Several previous studies demonstrate a decrease in the frequency of tropical cyclones (TCs) in future climate regimes. A series of high-resolution model simulations comparing current and future seasonal activity reproduce this result. Our goal is to understand precisely why this decrease in TC frequency takes place. An experimental design which isolates the thermodynamic portion of the climate-change signal sheds light on the processes involved in this change. By excluding changes in vertical wind shear and retaining an identical set of incipient disturbances, a direct comparison of current and future TCs can be undertaken. Both an active (September 2005) and inactive (September 2009) period of the TC season are simulated with a Weather Research and Forecasting (WRF) model domain that covers the North Atlantic basin using 6 km grid spacing. A 4-member physics ensemble is composed by varying microphysical and boundary layer parameterization schemes. 21st century temperature and moisture changes are derived from an ensemble of climate simulations from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) A1B scenario and added to analyzed initial and lateral boundary conditions, leaving wind fields unmodified.

Decreases in ensemble-mean TC counts (-18 to -28%) are consistently found. Physical processes that could result in such a change are further examined by comparing monthly- and spatially-averaged genesis-relevant parameters and then “case studies” of development (non-development) of similar initial disturbances in current (future) conditions. Larger moist entropy saturation deficits limit development of future initial vortices, as closing such deficits requires surface fluxes of heat and moisture that could otherwise be used to fuel intensification of the vortex in saturated conditions. Increased convective inhibition is also found in the future genesis environment. It is concluded that decreased TC frequency can result as a consequence of the thermodynamic component of climate change alone, without increased shear. Case studies of circumstances where a developing wave in the present-day environment did not undergo genesis in the future simulation demonstrate that it is the marginally-favorable environments that result in decreases in future storm counts.