Wind onset and withdrawal of Asian summer monsoon and their simulated performance in AMIP models

This study defines the concepts of wind onset and wind withdrawal to describe the abrupt seasonal variations of wind direction and circulation of the Asian monsoon. The patterns of wind onset and withdrawal show that the earliest wind onset in the tropical monsoon regions is found over equator around 70°–100°E and the southernmost South China Sea (SCS) and western Kalimantan, and the wind withdrawal shows a southward progression in tropics compared to the wind onset. A notable temporal boundary is found around 25°N in the subtropical western North Pacific (WNP), which may be related to the northward advance and southward retreat of the western Pacific subtropical high. The angle amplitudes of wind vectors in wind onset and withdrawal have distinct regional differences in Asian monsoon regions. Since the process of monsoon onset (withdrawal) may include several onsets of different variables without simultaneity, the relationships of the wind onset and withdrawal with the abrupt change of other variables (e.g. reversal of zonal wind, reversal of meridional wind, outgoing longwave radiation (OLR), precipitation) are investigated. The results indicate that the temporal discrepancies in different monsoon regions confirmed the asynchronous onsets. It also implies that the wind onset might be a good omen for monsoon precipitation in most regions since it is slightly earlier than rainy season onset. Seven Atmospheric Model Intercomparison Project (AMIP) models from Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) are validated against observations mentioned above. Generally, the simulations of the multi-model ensemble mean are better than any individual model results. And the simulations of wind withdrawal are better than those of wind onset. For wind onset, IAP-FGOALS-1.0g, MIROC3.2 (medres) and MPI-ECHAM5 simulate reasonably well. For wind retreat, most models can capture the behaviors in tropics. However, there are still some discrepancies in a few models to simulate the dates of sudden change of monsoon wind direction. Moreover, most of models cannot reproduce the onset and withdrawal of both rainfall and OLR. The relationship between these discrepancies and the shortcomings of precipitation simulation is crucial for further investigating in the future.