Role of SST in Hurricane Genesis

Sea Surface Temperature. Hurricanes can only form in extensive ocean areas with a surface temperature greater than 26.5 deg C. This is because the warm ocean water provides sensible heat and water vapor that fuels the intense convection of a hurricane, and assists the conversion of a cold-core tropical depression to a warm-core cyclone.

One of the remarkable relationships of tropical cyclone climatology is the existence of a threshold sea-surface temperature below which tropical cyclones do not form. The most favorable regions according to the sea temperature criterion include the northwest Atlantic (including the Gulf of Mexico) and the northwest Pacific. The cooler temperatures of the southwest Atlantic and northeast Atlantic apparently explain why no tropical cyclones form there. Although it is easy to understand a positive correlation of sea-surface temperature with tropical cyclone formation, it is not at all obvious why there should be such a sharp cutoff in formation at 26.5 deg C. Because of the exponential variation of saturation vapor pressure with temperature, there is a rapid increase of saturation specific humidity and equivalent potential temperature with temperature at a constant relative humidity. It is not surprising, therefore, that oceans with temperatures of 30 deg C should produce more tropical cyclones than with 28 deg C. But why should no storms form with sea temperatures below 26.5 deg C? Certainly convective instability and thunderstorms occur over water with much lower surface temperatures. At this point the answer is not known

Hurricanes (or tropical cyclones) are generated in the warm ocean regions shown below. Strictly speaking, the name hurricane is applied only to hurricanes that start in the Atlantic Ocean and that part of the Pacific Ocean east of the International Dateline. Hurricanes that occur in the Pacific Ocean west of the Dateline and the Indian Ocean are called typhoons. Typhoons of the Bay of Bengal and Arabian Sea are locally called cyclones. Lesser known names for hurricanes are "WillyWilly" in Australia, "Baguio" in the Philippines and "Cordonazo" in Mexico. South America is not affected by hurricanes since none form in the South Atlantic Ocean

Fig.1. Regions of cyclone origin and their relation to SST.