Cascading weather events amplify the coastal thermal conditions prior to
the shelf transit of Hurricane Sally (2020)
Abstract
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.