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 degrees latitude of the equator over the period 1981-2008. It is found that 98.3% of formations occur at SST values exceeding 25.5 degrees 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 degrees C. It is also found that tropical cyclones form over a narrow temperature range with 90.4% forming over SSTs between 27.5 degrees and 30.5 degrees C when the 48-h period is considered. Without consideration of the 48-h period, an SST threshold of 25.5 degrees C is representative with less than 2% of formations occurring below this value. When the 48-h period is considered, 26.5 degrees 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 degrees C) occur in the mean formation temperature. There is no detectable shift of the threshold temperature toward a higher value.