Journal of Climate, in press.

The effect of greenhouse-gas-induced changes in SST on the seasonality of tropical precipitation

John Dwyer
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY

Suzana J. Camargo
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY

Adam Sobel
Department of Applied Physics and Applied Mathematics, and Lamont-Doherty Earth Observatory, Columbia University, New York, NY.

Michela Biasutti
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY

Kerry A. Emanuel
Program in Atmospheres, Oceans and Climate, Massachusetts Institute of Technology, Cambridge, MA

Gabriel A. Vecchi and Ming Zhao
NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, NJ

Michael K. Tippett
Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY.

Abstract

This study investigates projected changes in the length of the tropical cyclone season due to greenhouse gas increases. Two sets of simulations are analyzed, both of which capture the relevant features of the observed annual cycle of tropical cyclones in the recent historical record. Both sets use output from the general circulation models (GCMs) of the CMIP3 or CMIP5 suites. In one set, downscaling is performed by randomly seeding incipient vortices into the large-scale atmospheric conditions simulated by each GCM and simulating the vortices’ evolution in an axisymmetric dynamical tropical cyclone model; in the other, the GCMs’ sea surface temperature (SST) is used as the boundary condition of a high-resolution, global atmospheric model (HIRAM). The downscaling model projects a longer season (in the late 21st century compared to the 20th) in most basins when using CMIP5 data, but a slightly shorter season using CMIP3. HIRAM with either CMIP3 or CMIP5 SST anomalies projects a shorter tropical cyclone season in most basins. Season length is measured by the number of consecutive days that the mean cyclone count is greater than a fixed threshold, but other metrics give consistent results. The projected season length changes are also consistent with the large-scale changes, as measured by a genesis index of tropical cyclones. The season length changes are mostly explained by an idealized year-round multiplicative change in tropical cyclone frequency, but additional changes in the transition months also contribute.