The Global Relation Between Sea Surface Temperature and Evaporation

The starting point for the analysis is the global relation between sea surface temperature and evaporation. Here, we use the monthly sea surface temperature data from the Hadley Centre (Rayner et al., 2003) and the monthly evaporation data computed from the NCEP reanalysis (Kalnay et al., 1996), from which the annual average fields have been computed. Each field is then zonally averaged over the ocean—on the T62 Gaussian grid of the NCEP data. The results for the period (1979-2001) for each hemisphere are plotted as lnE versus T, where E is the evaporation rate, and T is the sea surface temperature (SST), (Figs. 1 and 2). The data points correspond with the individual years of the 23 year record for each Gaussian grid. In the subtropics, especially in the southern hemisphere, the 23 years at each latitude are tightly clustered Similar results, obtained from the other two reanalysis products, ERA40 (Uppala et al., 2004) and NCEP2 (Kanamitsu et al., 2002) are given in Bye and Keay (2006). It is apparent that the evaporation has a maximum at about 26°C, which is the commonly stated threshold criterion for tropical cyclone (TC) development, and also that at higher temperatures E decreases. The physical processes operating in this tropical region will be considered in Discussion after the results of the analysis have been presented.

Fig. 1 ln E versus T Northern Hemisphere, from Bye and Keay (2006)

Fig. 1 ln E versus T Northern Hemisphere, from Bye and Keay (2006)

Fig. 2 ln E versus T Southern Hemisphere, from Bye and Keay (2006)

Fig. 2 ln E versus T Southern Hemisphere, from Bye and Keay (2006)

This global signal has prompted the investigation carried out below which is aimed at developing a method of estimating the likelihood of TC development using large scale fields of SST and E.

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