Hurricane Sally (2020) Shifts the Ocean Thermal Structure across the Inner Core during Rapid Intensification over the Shelf

Abstract

Prediction of rapid intensification in tropical cyclones prior to landfall is a major societal issue. While air-sea interactions are clearly linked to storm intensity, the connections between the underlying thermal conditions over continental shelves and rapid intensification are limited. Here, an exceptional set of in-situ and satellite data are used to identify spatial heterogeneity in sea surface temperatures across the inner core of Hurricane Sally (2020), a storm that rapidly intensified over the shelf. A leftward shift in the region of maximum cooling was observed as the hurricane transited from the open gulf to the shelf. This shift was generated, in part, by the surface heat flux in conjunction with the along and across-shelf transport of heat from storm-generated coastal circulation. The spatial differences in the sea surface temperatures were large enough to potentially influence rapid intensification processes suggesting that coastal thermal features need to be accounted for to improve storm forecasting as well as to better understand how climate change will modify interactions between tropical cyclones and the coastal ocean.

Spatially and Temporally Variable Impacts of Hurricanes on Shallow Sediment Structure

“…Storm surge caused by Hurricane Sally only reached around 1 m in the study area, and Dauphin Island experienced little overwash and no channel cutting (Berg & Reinhart, 2021;Frank-Gilchrist et al, 2023). Bottom current measurements at CP also did not exhibit strong offshore currents after Sally that would be expected with substantial surge ebb (Figure 2d, Figures S1a, S1b, S1e, and S1f in Supporting Information S1; Dzwonkowski et al, 2022). In fact, the strong offshore winds from the northeast as Sally approached landfall caused a substantial drop in water level in Mobile Bay (Figure 2e; Berg & Reinhart, 2021).…”

“…This study, however, demonstrates the benefits of coastal hydrological monitoring in addition to opportunistic pre‐ and post‐storm sediment sampling. Sediment sampling over a relatively large area (<500 km 2 ) and continuous current measurements (Figure 2d, Figure S1 in Supporting Information ; Dzwonkowski & Lockridge, 2021; Dzwonkowski et al., 2022) captured spatial variability in post‐storm sediment changes that were consistent with the offshore deflection of storm‐generated sand transport by the Mobile Bay ebb tidal delta, resulting in sand deposition in a narrow offshore area (Figures 1b and 3, Figures S6 and S7 in Supporting Information ). These results demonstrate the importance of sediment sampling to account for storm current interactions with bathymetry.…”

“…In fact, the strong offshore winds from the northeast as Sally approached landfall caused a substantial drop in water level in Mobile Bay (Figure 2e; Berg & Reinhart, 2021). Relatively strong onshore flow occurred immediately after landfall (Figure 2d, Figures S1b and S1d in Supporting Information S1; Dzwonkowski et al, 2022), indicating that sediment transport during Hurricane Sally likely did not occur via storm surge ebb.…”

“…Thin black lines show selected isobaths. Figure modified and adapted from Dzwonkowski et al., 2022 with permission (https://www.ametsoc.org/index.cfm/ams/publications/ethical-guidelines-and-ams-policies/ams-copyright-policy/).…”

“…Hurricane Sally crossed the Alabama shelf 15–16 September 2020, rapidly intensifying from a Category 1 to 1 knot short of a Category 3 hurricane with a well‐developed eyewall, producing maximum sustained winds of 48.9 m s −1 . The storm made landfall near Gulf Shores, AL, around 0345 CST on 16 September 2020 (Figure 1a; Berg & Reinhart, 2021; Dzwonkowski et al., 2022). Six weeks later, Hurricane Zeta made landfall in southeastern Louisiana at around 1600 CST on 28 October 2020, rapidly intensifying from a tropical storm to a Category 3 hurricane just before landfall (Figure 1c; Blake et al., 2021).…”