A Role of the Tropical Upper Tropospheric Trough in Early Season Typhoon Development

A synoptic model is developed to describe a role of the tropical upper tropospheric trough (TUTT) in early season typhoon development in the western Pacific low-level near-equatorial monsoon trough. The role is threefold: 1) The accompanying sub-equatorial ridge on the south side of the TUTT lies over the low-level trough and decreases the vertical shear; 2) the divergent Row on the south and cast side of a cyclonic cell in the TUTT increases the evacuation aloft to aid in depression development in a low-level trough; 3) a channel to large-scale westerlies is established for efficient outflow of the heat released by increased convection in the developing depression.