Low-Frequency Transitions of a Simple Monsoon System
Webster, Peter J.
Chou, Lang C.
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Seasonal, diurnal and synoptic time scales, plus a subseasonal modulation" of synoptic events, appear to dominate the temporal structure of monsoon systems. Observational studies indicate that the latter low-frequency variation modulates or groups synoptic disturbances, producing periods of intense activity (the "active" monsoon) separated by distinct lulls (the "break" monsoon). Together with the "onset" and "retreat" ofthe monsoon, the modulations introduce into the system time scales which are far more rapid than that which would be expected from the evolving latitudinal variation of insolation. As observations indicate that the seasonal cycle and low-frequency transients occur in large spatial scales a model is used which appears to simulate the large-scale mean seasonal structure of the monsoon. Such a model is a zonally symmetric moist primitive equation model coupled to an interactive and mobile ocean. With such a model the hypothesis is tested that the basic character of the low-frequency subseasonal transients of the simple monsoon system are a result of feedbacks between the hydrologic cycle and the differential heating between the interactive ocean and continental regimes. With multi-annual integrations of the joint ocean-atmosphere model, monthly and daily variations are studied with the aim of testing the hypothesis, With full hydrology and an ocean-continent contrast, the model monsoon deviates substantially from the smooth transitions noted in dry experiments. Not only do the onsets and retreats of the monsoon system accelerate but rapid, orderly transitions occur during the Northern Hemisphere summer. During such transitions, the meridional monsoon cell periodically migrates inland causing rising motion north of 30"N and subsidence near the coastal margin ,winch is the location of mean seasonal ascent. The transition is seen to possess biweekly time scales and show some characteristics of the monsoon "break". Similar transitions occur in subsequent years of the integration and differ in timing and intensity but maintain the same basic period. Comparison of the results with those of other studies are made and further studies detailed.
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