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 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.

Changes in tropical cyclone intensity prior to landfall represent a significant risk to human life and coastal infrastructure. Such changes can be influenced by shelf water temperatures through their role in mediating heat exchange between the ocean and atmosphere. However, the evolution of shelf sea surface temperature during a storm is dependent on the initial thermal conditions of the water column, information that is often unavailable. Here, observational data from multiple monitoring stations and satellite sensors were used to identify the sequence of events that led to the development of storm-favorable thermal conditions in the Mississippi Bight prior to the transit of Hurricane Sally (2020), a storm that rapidly intensified over the shelf. The annual peak in depth-average temperature of >29°C that occurred prior to the arrival of Hurricane Sally was the result of two distinct warming periods caused by a cascade of weather events. The event sequence transitioned the system from below average to above average thermal conditions over a 25-d period. The transition was initiated with the passage of Hurricane Marco (2020), which mixed the upper water column, transferring heat downward and minimizing the cold bottom water reserved over the shelf. The subsequent reheating of the upper ocean by a positive surface heat flux, followed by a period of downwelling winds, effectively elevated shelf-wide thermal conditions for the subsequent storm. The climatological coupling of warm sea surface temperature and downwelling winds suggest regions with such characteristics are at an elevated risk for storm intensification over the shelf.

Extensive research has shown that wind has a strong influence on estuarine circulation and salt transport. However, the response to wind forcing in estuarine systems presents challenges due in part to the complexities of realistic forcing conditions, system states, and geomorphologies. To further advance the understanding of estuarine responses to wind forcing, a comprehensive analysis of stratification and mixing during a typical southeast wind event was conducted in Mobile Bay, a microtidal, wide, shallow, and river-dominated estuary in the northern Gulf of Mexico. An analysis of the vertical salinity variance and its associated budget terms shows that the system generally becomes less stratified and fully mixed across much of the system; however, there was significant spatial heterogeneity in physical processes driving the evolution of the water column stratification over the course of the event. Surprisingly, in some regions of the bay, dissipation of salinity variance was secondary to contributions from straining and advection. Furthermore, local wind stress and remote wind driven Ekman transport affected stratification responses and their relative impacts varied spatially across the estuary. Direct turbulent mixing from local wind stress and straining dominated the stratification responses away from the main tidal inlet where estuarine-shelf exchange (i.e., current velocity structure and advection of salinity) from Ekman transport controlled the vertical mixing. This detailed case study highlights the complexity of wind influences in a system like Mobile Bay, a representative typical of the northern Gulf of Mexico and other coastal region.