Relative Impacts of Tropical and Extratropical Forcing on Atlantic Tropical Cyclones in Numerical Model Simulations

This study examines the relative impacts of the tropical sea surface temperature anomalies (SSTAs) and the extratropical processes. Seasonal simulations are conducted for 2005 and 2013 from late July to the end of October using the WRF model, in which the NCEP FNL analysis provides the SST, initial and lateral boundary conditions. While 2005 is one of the most active seasons in terms of Atlantic tropical cyclone activity, tropical cyclone activity is below normal in 2013, which has been attributed to the impacts of extratropical forcing in previous studies. The simulations capture the contrast in tropical cyclone activity between the two years. In the third experiment, the WRF model is driven by SST from 2005, and the lateral boundary conditions are taken from 2013. Despite the different SST, tropical cyclone activity in the mixed experiment is similar to the 2013 simulation and is substantially suppressed compared to 2005. Further analyses suggest that the lateral boundary forcing plays a more dominant role in perturbing the mid-upper troposphere than the local sea surface temperature. This study highlights that the remote forcing can play an important role in modulating the Atlantic TC activity and should be considered in the seasonal prediction schemes of TC activity.