Similarity and difference between the south Atlantic convergence zone and the baiu frontal zone simulated by an AGCM

摘要

Features of the South Atlantic convergence zone (SACZ) and the Baiu frontal zone (BFZ) simulated by an AGCM (T106L56: a primitive equation spectral model which has 56 sigma-levels and triangular spectral truncation at wave-number 106) are studied. The 24-year integration, from 1979 to 2002, by the model constrained by observed sea-surface temperature and sea-ice distribution, is used for this study. The detailed analysis is made for the typical case of SACZ and BFZ selected from data in 1985-1996. The South American monsoon circulation and the associated precipitation are reasonably reproduced. The precipitation zone of the SACZ, which extends southeastward from the southern part of Brazil to the South Atlantic, is sustained along the southwestern rim of the South Atlantic subtropical anticyclone during the southern summer. The Asian monsoon circulation and the associated precipitation are also reasonably reproduced. The precipitation zone of the BFZ, which extends northeastward from the southern part of China to Japan is formed along the northwestern rim of the North Pacific subtropical anticyclone in June. Many common features are found in the simulated SACZ and BFZ, in regard to the frontal structure and the associated synoptic- and meso-alpha-scale disturbances, and the relation to the respective subtropical anticyclone. However, an important difference is seen between their geographical environments. The cold south Atlantic in the poleward side of the SACZ provides the significant baroclinicity for SACZ, while the existence of warm land mass to the poleward side of the BFZ brings on weak baroclinicity of the BFZ. Another significant difference is seen in the large-scale environmental condition. The SACZ is influenced, in essence, by the trade winds and the subtropical anticyclone over the South Atlantic. The BFZ is strongly influenced by the moisture transport due to the Indian monsoon westerly and variation of the North Pacific subtropical anticyclone related to the convective activity over the western North Pacific.