Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century

Abstract

A recently developed technique for simulating large [O(10(4))] numbers of tropical cyclones in climate states described by global gridded data is applied to simulations of historical and future climate states simulated by six Coupled Model Intercomparison Project 5 (CMIP5) global climate models. Tropical cyclones downscaled from the climate of the period 1950-2005 are compared with those of the 21st century in simulations that stipulate that the radiative forcing from greenhouse gases increases by over preindustrial values. In contrast to storms that appear explicitly in most global models, the frequency of downscaled tropical cyclones increases during the 21st century in most locations. The intensity of such storms, as measured by their maximum wind speeds, also increases, in agreement with previous results. Increases in tropical cyclone activity are most prominent in the western North Pacific, but are evident in other regions except for the southwestern Pacific. The increased frequency of events is consistent with increases in a genesis potential index based on monthly mean global model output. These results are compared and contrasted with other inferences concerning the effect of global warming on tropical cyclones.

Keywords: climate change; natural hazards.

Fig. 1.

Global annual frequency of tropical cyclones averaged in 10-y blocks for the period 1950–2100, using historical simulations for the period 1950–2005 and the RCP8.5 scenario for the period 2006–2100. In each box, the red line represents the median among the six models, and the bottom and tops of the boxes represent the 25th and 75th percentiles, respectively. The whiskers extend to the most extreme points not considered outliers, which are represented by the red + signs. Points are considered outliers if they lie more than 1.5 times the box height above or below the box.

Fig. 2.

Change in track density, measured in number of events per 4° × 4° grid box per year, averaged over the six models. The change is the average over the period 2006–2100 minus the average over 1950–2005. The white regions are where fewer than five of the six models agree on the sign of the change.

Fig. 3.

As in Fig. 1, but for the power dissipation index. Units are .

Fig. 4.

Change in Power Dissipation Index averaged over the six models, per 4° latitude grid box. This is defined as the difference between power dissipation averaged over the period 2006–2100 and that averaged over 1950–2005. Units are per 4° × 4° square, and white areas show regions in which fewer than five of the six models agree on the sign of the change.

Fig. 5.

Annual downscaled global tropical cyclones (green) and GPI given by Eq. 2 (red). Both quantities have been averaged over the six models. The green shading shows one SD up and down among the six downscaled storm counts.