The Seasonal Process Of Onset Of The Asian Summer Monsoon And Related Mechanisms

The seasonal process and mechanisms of onset of the Asian summer monsoon are investigated in this study, and the conclusions are as followed:1. The earliest onset of the Asian summer monsoon occurs over the tropical eastern Indian Ocean, which is mainly characterized by a stepwise advance, which is closely related to the evolution of the cross-equatorial flow. It will provide a precondition for the onset and the northward advance of the summer monsoon over the tropical eastern Indian Ocean when the cross-equatorial flow strengthens continuously and advances northwards, which is relative to the cyclone circulation caused by the heating of the strong surface sensible heat flux over the Indian Peninsula and the surface latent heat flux over the ocean to the south of it. The cyclone circulation is in favor of the continuous intensification of the southwesterly over the tropical eastern Indian Ocean but has an opposite effect on the counterpart over the tropical western Indian Ocean from the end of March to late May, and as a result the cross-equatorial flow takes place the earliest over the tropical eastern Indian Ocean.2. Three stages and two abrupt jumps exist in the climatological onset process of Bay of Bengal (BOB) summer monsoon: (1) on Pentad 23, westerly and convection start to build up in the southeastern part of BOB. (2) When the active convection emerges over the Indochina Peninsula, affected by the vertical asymmetric heating caused by the great amount of condensation latent heat release, the BOB cyclone circulation intensify rapidly as well as the moisture converges greatly in the BOB, which results in the active convection over the most areas of the BOB and the first jump appears on Pentad 28 consequently. (3) Influenced by the onset of the Indian summer monsoon, the second jump is found on Pentad 31.3. The onset of the vigorous convection over Indochina Peninsula and its adjacent areas is of very importance to the onset of the Indian summer monsoon. In consequence of the active convection over the Indochina Peninsula, the divergence at upper levels over the Indochina Peninsula-South China Sea intensifies and extends, then the convergence center over the Arabian Sea and the Arabian Peninsula comes to a westward shift, in such a way the subsidence in the troposphere over the Arabian Sea weakens, so does the low-level anticyclone circulation and the northerly wind, which makes for the fast northwards advance of the Somali jet flow after crossing the equator. While the significant increase of the vorticity on the left side of the jet flow is in favor of the rapid intensification of the heat low over the Arabian Peninsula and the lowering of the low-level geopotential height over the Arabian Sea, further more, the heating field with heating at upper levels and cooling at lower levels over the Arabian Sea also favors the lowering of the lower-level geopotential height. Affected jointly by the factors mentioned above, the low-level geopotential height lowers over the Arabian Sea, thus the negative meridional geopotential gradient strengthens and the westerly develops. At the same time the strengthened heat low over the ArabianPeninsula also leads to an eastward acceleration of the westerly, so the Indian summer monsoon onset starts on Pentad 31.4. The timing of the stepwise onset and advance of the Asian summer monsoon from 1979 to 2001 is identical to the result from the climatological analysis, indicating that the onset of the tropical eastern Indian Ocean acts as a very important role in onset process of the Asian summer monsoon. Great interannual variability appears in the onset timing of the Asian summer monsoon subsystems and their onset behaviors differ in different years.The results of this paper suggest that close relationships and interactions exist between the subsystems of the Asian summer monsoon.