterly waves make the forecast of easterly waves more difficulty and important.In this article we will pick up the processes of easterly waves from1999to2013. Using the methods of statistical classification, synthetic analysis, diagnostic analysis and numerical simulation, the thermal and dynamical structures, along with the circulation backgrounds and moisture conditions of the easterly waves will be analyzed. We come to these conclusions:Ⅰ According to the differences of the movement routines and the generating sources, the low-level and deep easterly waves effecting South China coastal can be divided into three categories:south path easterly wave, westward easterly wave and offshore easterly wave.Ⅱ The large scale circulation background fields of the three types of easterly waves are different:firstly, the westward easterly wave is associated with a stronger South Asia High and a stronger Subtropical High than the south path and the offshore easterly wave; and secondly, the subtropical high ridge line of the westward easterly wave locates more southward.Ⅲ The positive vorticity as well as the vertical helicity is good indicators to the strength and the Vertical stretching height of the easterly waves. On the vorticity field, the trough area has positive vorticity, while the other, negative. Dividing from height of the strongest vorticity, below the height there is strong convergence, while above it, with strong divergence. The allocation situation of the divergence and vorticity field provides a favorable dynamic condition for the generation and development of easterly wave. During the prosperous period, the trough area has upward movement through the whole layer of the atmosphere, the stronger the upward movement, the more precipitation it produces.Ⅳ Three parts of different ocean areas are associated with the generating and moving of the easterly wave. Through the analysis of the sea surface temperature of the three parts ocean areas and found that the movement of the easterly wave has a tendency to move towards the warmer part of the sea.Ⅴ The three kinds of easterly wave have different vapor sources. The vapor of westward easterly wave comes from the western Pacific Ocean, but the offshore easterly wave and the south path easterly wave has two vapor sources:one is the western Pacific, the other is the bay of Bengal and the south China sea.Ⅵ By the numerical simulation of an easterly wave process on August3-4,2001off the coast of Zhejiang and Fujian, we found that the strong precipitation process of this deep easterly wave occurred behind the trough-line, which is different to the predecessors’ research. In the sensitivity test,we found that the easterly wave intensified in the bottom of the trough region by the rise of sea surface temperature:the lower atmosphere had a convergence while the upper atmosphere, divergence. The rise of sea surface temperature also made the strong precipitation locate even more Northern.