The Influence of Coupled Model Sea Surface Temperature Biases on Tropical Cyclone Environmental Conditions

Most coupled climate models within CMIP5 struggle to accurately simulate all aspects of the climatological global sea surface temperature (SST) distribution, especially in regions of coastal and equatorial upwelling. Of particular interest in this project are the tropics and the potential impacts of model SST biases on environmental conditions that can influence TC characteristics. To evaluate this, two atmospheric general circulation models (AGCMs), the ECHAM5 and CAM5, were forced with observed SSTs (control run) and observed SST plus the climatological, multi-model (31) mean CMIP5 monthly SST bias (bias run), both for the period 1979-2004. A set of 16 ensemble members was generated for each model by using slightly different initial atmospheric conditions. The output from these two sets of model runs was then utilized to evaluate the Tropical Cyclone Genesis Index (TCGI; Tippett et al. 2011) and the hurricane Potential Intensity (PI; Emanuel 1988, Bister and Emanuel 2002) for the globe. Results using monthly mean atmospheric conditions from the control runs of ECHAM5 and CAM5 were found to produce similar results with those obtained using daily NCEP reanalysis data. Comparing results from the bias and control runs, the CMIP5 climatological SST biases are found to result in statistically significant regional variations in both the TCGI and PI. In some locations, the proxies for TC frequency (TCGI) and intensity (PI) are enhanced, in others they are reduced. The sensitivity of the TCGI to the individual component variables was also examined and tested for statistical significance.