The Role of Land–sea Distribution and Orography in the Asian Monsoon. Part I： Land–sea Distribution

The Role of Land--sea Distribution and Orography in the Asian Monsoon. Part I: Land--sea Distribution

A number of AGCM simulations were performed by including various land--seadistributions (LSDs), such as meridional LSDs, zonal LSDs, tropicallarge-scale LSDs, and subcontinental-scale LSDs, to identify their effectson the Asian monsoon. In seven meridional LSD experiments with thecontinent/ocean located to the north/south of a certain latitude, the LSDsremain identical except the southern coastline is varied from 40^o to4^oN in intervals of 5.6^o. In the experiments with the coastlinelocated to the north of 21^oN, no monsoon can be found in the subtropicalzone. In contrast, a summer monsoon is simulated when the continent extendsto the south of 21^oN. Meanwhile, the earlier onset and strongerintensity of the tropical summer monsoon are simulated with the southwardextension of the tropical continent. The effects of zonal LSDs wereinvestigated by including the Pacific and Atlantic Ocean into the modelbased on the meridional LSD run with the coastline located at 21^oN. Theresults indicate that the presence of a mid-latitude zonal LSD induces astrong zonal pressure gradient between the continent and ocean, which inturn results in the formation of an East Asian subtropical monsoon. Thecomparison of simulations with and without the Indian Peninsula andIndo-China Peninsula reveals that the presence of two peninsulas remarkablystrengthens the southwesterly winds over South Asia due to the tropicalasymmetric heating between the tropical land and sea. The tropical zonal LSDplays a crucial role in the formation of cumulus convection.