The influence of climate state variables on Atlantic Tropical Cyclone occurrence rates

We analyzed annual North Atlantic tropical cyclone (TC) counts from 1871-2004, considering three climate state variables - the El Niño/ Southern Oscillation (ENSO), peak (August-October or 'ASO') Sea Surface Temperatures (SST) over the main development region ('MDR': 6-18°N, 20-60°W), and the North Atlantic Oscillation (NAO) - thought to influence variations in annual TC counts on interannual and longer timescales. The unconditional distribution of TC counts is observed to be inconsistent with the null hypothesis of a fixed rate random (Poisson) process. However, using two different methods, we find that conditioning TC counts on just two climate state variables, ENSO and MDR SST, can account for much or all of the apparent non-random variations over time in TC counts. Based on statistical models of annual Atlantic TC counts developed in this study and current forecasts of climate state variables, we predicted m = 15 ± 4 total named storms for the 2007 season.