The Role of the General Circulation in Tropical Cyclone Genesis.

This study investigates the role that large scale circulation patterns play in tropical cyclone genesis. The data available for this study were largely from stations within the eastern hemisphere, accordingly the paper addresses the problem of defining the important flow features observed prior to cyclone genesis in the northwest Pacific and Australian region. A combination of case study and compositing has been used to identify those weather systems which interact with a pregenesis tropical cloud cluster, enhancing its development potential. A comparison of pre cyclone genesis conditions with those observed during non cyclone genesis periods reveals more vigorous Hadley circulations between the subtropical ridges in both hemispheres and the ITCZ. The acceleration of the winter hemisphere Hadley cell in response to cold surge forcing appears to be very dominant. In order to better understand the planetary scale adjustment processes resulting from these winter hemisphere surge situations, a series of simple numerical model experiments have been carried out. The shallow water equations have been solved for an arbitrary time dependent forcing in either wind or height field. Spherical geometry and a realistic pole to pole zonal wind profile have been used. These experiments indicate that energy propagation from the midlatitudes to the deep tropics is dependent upon the scale of the forcing and the background wind field. For northern hemisphere midlatitude forcings which are equivalent barotropic in nature, it is possible to generate large amplitude responses in the southern hemisphere regardless of the intervening wind fields. An important role of the tropics in all simulations appears to be to provide a region where 'down the pressure gradient' flow may readily occur, assisting in the adjustment processes to planetary scale forcings.