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.

Seasonal cycle is the largest source of variability for sea surface salinity (SSS) and has a significant influence on the upper-ocean stratification and water-mass formation. The advent of the Argo profiling floats and L-band passive microwave remote sensing in the past one and half decade has significantly improved the sampling of seasonal variations of SSS over the global ocean. Assessing the seasonality of SSS using these recent measurements is important for understanding its relationships with freshwater forcing and ocean dynamics as well as for identifying potential limitations of the SSS observing system. Here we utilize a suite of SSS products from recent satellite and in-situ platforms to revisit seasonal variations of SSS under different freshwater forcing conditions. The result shows that, although the annual harmonic is the most characteristic feature of the seasonal cycle, the semiannual harmonic is not negligible, especially in regions influenced by monsoon and major rivers. The annual and semiannual harmonics account for 70–80 % and 10–16 % of the total observed variance respectively, which together drive the SSS seasonality. The range of seasonal SSS is approximately ±0.05 practical salinity scale (pss) in the subtropical SSS maximum regions, but greater than ±0.25 pss in the tropical SSS minimum regions. However, the seasonal variations of satellite SSS in the 20-40°N latitude range showed erroneous annual and semiannual phases compared with in situ products, the cause of which needs further examination.