The Threshold Sea Surface Temperature Condition for Tropical Cyclogenesis

The analyzed value of sea surface temperature (SST) is examined for all global cases of tropical cyclone formation within 35° latitude of the equator over the period 1981–2008. It is found that 98.3% of formations occur at SST values exceeding 25.5°C. This practical threshold is relatively insensitive to the exact value of maximum wind speed used to define formation. The threshold is sensitive, however, to short-term variations in SST during development. By expanding the time period such that thresholds are calculated based on the maximum SST during the 48-h period leading to genesis, it is found that 99.5% of cyclone formations occur above 25.5°C. It is also found that tropical cyclones form over a narrow temperature range with 90.4% forming over SSTs between 27.5° and 30.5°C when the 48-h period is considered. Without consideration of the 48-h period, an SST threshold of 25.5°C is representative with less than 2% of formations occurring below this value. When the 48-h period is considered, 26.5°C is the equivalent threshold. The response to warming of the global oceans is also examined. Dividing the 27-yr dataset into an earlier versus a later 13.5-yr period, positive but small changes (+0.2°C) occur in the mean formation temperature. There is no detectable shift of the threshold temperature toward a higher value.