It is often stated that tropical cyclogenesis occurs within pre-existing disturbances. One type of pre-existing disturbance is an easterly wave. In his study of the development of easterly waves, Burpee (1972) found a reversal of the meridional potential vorticity (PV) gradient over the African continent.
The presence of this PV sign reversal satisfies the Charney-Stern necessary condition for instability of the mean flow. Burpee suggested that this region was the energy source for African easterly waves. Recently Molinari et al. (1997) found a sign reversal of the meridional PV gradient in the Caribbean and eastern Pacific. They speculate that easterly waves can be reinvigorated or develop within the Caribbean sign reversal. These papers reflect the potential importance of sign reversals in spawning the precursor disturbances to tropical cyclogenesis.
The emphasis of the current work is to present a global climatology of PV sign reversals using ten years of ECMWF analyses. The relationship between the PV gradient sign reversal climatology and deep tropical convection, as measured by outgoing longwave radiation (OLR), will also be examined. Dickinson (1997) showed that during TOGA COARE, PV gradient sign reversals can develop on the poleward side of deep convection. These sign reversals were often found in a region of rapid poleward decrease of deep convection. Using OLR as a surrogate for diabatic heating, we hope to estimate the contribution to the PV production by the horizontal and vertical diabatic terms in the PV equation.
References:
Burpee, R. W., 1972: The origin and structure of easterly waves in the lower troposphere of North America. J. Atmos. Sci., 29, 77 - 90.
Dickinson, M. J., 1997: Potential vorticity, easterly waves, and tropical cyclogenesis. Preprint, 22nd Conf. on Hurricanes and Tropical Meteorology, Ft. Collins, Colorado, Amer. Meteor. Soc., 549-550.
Molinari, J., D. Knight, M. Dickinson, D. Vollaro, and S. Skubis, 1997: Potential vorticty, easterly waves, and eastern Pacific tropical cyclogenesis. Mon. Wea. Rev., 125, 2699 - 2708.