Tropical Cyclone Activity Downscaled from NOAA-CIRES Reanalysis, 1908-1958

A recently developed technique for deducing tropical cyclone activity from global reanalyses and climate models is applied to a reanalysis of the global atmosphere during the period 1908-1958. This reanalysis assimilates only sea surface temperature, sea ice, and surface pressure observations, which are relatively homogeneous over the period. The downscaling technique has been shown to produce results in good agreement with observations of tropical cyclones when driven by reanalyses over the period 1980-2006, a period when global tropical cyclone frequency was robustly observed. When applied to the 1908-1958 reanalysis, the derived global frequency of tropical cyclones shows no significant trend over the period, while the frequency of events in the southern hemisphere shows a statistically significant decline and that of the northern hemisphere shows a marginally significant increase. There are statistically significant increases in frequency over the period in the North Atlantic, eastern North Pacific, and northern Indian Oceans, while frequency declines in the western North Pacific. Power dissipation estimates from best-track data are highly correlated with the power dissipation of downscaled events in the Atlantic, though the amplitude of the variability and trends of the downscaled power dissipation are smaller than those of the best-track estimates by about a factor of two. A recently developed genesis index applied to the reanalysis data is highly correlated with downscaled event frequency on regional spatial scales, but is largely uncorrelated at the scale of the globe and even on the scale of large tropical cyclone-producing regions such as the western North Pacific. Finally, while it is tempting to believe that specification of sea surface temperature is sufficient for capturing most aspects of the general state of the atmosphere relevant to tropical cyclones, we show, using simple arguments, that failure to account for changing radiative properties of the atmosphere can distort the response of tropical cyclone activity to changing distributions of sea surface temperature; moreover, models appear to systematically underestimate the response of near-tropopause temperatures to changing surface temperature, and this too can affect the response of potential intensity.