Climate Variation and Prediction of Rapid Intensification

One of the greatest challenges in tropical weather forecasting is the rapid intensification (RI) of the tropical cyclone (TC), during which its one-minute maximum sustained wind speed increases at least 30 knots per 24 hours. Here we identify and elucidate the climatic conditions that are critical to the frequency and location of the RI on annual, intraseaonal, and interannual time scales. Whereas RI and formation share common environmental preferences, we found that the proportion of TCs with RI varies annually and from year to year. In August, only 30% of TC actually experiences RI, in contrast to the annual maximum of 47% in November. The proportion of RI in July-September is higher during El Nino years (53%) than the corresponding one in the La Nina years (37%). Three climate factors may contribute to the increase in the proportion of RI: the southward shift in the monthly or seasonal mean location of the TC formation, the increase in the low-level westerly-induced meridional shear vorticity, and the decrease in northerly vertical shear. When the mean latitude of TC formation increases, the mixed-layer heat content decreases while TC's inertial stability increases; both are more detrimental to the RI than to TC formation because the RI requires large amount of latent heat energy being extracted efficiently from the ocean mixed layer and requires accelerated low-level radial inflow that carries latent heat reaching the inner core region.

We further demonstrate that the RI frequency in the Philippine Sea and South China Sea can be predicted 10 to 30 days in advance based on the convective anomalies in the equatorial western Pacific (5ºS-5ºN, 130º-150ºE) on intraseasonal time scale. The NINO3.4 SSTA in June is a potential predictor for the peak TC season (July-September) RI activity in the southeast quadrant of the western North Pacific (0-20ºN, 140-180ºE).

The RI is an essential characteristic of category 4 and 5 hurricanes and super typhoons because all category 4 and 5 hurricanes in the Atlantic basin and 90% of the super typhoons in the western North Pacific experience at least one RI process in their life cycles. Over the past 40 years, the annual total of RI in the western North Pacific shows pronounced interdecadal variation but no significant trend. This result implies that the number of supper typhoons has probably no upward trend in the past 40 years. Our results also suggest that when the mean latitude, where the tropical storms form, shifts southward (either seasonally or from year to year) the proportion of super typhoon will likely increase. This shift is determined by large scale circulation change rather than local SST effects. This idea differs from the notion that increasing SST can lead to more frequent occurrence of Category 4 or 5 hurricanes through changing thermodynamics.