Vortex Tracking and Tropical Cyclone Genesis Factors in Simulations of the Last Glacial Maximum

Orbital geometry determines the seasonal distribution of insolation received at the top of the atmosphere, varying over an approximately 21,00 year cycle. We examine the effects of this cycle on the tropical climate in a series of 1° x 1° CCSM simulations for three scenarios: a control case representative of the 20th century, a period in the mid-Holocene (6,000 years ago) when the perihelion occurs during the peak of the modern Northern Hemisphere hurricane season, and 21,000 years ago during the Last Glacial Maximum (LGM). A variety of General Circulation Models (GCMs) simulate the impact of climate change on tropical cyclones (TCs) in our current and for future climates. We are interested in how these same models simulate TCs in different climates.

Utilizing a detecting and tracking algorithm used for seasonal forecasts of TCs in GCMs, we determine the sensitivity of the genesis and frequency of TCs. We provide an analysis of the change in significant environmental factors (e.g. vorticity, wind shear, and available moisture) during periods of storm intensification. In addition, we compare the TC seasonal frequency with genesis factors. The orbital geometry during the mid-Holocene yields a moderate seasonality in Northern Hemisphere temperature and TC occurrence. In addition, a shift in the timing of peak hurricane season relative to our current climate is also seen. Although orbital geometry during the LGM is comparable to that in our current climate, yielding similar seasonal TC variability, cooler climatic conditions cause a decrease in the total frequency.