A New Index for Tropical Cyclone Development from Sea Surface Temperature and Evaporation Fields

Abstract

In this study, we present a new index (called the H-index) which is the spatial mean gradient of the logarithm of the surface wind speed with respect to sea surface temperature (SST), and can be easily computed from large scale fields of evaporation and SST Two independent physically based arguments indicate that tropical cyclones would tend to be spun-up in regions of negative H of large magnitude (< -1). In these regions which only occur in the tropics, significant releases of kinetic energy (KE) into the atmosphere from the ocean mixed layer due to convective instability give rise to warm core systems which may develop into intense TCs. The monthly mean histograms of H evaluated from reanalysis and SST data and averaged over the period 1979-2005 for three generation regions (the West Pacific, the Atlantic and the East Pacific) show a remarkable symmetrical pattern in which the standard deviation increases during the active season. This property is therefore ideally suited to being used as a predictor for TC development. The symmetry of the distributions indicates in addition that there are counter-balancing regions of large positive H within the generation regions where the KE is reabsorbed into the potential energy of the mixed layer, and the TCs would be spun-down. The inter-monthly variability of TC counts in the three generating regions during the period 1979-2005 was found to be well predicted by the variability of the standard deviation of H, giving confidence that the method may be used to simulate changes in the occurrence of TCs under global warming using the results of climate models that do not explicitly resolve TCs. The CSIRO Mk3 coupled climate model results predict increases in TC counts of 20%, 50% and 100% respectively in the West Pacific, the East Pacific and the Atlantic, and also a change in their seasonal distribution. In both the West Pacific and the Atlantic a secondary early seasonal maximum occurs in the period 2051-2080, which essentially advances the season in the West Pacific and lengthens the season in the Atlantic.