Tropical Cyclone Movement and Surrounding Flow Relationships

This paper presents results of a comprehensive study of the relationship between the movement of tropical cyclones and the large-scale circulation which surrounds them. Cyclones have been stratified by direction and speed of movement, latitude, intensity change and size (as determined by the radius of the outermost closed surface isobar) in three ocean basins: the northwest Pacific, the west Atlantic and the Australian-South Pacific region. Twenty-one different stratifications are available in the northwest Pacific, 13 in the west Atlantic and 6 in the Australian-South Pacific area. Cyclone movement and surrounding flow relationships were studied at different pressure levels and a variety of radii. Pressure-weighted layer-averages were also analyzed in search of such relationships. Results show an important relationship between surrounding large-scale flow and tropical cyclone movement. For all stratifications, the winds in the mid-troposphere (500–700 mb) at 5–7° latitude radius from the cyclone center have the best correlation with cyclone movement. Tropical cyclones in the Northern Hemisphere move ∼10–20° to the left of their surrounding mid-tropospheric flow at 5–7° latitude radius, and those in the Southern Hemisphere move ∼10° to the right. It is also found that cyclones, in general, move ∼1 m s−1 faster than this flow. These general relationships appear to be modified by the vertical shear of the environmental wind, the zonal component of the cyclone velocity and other characteristics of the cyclone. The mean tropospheric flow (surface to 100 mb) at 5–7° latitude radius also correlates well with cyclone movement in most cases. For cyclones embedded in an environment with relatively small vertical wind shear, the mid-tropospheric flow is as good a descriptor of cyclone motion as the mean tropospheric flow. The average wind between the upper (200 mb) and lower (900 mb) troposphere also appears to correlate reasonably well with cyclone movement.