Changes in the Tropical Cyclone Steering Flow Along the U.S. Southeast and Gulf Coasts

The large-scale environmental steering flow determines, to first order, the direction and speed of tropical cyclones. As the hemispheric meridional temperature gradient decreases with increased anthropogenic warming, we hypothesize that the tropical cyclone steering flow may weaken concurrently, and increase the frequency of slow-moving tropical cyclones making landfall on U.S. coasts. We analyze 38 years of large-scale deep layer mean (DLM) steering flow during the North Atlantic hurricane season along the southeast and Gulf coasts of the U.S. by splitting DLM wind speed reanalysis data into two 19 year periods and examining differences in DLM wind speeds between the two periods using multiple methods. We discover that, overall, the DLM steering flow along the U.S. coasts was weaker in the 1998-2016 time period compared to the 1979-1997 time period. Further analysis revealed that the DLM wind speed changes varied within the hurricane season. The months of June and November experienced the most significant decreases in DLM wind speed, and although the DLM steering winds remained at seasonally low values in July and August, these two months experienced slight increases in DLM wind speed. Our findings suggest that the time frame for an increased likelihood of slow-moving tropical cyclones impacting the southeastern U.S. may be expanding.