Projecting Tropical Cyclone Activity with a Model- and Basin-Independent Detection and Tracking Scheme using CMIP5 Models.

A recently validated technique (OWZP) for detecting and tracking tropical cyclones (TCs) in coarse resolution climate models that circumvents model-resolution was applied to a selected group of 12 models from the Coupled Model Intercomparison Project Phase 5 experiemnts (CMIP5) to assess potential changes in TC track characteristics in the Southern Hemisphere (SH) and the Western North Pacific (WNP) due to greenhouse warming. Current-climate simulations over the period 1970-2000 were first evaluated against the observations using measures of TC genesis location and frequency, as well as track trajectory and lifetime. Results showed substantial model skill in reproducing a realistic TC climatology over the evaluation period, with comparisons to prior studies highlighting the need for an objective detection scheme. Concerns raised regarding model interdependency were addressed with a smaller subset of models chosen for each basin.

We next determined projected changes in TC track density between the current- and future-climate (2070-2100) simulations under the Representatives Concentration 8.5 Pathways (RCP8.5). Overall, projection results showed a substantial decrease in track density over most parts of the SH (~1-3 per decade) and lower and western segments of the WNP (~20% decrease) by the end of the twentieth-century. These decreases were attributed to significant reductions in TC numbers (~15-42%) driven by changes in large-scale environmental parameters such as relative vorticity, environmental vertical wind shear and relative humidity. In the eastern and upper segments of the WNP basin significant increases in TC activity were found, eclipsing that which was expected solely from the increase in TC numbers (particularly in Korea and Japan). Likewise, projected TC activity off the eastern coast of Australia was also found to marginally increase (~1 per twenty years). Hypotheses behind these increases in TC activity are discussed within.