A new hurricane index for the Caribbean

A new index for hurricane development is introduced which indicates regions of the tropical ocean where tropical cyclones are likely to be generated and regions where existing cyclones would be spun-down. The index physical basis, which can be computed from the fields of sea surface temperature (SST) and evaporation, is briefly discussed. The principal result is that regions favorable to the development of tropical cyclone development are characterized by strong negative spatial gradients of evaporation with respect to SST whereas cyclones are likely to be spun-down in regions of strong positive gradients. The application to the Caribbean region indicates a corridor of favorable conditions in the central Caribbean, possibly promoted by the presence of easterly waves, and a southern unfavorable zone that shelters the Venezuelan coast from hurricane impact. The results show in detail the reasons for the variability in hurricane seasons, using 1983 and 2005 as examples. On the large scale, the sign and strength of the SST dipole anomaly between the Pacific and the Atlantic oceans appears to be the controlling influence, a positive anomaly in the Atlantic Ocean leading to hurricane formation. The monthly standard deviation of the hurricane index simulates reasonably well the historical fluctuations in hurricane occurrence in 1979-2005, and using the results of a coupled climate model, it predicts that the hurricane season in 2051-2080 will be lengthened to include an early season maximum in June and another in September-October in contrast to the current single maximum in September.