Abstract
Almost 70 years ago a sea surface temperature (SST) threshold of 26°–27°C, below which tropical cyclones (TCs) did not form, was proposed, based on a qualitative assessment of warm-season global SST and known TC formation regions. This threshold was widely accepted without further testing, until a recent study suggested a threshold of 25.5°C. That study is revisited here by reexamining the SST for all global TC formations from 1981 to 2008 using (i) a broader range of SST threshold values, (ii) an improved method for identifying subtropical storms—any storm that forms poleward of the subtropical jet (STJ), and (iii) a range of TC formation gestation periods, which refers to a time interval prior to formation in which the SST threshold is exceeded for at least one 6-h period. Consequently, thresholds reported in this paper are expressed as a combination of SST and gestation period.
Using the STJ position to identify and exclude subtropical storms, the threshold of 25.5°C SST–48-h gestation period was found to be robust, but conservative. An examination of TCs of questionable validity (e.g., weak, short lived, and/or storms that formed with baroclinic influences) revealed a further 26 storms (1.2%) that could arguably be excluded from the analysis. With these storms removed, several SST–gestation period threshold combinations were found to be valid, including 25.5°C–18 h and 26.5°C–36 h. A practical threshold combination of 26.5°C–24 h is proposed as only two additional storms failed to meet this threshold, which supports the often-quoted 26.5°C SST necessary condition for TC formation.
