Hydrological cycle associated with monsoon depressions
Date
1999
Authors
Yoon, Jin-Ho
Major Professor
Advisor
Chen, Tsing-Chang
Committee Member
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Abstract
The contribution from different scale weather disturbances—30-60, 12-24 day monsoon modes, and monsoon depressions—on the regional watervapor balance over the Indian subcontinents was studied using the ECMWF reanalysis data and GPI (Goddard Precipitation Index) blended with the 24-hour surface station precipitation data. Also, the interaction between two ISOs (Intra-Seasonal Oscillation) and monsoon depressions is shown by the water vaporbudget analysis. Finally, the contribution of monsoon depressions to the interannual variation of monsoon rainfall is estimated. Major findings of the present study can be summarized as follows. 1) Monsoon depression brought about half of the monsoon rainfall during the summer season. It was reported that occurrence frequency of the monsoon depression has an interannual variation highly correlated with the sea surface temperature (SST) over the NIN03 region. However, the change in the occurrence of the monsoon depression cannot be the decisive factor of the interannual variation of the Indian monsoon. The reason why the monsoon depression cannot decide the interannual variation of the Indian monsoon rainfall is the interannual variation of occurrences of the monsoon depression results from the large scale circulation of the Western Tropical Pacific and South China Sea WTP-SCS), instead of India. Also, other mechanisms (e.g., soil moisture) might have some contribution to the Indian monsoon. It was shown from our water vapor budget analysis that the amount of the evaporation (E) is not negligible. Therefore, future research will be needed to determine the effects of change on the evaporation (E). 2) Monsoon depression is an intense weather system.
However, monsoon depressions are synoptic scale disturbances imbedded in the monsoon circulation. Therefore, the large-scale environment change induced by the ISOs can modulate the hydrological cycle of monsoon depressions. During active or couple cases, the strong precipitation is maintained by the enhanced convergence of water vapor flux toward the Indian subcontinent. The intensity of the 30-60 day monsoon mode is stronger that that of the 12-24 day mode, because the convergent water vapor flux of the 30-60 day mode is more active than that of the 12-24 day mode. One advantage of the present study is the data availability over the oceans near India. Precipitation estimation (GPI) from the GSFC/NASA provides us with a relatively uniform dataset over the analysis domain. To improve its realism, we will employ the blending with the 24-hour surface precipitation from surface observations over the Indian monsoon region. Therefore, fairly uniform and realistic precipitation data provide the hydrological process of the Indian monsoon clearly.
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