初夏の東アジアおよび西部北太平洋域にみられる対流活動と循環場の10-25日季節内変動

Abstract

The space-time evolution of intraseasonal convection and circulation anomalies in the 10-25-day period range over East Asia and western North-Pacific Monsoon regions are explored for four early summer seasons from 1991 to 1994. The paper primarily focuses on the linkage between tropical convection over the South China Sea, and lower-tropospheric circulation over the East Asian subtropics on this time scale. The composite results show that quasi-periodic fluctuations of convection on the 10-25-day time scale over the South China Sea, are associated with large-scale circulation in the Asian-Pacific region. The development of subtropical circulation and convection takes place through changes in large-scale circulation on this time scale. 10-25-day enhanced (suppressed) convection occurs in conjunction with well-organized cyclonic (anti-cyclonic) circulation anomalies over the South China Sea. A downstream wave train extending from the South China Sea into the North Pacific, is built up in the lower troposphere associated with this convective activity. This wave train is probably interpreted as a Rossby mode response to the anomalous heating (cooling). Following the peak of enhanced convection, the East Asian subtropical anomalous anti-cyclone maintains its strength as part of this wave train, and couples with enhanced subtropical convection along the northern flank of this anti-cyclonic cell. This development of convection in the subtropical frontal zone is due to an increase in the low-level westerlies at the northern flank of this anti-cyclonic circulation. In addition, this subtropical anomalous anti-cyclone moves southwestward from the subtropics into the South China Sea region, and appears to initiate subsequent suppressed convection. In contrast, the southwestward migration of subtropical cyclonic anomalies also occurs following the inactive convection over the South China Sea, and this behavior appears to trigger the subsequent active convection over this region. These features suggest that mutual interactions between the tropics and the subtropics on this time scale play important roles on the variability of monsoon convection and circulation over East Asia, and western North Pacific. In the upper level strongest divergent outflow (convergent inflow) occurs in the convective (non-convective) region and the significant wave train is maximally intensified in the mid-latitude at the convective peak. Upper-level circulation also appears to be forced by heating (cooling) located over the South China Sea region. These circulation features confirm that 10-25-day variations of convection over the South China Sea are one of the effective forcings for the large-scale circulation during the early summer season.