The Outbreak Of The Asian Summer Monsoon And To Promote The Process And Subtropical Morphological Variation

The NCAR/NCEP reanalysis data are employed to study the relationship between the monsoon onset vortex (MOV) over the Bay of Bengal (BOB) and the variation of the subtropical high (SH) during the Asia summer monsoon (ASM) onset. The mechanism of the short-term variation of the SH pattern responsible for the ASM onset is revealed. The kinetic model of the short-term variation of the SH has been developed and its universality has been verified. The effect of the Tibetan Plateau (TP) and the land-sea distribution in the tropics on the change of the SH ridge in the ASM area are also examined with mean climatical data. It has been further confirmed that the existence of the huge topography and the sub-land-sea thermal contrast in the tropics provides large-scale background for the ASM onset process. The onset occurs first in the BOB, next in the South China Sea (SCS), and in India at last. With the analysis of the relationship between the climate mean wind and precipitation field, the concept of ASM precipitation is defined and the uniform criterion of the ASM onset is established. This criterion proves to be able to precisely describe the summer monsoon onset and its advancing process. Further dynamic analysis reveals the dynamic mechanism for the late occurrence of ASM precipitation in Indian monsoon area and the abrupt occurrence of Indian summer monsoon onset. The work of this dissertation can be summarized in the following five parts: (1) The break of the SH ridgeline in the low level of the troposphere in BOB area during the pre-onset period are found by examining the circulations of the top, middle and low level of the troposphere and the evolution of the SH ridge over the Asia monsoon area in May, 1998, suggestting the MOV over the BOB prior to the ASM onset might be the key factor. When the MOV moves northward, it has different effects on the development of the SH ridgeline in the lower troposphere to the east of the BOB and to the west of BOB: the SH ridgeline east of the BOB moves eastward and southward while the SH ridgeline west of the BOB moves northward, which will result in the transformation of the SH ridgesurface east of the BOB from the winter pattern into the summer one while the SH pattern west of the BOB still maintain winter pattern. The respective diagnosis of the kinetics equation and complete vorticity equation reveals two kinds of physics mechanism with which the MOV affects the development of the SH pattern: dynamic effect and the force of vorticity source. The dynamic model of the SH pattern development during the ASM pre-onset period is established and this model is proved to be universally applicable with about 20-year's verification. (2) The regular development pattern of the SH ridgeline (ridge-surface) during the ASM pre-onset period is identified. The short-term variation of the SH ridgeline (ridge-surface) consists of two parts: one results from the seasonal northward movement of the sun, the other from the local process due to the local spatial inhomogeneous diabatic heating. (3) The effect of the TP and tropical land-sea distribution on the SH pattern development is verified with the reanalysis data. TP directly leads to the occurrence of the ASM onset first over the BOB area, while the Indo-China peninsula and the India peninsula cause the ASM onset to occur over the SCS and India one after another due to their different kinds of spatial in-homogeneous diabatic heating above to the atmosphere. (4) The concept of the ASM precipitation is defined by analyzing the relationship between the ground wind field and the precipitation. The universal criterion of the ASM onset is formulated with the light of this definition. This universal criterion is proved to be able to describe the onset process of the ASM in the tropics and the advancing process over the mainland China and the west tropical Pacific. With this advancing process of the ASM precipitation, the Indian summer monsoon is proved to be a relatively independent sub-system, advancing from the south to the north. (5) The convergence circulation of the top level of the troposphere, which causes the descent stream to stop the convection over the peninsula, is the key factor that prevents the ASM precipitation spreading from the BOB area to the India peninsula. With the sun moving northward, the convergent circulation converts to divergent one, which pumps the air from the middle and low troposphere and enforces the convection over the peninsula. This pumping effect over the top troposphere results in the occurrence of the ASM precipitation and Result in the rapidly northward advance of the precipitation area.