The effects of early season ENSO on peak season tropical cyclones in the Northern Hemisphere

36 years (1973–2008) of best-track data for tropical cyclones in the western North Pacific (WNP), eastern North Pacific (ENP) and North Atlantic are analyzed with Sea Surface Temperature anomaly (SSTA) averages in the Niño-3.4 region. Tropical cyclone characteristics including formation region, maximum intensity and track tendency have been shown to be tied to the strength of the SSTAs - particularly for Strong El Niño/La Niña events in the WNP, but also for the eastern North Pacific (ENP) and North Atlantic. Previous work has focused on tropical cyclone characteristics with respect to the SSTA during the same time period. This paper compares the three-month SSTAs during the May, June, July (MJJ) season with tropical cyclone formation and development trends during the Northern Hemisphere peak season of August, September and October (ASO). Due to the strong correlation between the MJJ seasonal SSTAs and the seasonal SSTA during the peak tropical cyclone months (ASO), it is possible to predict general trends for tropical cyclones forming 01 August through 31 October up to three months in advance.

This study shows that among the categorizations of the Niño 3.4 SSTA, Strong El Niño years, which are defined as years with SSTAs greater than 0.5°C, have the most statistically significant impact on the numbers of tropical cyclones, their formation location and their maximum intensity in all three basins. The yearly averages for these characteristics are correlated to the yearly MJJ SSTAs for each basin. The WNP has the strongest correlations, followed by the ENP. The North Atlantic has the weakest yearly correlation between tropical cyclone characteristics and MJJ SSTAs. There is a strong relationship between higher SSTAs in MJJ and tropical cyclone development farther south and east in the WNP. Also, a higher occurrence of typhoons is observed during warmer years whereas lower maximum wind speed is shown in the colder years in the WNP. In the ENP, tropical cyclones tend to form farther south and have a larger life span in warmer years. Finally, Strong El Niño years are tied to fewer hurricanes and lower maximum wind intensity in the North Atlantic.