Similarities as well as Differences between Summer Monsoons over Southeast Asia and the Western North Pacific

[摘要] The life cycles of SEAM (Southeast Asian Monsoon) and WNPM (Western North Pacific Monsoon) are monitored by the climatological pentad mean OLR and 850 hPa [u, υ], as well as utilizing the vertically integrated moisture flux vector [Qλ, Qψ] and associated precipitation minus evaporation (P - E). SEAM reaches its peak in late July in conjunction with full establishment of a gigantic heat low over the Asian Continent due to surface heating. The height of WNPM occurs two to three weeks later due to the delayed intensification of the monsoon trough not directly by in situ surface warming, but indirectly by remote influence of the Asian heat low. SEAM owes its existence to the westerly moisture influx from the Arabian Sea and further upstream. On the other hand, the easterly moisture influx from the Eastern Pacific is the major contributor to WNPM, relegating the role of moisture import from the SEAM domain to that of secondary importance. The climatological pentad mean data are then partitioned into symmetric and asymmetric components with reference to the equator. SEAM is characterized by dominance of the asymmetric mode, an implication that it is essentially an asymmetric monsoon system regulated by north-south heat contrast. The contribution of the asymmetric mode to the summer mean moisture budget of SEAM is as high as 82%, while the corresponding contribution due to the symmetric mode is only 18%. In comparison, the asymmetric and symmetric modes equally contribute to WNPM and as such, it is an asymmetric and symmetric mixed, hybrid monsoon system. The summer mean moisture budget of WNPM is accomplished by almost equally significant contributions of the symmetric and asymmetric modes, i.e., 58% from the former, while 42% from the latter mode. The symmetric mode exerts a strong influence on the maintenance of the climatological equatorial Walker circulation. It is the asymmetric Hadley circulation that furnishes moisture to equatorial convections over the updraft portion of the Walker circulation in the eastern Indian ocean. Activated convections then enhance the symmetric zonal flow along the equator, which is westerly over the Indian Ocean while easterly over the western Pacific.

[学科分类] 大气科学