Ocean mixing during Hurricane Ida (2021): The impact of a freshwater barrier layer

Tropical cyclones are one of the costliest and deadliest natural disasters globally, and impacts are currently expected to worsen with a changing climate. Hurricane Ida (2021) made landfall as a category 4 storm on the US gulf coast after intensifying over a Loop Current eddy and a freshwater barrier layer that extended from the coast to the open ocean waters off the continental shelf. An autonomous underwater glider sampled this ocean feature ahead of Ida. We use this data with 1-D shear driven mixed layer models to investigate the sensitivity of the upper ocean mixing to a barrier layer during Ida’s intensification period. We show that the freshwater barrier layer inhibited cooling by as much as 56% and resulted in increased enthalpy flux to the atmosphere by >20% as the storm made landfall. This highlights the utility of sustained observations to support coupled ocean and atmosphere hurricane forecasts.