A regional coupled air-ocean wave model and the simulation of the South China Sea summer monsoon in 1998

摘要

Wind-generated ocean waves play a key role in the transfer of momentum, heat, and water vapor between air and sea. In order to reveal the influences of ocean waves on the South China Sea summer monsoon (SCSSM) and to discuss the related mechanisms, a regional atmospheric climate model (with 9 layers in P-a coordinate) is coupled to the third generation ocean wave model using the sea-state-dependent roughness, and the coupled model (referred to as P-sigma RCM-WAM) is applied to simulate the SCSSM-developing processes as applicable for 1998. Four simulations are performed and their results compared with one another. The difference between the fully coupled and uncoupled models in terms of the pentad mean wind fields at the 850 hPa level shows a cyclonic circulation over the northern South China Sea (SCS) and southern China before the SCSSM onset. During the onset of the SCSSM the cyclonic circulation changes to a prevailing southwesterly flow over the northern SCS, thus accelerating the SCSSM onset. After the onset the differential wind field shows a divergent pattern. The evolution of precipitation is also modified by the coupling. In response to the air-ocean wave (AW) coupling, the SCSSM rainfall increases in the northern SCS during the SCSSM onset; after the onset the rainfall decreases over the SCS and increases over South China. Considering the atmospheric fields at seasonal timescales, the fully coupled model better simulates the intensity of the 500 hPa geopotential height field and obtains the precipitation with the spatial pattern and amount that are closer to the observations, especially in June, compared with the uncoupled model. The AW feedback in the momentum coupling process results in an anticyclonic differential wind field at the 850 hPa level near the coast. During the SCSSM onset, the thermal positive feedback is dominant in causing changes in the heat fluxes and intensifies the 850 hPa wind field over the Bay of Bengal (BOB) and northern SCS. The BOB and the SCS are both sensitive regions for the onset of SCSSM in the momentum and thermal coupling experiments. Copyright (C) 2006 Royal Meteorological Society.