Large Scale Control of Seasons with Extreme Tropical Cyclone Activity in the North Atlantic

The North Atlantic basin has particularly large interannual variability in tropical cyclone activity. The most extreme years, such as 2005 and 2020, see particularly large deviations, with much larger-than-average numbers of storms, including some intense ones, and large losses as some of them make landfall. Existing genesis potential indices, which relate the probability of cyclogenesis to large-scale conditions, do not explain such extreme seasons; given the environmental predictors(conditions), they predict numbers of storms that are above average, but not nearly as large as observed. This study aims to understand what is missing from these indices that could be responsible for such extreme years. Genesis indices are dependent on both thermodynamic and dynamic components, currently monthly averaged large scale data is used to estimate these genesis indices. Genesis indices may be sensitive with different temporal scales of large scale flow, hence here we investigate sensitivity of different temporal scales of large scale variables with TC count. High resolution downscaled WRF and ensembles of synthetic tracks from one or more statistical-dynamical downscaling models, forced by observation-based reanalysis data, are also analyzed to see whether the high resolution and stochastic variability in these models might help explain the observations for extreme years 2005 and 2020 in the NA Basin.