POTENTIAL VORTICITY, EASTERLY WAVES, AND EASTERN PACIFIC TROPICAL CYCLOGENESIS

A significant sign reversal in the meridional potential vorticity grad ient was found during the summer of 1991 on the 310-K isentropic surfa ce (near 700 mb) over the Caribbean Sea. The Chamey-Stern necessary co ndition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of Afric an waves or actual generation of easterly waves in the Caribbean. Duri ng the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, conve ctive heating measured by outgoing longwave radiation data, and easter n Pacific cyclogenesis all varied on the timescale of the Madden-Julia n oscillation (MJO). It is hypothesized that upstream wave growth in t he dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstr eam tropical cyclogenesis.