On the variability of tropical cyclones and category shifts

Relationships between oceanic and atmospheric climate conditions and the frequency of occurrence of Tropical Cyclones are becoming better understood. To that end, Colorado State University, the pioneer, the National Oceanic and Atmospheric Administration and North Carolina State University have all gone public with different forecasts of the number of hurricanes that will form in the North Atlantic and additionally, how many will make land-fall. The forecasts have had varying degrees of success but have been qualitatively successful. However, the frequencies of occurrence of the total number of different category events and their relative strength in persistence have not been well revealed or understood. Albeit, several recent studies have reported that Tropical Cyclones in the Atlantic and Pacific Oceans are likely to be more frequent for the next decade or two, and moreover, have become stronger overall. This suggests that there has been a shift towards higher category storms over the past several decades. This study reports on the statistics of the frequency of occurrences of Tropical Cyclones as well as on the statistics of the shifts within the family of categories. Overall trends are presented for Tropical Cyclones in general, for all Saffir-Simpson categories and then for the two major category events. Relationships with climate systems are also causally evaluated.