Tropical Cyclone Genesis Factors in a Simulation of the Last Two Millennia: Results from the Community Earth System Model

Using a coupled global climate model, Community Earth System Model (CESM), the authors investigate the response of tropical cyclone (TC) genesis factors (i.e., potential intensity, vertical wind shear, midtropospheric moisture content, and absolute vorticity) to external forcings in the last two millennia (L2M). They then examine how the large-scale conditions that favor TC activity varied using a genesis potential index (GPI). These large-scale genesis factors generally exhibit no long-term trend in the simulation of the L2M prior to the industrial revolution, and the spread in the interannual variability lies within a small window. The estimated TC activity is highly variable from region to region on multidecadal time scales. Conditions appear to be more favorable for TC genesis in the twentieth century in the Northern Hemisphere relative to earlier centuries of the L2M. Additionally, conditions in this simulation are not more favorable for TC formation during the Medieval Climate Anomaly (AD 1000–1200) relative to the Little Ice Age (AD 1500–1700) except in the eastern North Pacific and south Indian Ocean. Although a comparison of conditions simulated in the model with proxy-based reconstructions of prehistoric storm activity finds agreement during several active periods in the western North Pacific, the time series of simulated genesis factors does not match that of proxy reconstructions over the entire interval in either the western North Pacific or North Atlantic; this discrepancy likely arises from uncertainties in both the model and reconstructions.