SST explains a much smaller percentage of tropical cyclone intensification rates (kt (36hr)^-1) in the Atlantic compared to the Pacific. Shown are scatter-plots of tropical cyclone intensification rate and pre-storm SST, both taken at 6-hour intervals along each storm’s track for 1998-2012 in the a) western Pacific (100°-180°E, 0°-30°N), b) eastern-central Pacific (90°-180°W, 0°-30°N), and c) Atlantic (20°-100°W, 0°-30°N). Numbers in each plot indicate the percentage of the variance in intensification rate that is explained by SST, calculated as the square of the correlation coefficient. Click to enlarge.

Predictions of hurricane intensity have improved only modestly in recent decades despite significant reductions in forecasted track errors. Part of the reason is that a hurricane’s future path is strongly influenced by the large-scale atmospheric circulation, which has become more predictable with advances in numerical weather prediction. In contrast, there are many factors that contribute to intensity changes, such as sea surface temperature (SST), vertical wind shear, humidity, outflow temperature, and small-scale atmospheric variability. It is therefore important to understand each of these parameters and their relationships with hurricane intensity. SST is the most important predictor at lead times longer than 24 hours because of its influences on air temperature, humidity, atmospheric stability, and wind shear.

In a recent paper published in Monthly Weather Review, the authors show that SST explains a significantly smaller percentage of the variance in tropical cyclone intensification in the Atlantic (4%), compared to the western North Pacific (12%) and eastern-central Pacific (23%). They find that several factors are responsible for these inter-basin differences. In the Atlantic, tropical cyclones experience the weakest variability of SST along their tracks, due to small spatial variability of SST and the tendency for storms to travel along mean SST gradients instead of across them. These factors limit the strength of the relationship between SST and hurricane intensification.

Storms in the Atlantic also tend to be strongest in the western basin, where the SST is warmest, as compared to other basins, where SST is not as well correlated with intensity. Since stronger storms are less likely to intensify further, warmer underlying SST in the Atlantic does not generally translate to higher intensification rates. In the Atlantic, storms also move more slowly in the west, where SST is higher. These combined factors generate longer lasting and more vigorous storm-induced upper-ocean mixing and SST cooling, acting as a negative feedback on storm intensity. The results from this study may help to explain why operational statistical intensity forecast models explain less of the observed variance in tropical cyclone intensities in the Atlantic (45%) compared to the eastern-central and western North Pacific (about 60% each).