Future Change of North Atlantic Tropical Cyclone Tracks: Projection by a 20-km-Mesh Global Atmospheric Model

Possible future change in tropical cyclone (TC) activity over the North Atlantic (NA) was investigated by comparison of 25-yr simulations of the present-day climate and future change under the A1B emission scenario using a 20-km-mesh Meteorological Research Institute (MRI) and Japan Meteorological Agency (JMA) atmospheric general circulation model. The present-day simulation reproduces many essential features of observed climatology and interannual variability in TC frequency of occurrence and tracks over the NA. For the future projection, the model is driven by the sea surface temperature (SST) that includes a trend projected by the most recent Intergovernmental Panel on Climate Change (IPCC) multimodel ensemble and a year-to-year variation derived from the present-day climate. A major finding is that the future change of total TC counts in the NA is statistically insignificant, but the frequency of TC occurrence will decrease in the tropical western NA (WNA) and increase in the tropical eastern NA (ENA) and northwestern NA (NWNA). The projected change in TC tracks suggests a reduced probability of TC landfall over the southeastern United States, and an increased influence of TCs on the northeastern United States. The track changes are not due to changes of large-scale steering flows; instead, they are due to changes in TC genesis locations. The increase in TC genesis in the ENA arises from increasing background ascending motion and convective available potential energy. In contrast, the reduced TC genesis in the WNA is attributed to decreases in midtropospheric relative humidity and ascending motion caused by remotely forced anomalous descent. This finding indicates that the impact of remote dynamical forcing is greater than that of local thermodynamical forcing in the WNA. The increased frequency of TC occurrence in the NWNA is attributed to reduced vertical wind shear and the pronounced local warming of the ocean surface. These TC changes appear to be most sensitive to future change in the spatial distribution of rising SST. Given that most IPCC models project a larger increase in SST in the ENA than in the WNA, the projected eastward shift in TC genesis is likely to be robust.