Formation Mechanism of the Simulated SPCZ and Baiu Front Using a Regional Climate Model

Abstract

The formation mechanisms of both the South Pacific convergence zone (SPCZ) and the baiu front are investigated using a regional climate model. Some idealistic numerical experiments are carried out assuming zonally uniform and temporally constant atmospheric fields obtained from ECMWF analysis data as initial and lateral boundary conditions. A rainfall zone similar to the SPCZ is reproduced using a zonal mean atmospheric field of the Southern Hemisphere (SH) summer. The simulated SPCZ in the idealized model framework is highly sensitive to the variation of SST during 1997?98 in a manner similar to observation. The SPCZ is extremely weak in an experiment under the zonal mean field of the Northern Hemisphere (NH) early summer. Experiments with a different intensity of zonal wind speed and baroclinicity suggest that a mild zonal wind (weak baroclinicity) weakens the precipitation of the SPCZ and even occasionally suppresses precipitation when it is too weak. The heat contrast between the Australian continent and the South Pacific Ocean contributes to form another rainfall zone when the zonal flow is very weak. Under these conditions, the SPCZ becomes unclear, and the rainfall zone appears from the southeastern part of Australia to the east of New Zealand. The latter rainfall zone will be intensified if the orography in the Australian continent is magnified. This rainfall zone is formed by the heat contrast between land and ocean and is somewhat similar to the baiu front. A continent as large as Eurasia creates a better-defined rainfall zone, even under stronger zonal flow. The baiu front seems to be a rainfall zone caused by the heat contrast between land and ocean that differs from the SPCZ in its formation mechanism.