Impact of compounding atmospheric events on shelf heat content in a
river-influenced shelf system: Potential implications for Hurricane
Michael
Abstract
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.