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.

Hurricane Michael in 2018 was one of the strongest storms to impact the coastal U.S. and was unusual given that it occurred in October (i.e., late in the hurricane season) and intensified over the continental shelf. A potential contributor to this extreme event was thought to be anomalously high heat content on the shelf of the Mississippi Bight, a shelf region significantly impacted by freshwater discharge. Using available long-term time series of regional meteorological and oceanic measurements, water column conditions during the run-up to the rapid intensification of Hurricane Michael were compared to historical conditions in the region. Data for the water column heat content in the western Mississippi Bight were available at a mooring site on the 20 m isobath (Site CP) during August-October of 2018. Unusually high heat content was observed, relative to the typical summer conditions of previous years (N = 13), which resulted from the compounding effects of atmospheric events in the preceding months (August and September): a series of smaller mixing events (e.g., passage of Tropical Storm Gordon) in conjunction with a regional heatwave during most of September. Tropical Storm Gordon traveled across the shelf of the Mississippi Bight, disrupting the stratified water column as observed at Site CP. The stratification breakdown mixed the upper water column heat content deeper into the water column and subsequently allowed for the anomalously warm atmospheric conditions in September to effectively transfer heat deeper into the water column. While the mooring site was significantly distant (250 km) from the center of Hurricane Michael, these processes observed in the western Mississippi Bight likely occurred in the eastern portion of the basin as well. As a result of these compounding atmospheric effects, the shelf water column was primed to support the intensification of following tropical storms, which highlights the need for coupled oceanic-atmospheric forecast models to capture the interaction between the ocean and atmosphere and its effect on water column conditions on the shelf.

As tides propagate inland, they become distorted by channel geometry and river discharge. Tidal dynamics in fluvial-marine transitions are commonly observed in high-energy tidal environments with relatively steady river conditions, leaving the effects of variable river discharge on tides and longitudinal changes poorly understood. To study the effects of variable river discharge on tide-river interactions, we studied a low-energy tidal environment where river discharge ranges several orders of magnitude, the diurnal microtidal Tombigbee River-Mobile Bay fluvial-marine transition, using water level and velocity observations from 21 stations. Results showed that tidal attenuation was reduced by the width convergence in seaward reaches and height convergence of the landward backwater reaches, with the channel convergence change location ~40-50km inland of the bayhead and seaward of the largest bifurcation (~rkm 90-100). River events amplified tides in seaward regions and attenuated tides in landward regions. This created a region of river-induced peak amplitude seaward of the flood limit (i.e., bidirectional-unidirectional current transition) and passed more tidal energy inland. Tidal currents were attenuated and lagged more with river discharge than water levels, making the phase lag dynamic. The river impacts on the tides were delineated longitudinally and shifted seaward as river discharge increased, ranging up to ~180 km. Results indicated the location and longitudinal shifts of river impacts on tides in alluvial systems can be estimated analytically using the ratio of river discharge to tidal discharge and the geometry convergence. Our simple analytical theory provides a pathway for understanding the tide-river-geomorphic equilibrium along increasingly dynamic coasts.