Impact of ocean conditions on hurricane development and forecast: examples of Hurricanes Maria (2017) and Michael (2018)

Matthieu Le Henaff, University of Miami, Cooperative Institute for Marine and Atmospheric Studies, Miami, United States, Ricardo M. Domingues, Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, FL, United States, Gustavo Jorge Goni, NOAA/AOML, Miami, FL, United States, George R Halliwell Jr, NOAA Miami, Miami, FL, United States, Francis Bringas, NOAA Atlantic Oceanographic and Meteorological Laboratory, Miami, FL, United States, Hyun-Sook Kim, Atlantic Oceanic and Meteorologic Laborabory, Miami, United States, Jili Dong, National Centers For Environmental Prediction-Environmental Modeling Center, College Park, MD, United States, Avichal Mehra, NOAA NWS NCEP Environmental Modeling Center, College Park, United States and Vijay Tallapragada, NOAA/NCEP/EMC, College Park, United States
The 2017 and 2018 hurricane seasons in the Atlantic Ocean showed major storms making landfall in the Antilles and in continental U.S. Accurately forecasting major hurricanes, in particular their intensity, is a critical societal need to support decision making related to marine safety, and to the protection of lives and property. We used satellite and in situ observations, as well as a coupled ocean-hurricane model, to investigate the impact of the observed ocean conditions on the intensification of Hurricanes Maria in 2017 and Michael in 2018. In 2017, underwater gliders deployed in the Caribbean Sea and in the Tropical North Atlantic revealed the presence of a thick barrier layer, i.e. a strong stratification at the top of the ocean due to freshwater. By reducing the ocean cooling under the storm, barrier layers are known to favor storm intensification. Similarly, gliders that were located close to Hurricane Michael’s track revealed preexisting barrier layer in the area. The ocean-hurricane model, based on HYCOM and HWRF models, was used to estimate the impact of the observed ocean conditions, and of individual observing platforms (through data-assimilation), on the storm intensity forecast. One key finding is that the ocean conditions played a role in the intensification of both hurricanes. In addition, the assimilation of ocean observations contributed to improved ocean representation, and consequently to improved hurricane forecast. More specifically, satellite altimetry allows identifying ocean features such as currents and mesoscale eddies, Argo floats allow for correcting model temperature and salinity large scale biases, and glider data provide continuous temperature and salinity profiles in areas of intense mesoscale activity. In particular, gliders deployed in the Caribbean Sea helped improve the wind intensity forecast just before Maria made landfall in Puerto Rico. Specific results from Michael revealed that the storm rapidly intensified in the Gulf of Mexico, as it travelled over: (1) a warm-core eddy shed by the Loop Current; (2) low-salinity areas due to the Mississippi plume; and (3) unusually warm waters in the northeastern Gulf. Results from the coupled ocean-hurricane model forecast revealed that these ocean conditions played a significant role in the intensification of Michael.