Coastal Ocean Thermal Structure and Rapid Intensification of Hurricane Ida over the Western Louisiana Shelf

Hurricane Ida, a major hurricane that made landfall on the Louisiana coast, caused severe to catastrophic damage in Louisiana. The National Centers for Environmental Information estimated that the Ida’s winds, rains, and storm surges caused a total of $75 billion in damages in the United States. In 2020 and 2021, the intensification of tropical cyclones over the Northern Gulf coasts shelves led to tropical making landfalls as major hurricanes. While the prediction of rapid intensification over the shelves is improving (the case of Hurricane Idalia 2023), observations of the interactions between the air-sea fluxes, the underlying thermal oceanic conditions and rapid intensification over the shelf regions are limited. This study, therefore, focusses on observations from a moored buoy on the 20 m isobath west of the closest approach of Hurricane Ida. Observations from the moored buoy were put into a broader context utilizing satellite derived products and model datasets.

Preliminary results show that high sea surface temperatures (SSTs) waters and high surface heat fluxes prevailed on the shelf as the hurricane transitioned from the open gulf to the shelf. SSTs, were 29.5℃ during the passage of Ida, but dropped to 28℃ two days after the storm. These high SSTs were maintained despite an overall surface heat flux of 600 W.m^-2 to the atmosphere during the storm. This is in part due to the coastal oceanographic pre-conditions and the shelf transport of heat from hurricane induced coastal circulation. Ocean heat content on the shelf in the Gulf of Mexico can be lower than the empirical threshold of 50 kJ/cm^-2 for tropical cyclone intensification that has been found for the deep ocean, because there is no main thermocline or cold pool, to further decrease SST under vertical mixing and upwelling.