Regional Variations in the Rate Tropical Depressions Become Tropical Cyclones

Projected changes in future tropical cyclone (TC) formation are almost entirely dependent on climate models. Understanding projections requires knowledge of the changing climatic conditions that support TC formation in these models and in the real world. In this study we attempt to identify the differences in necessary climatic conditions between early and late stages of TC development, as a means to better understand why TC formation rates change in future climate projections. The early and late development stages are defined, respectively, as the formation of a tropical depression (TD), and the further intensification to TC.

The Okubo-Weiss-Zeta-Parameter (OWZP) TC detection algorithm is adapted to identify a tropical depression (TD) proxy. Applied to 34 years of European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) data, the proportion of these TDs that do not become TCs (TD failure rate) is examined. Large variations in TD failure rate were found between, and within, the major TC ocean basins. One variation that applies globally is larger TD failure rates at higher latitudes, e.g., equatorward and poleward of 15° the TD failure rate is 25% and 38% respectively. A striking regional difference is present in the southern hemisphere poleward of 15°, where about 50% of TDs fail in the South Pacific Ocean, compared to 30% in the South Indian Ocean. These variations are mostly due to differences in climatic conditions that can be summarised by two sets of threshold values. The first set defines where the TD-proxies form, and the second set in conjunction with the first roughly defines where the majority of proxy TDs become TCs. The results suggest that the proxy TDs form in favourable thermodynamic environments, but the further development to a TC requires more specific favourable conditions that allow the juvenile TD vortex to intensify. The results highlight the increasing importance of in-situ energy extraction from the sea surface, and the vulnerability of the vortex to shear as it becomes reliant on this energy source. In regions of very high potential intensity the developing vortices may be more resilient to wind shear, consistent with observational studies that show hurricanes are more resilient to shear over warm seas.