Large-Scale Common Features of Sub-Tropical Convergence Zones (the Baiu Frontal Zone, the SPCZ, and the SACZ) Part II : Conditions of the Circulations for Generating the STCZs

Abstract

The Baiu/Meiyu frontal zone (BFZ), the South Pacific convergence zone (SPCZ), and the South Atlantic convergence zone (SACZ) are significant convergence zones in the sub-tropics. These zones (hereafter Sub-Tropical Convergence Zones ; STCZs) are significant sub-tropical frontal zones characterized with strong moisture convergence, frontogenesis in equivalent potential temperature fields, and generation of convective instability. To clarify the reasons why the STCZs form in the specific areas, we investigated large-scale circulations in the sub-tropics globally compared with those around the STCZs. Moreover, the large-scale circulations around the STCZs were compared between active and inactive periods of the STCZs. The STCZs appeared where the following two conditions were quasi-stationarily satisfied ; (1) sub-tropical jets flow in the sub-tropical latitudes (30°-35°) and (2) low-level poleward flows prevail along the western peripheries of the sub-tropical highs. Over the areas where one or two of the conditions were not satisfied, no, or only weak, precipitation areas appear in the sub-tropics. The STCZs were intensified (weakened) when the two conditions were satisfied (not satisfied). Moreover, significant precipitation zones similar to the STCZs also temporarily appeared elsewhere in the sub-tropics, if the two conditions were satisfied there even for a short period. The poleward flows were indispensable for the strong moisture convergence, the frontogenesis in equivalent potential temperature fields, and the generation of convective instability, in the sub-tropics, which are the characteristics of the sub-tropical frontal zones. The poleward flows were formed geostrophically in the eastward pressure gradient between the sub-tropical highs and heat lows developed in the high-pressure-zones in the sub-tropics. Monsoon convections and land-surface heating over the continents were important as heat sources to form the heat lows.