Momentum Transports Associated with Tropical Cyclone Recurvature

Abstract

This study attempts to investigate the linear momentum budget responsible for tropical cyclone (TC) recurvature. Using the operational analyses from the U.K. Meteorological Office global model, the environmental flow associated with recurving TCs over the western North Pacific for the years 1991?95 is composited. In general, the dominant features include an eastward-retreating subtropical ridge (STR) and an approaching westerly trough during the recurvature. While the southeasterly flow associated with the STR changes to southerly in the northeast quadrant of the TC in the mid- to upper troposphere, the southwesterly flow associated with the trough penetrates into the northwest quadrant of the TC, especially in the upper troposphere. These changes of the environmental flow throughout the recurvature alter the linear momentum of the TC from southeasterly to southwesterly. To understand the dynamical processes responsible for the linear momentum changes, individual momentum tendency terms are calculated. In the zonal direction, throughout the recurvature, the southerly component from the environment is found to play an important role for the increase of the net zonal momentum tendency through the earth momentum advection and the zonal component of the Coriolis force. The contribution of the environmental westerlies through the advection of westerly momentum is very small and is eventually cancelled by the negative advection associated with the southerly flow. The net positive meridional force appears to be the main contributor of the meridional tendency component. The negative meridional momentum advection term during and after recurvature only weakens the net meridional momentum tendency at the later stage.