Toward a unified system for understanding, predicting and projecting regional hurricane activity

A family of high-resolution (50km and 25km atmospheric/land resolution) global coupled climate models provide a unified framework towards the understanding, intraseasonal-to-decadal prediction and decadal to multi-decadal projection of regional and extreme climate, including tropical cyclones. Initialized predictions of global hurricane activity show skill on regional scales, comparable to the skill on basin-wide scales, suggesting that regional seasonal TC predictions may be a feasible forecast target. The 25km version of the model shows skill at seasonal predictions of the frequency of the most intense hurricanes (Cat. 3-4-5 and Cat. 4-5). It is shown that large-scale systematic errors in the mean-state are a key constraint on the simulation and prediction of variations of regional climate and extremes, and methodologies for overcoming model biases are explored. Improvements in predictions of regional climate are due both to improved representation of local processes, and to improvements in the large-scale climate and variability from improved process representation.

These models are used to explore the the response of tropical cyclones, both globally and regionally, to increasing greenhouse gases and to internal climate variations. The 25km model in generally shows a more faithful representation of the impact of climate variability on hurricane activity than the 50km model. The response of the total number and the total power dissipation index of tropical cyclones to increasing greenhouse gases can differ substantially between models of two atmospheric resolutions, 50km and 25km - with the 25km version of the model showing a larger increase in power dissipation from increasing greenhouse gases, principally because - in contrast to that of the 50km model - its global hurricane frequency does not decrease with increasing CO2. Some thoughts on the reasons behind those differences will be offered. The 25km model shows an increase in the frequency of intense tropical cyclones under increasing greenhouse gases, including the development of a substantial number of "Category 6" cyclones (storms with maximum winds exceeding 80m/s) that are not present in the control climate.