OBSERVED OCEAN-ATMOSPHERE INTERACTIONS DURING TROPICAL CYCLONE MICHAEL'S RAPID INTENSITY CHANGES

During five NOAA WP-3D aircraft missions that observed tropical cyclone Michael in October 2018, airborne expendable oceanic probes were deployed and collocated with dropsonde profilers to estimate the bulk enthalpy fluxes. In addition, satellite-tracked drifters and APEX-EM floats were deployed from the USAF WC-130J across 26^oN in front of Michel. The measurement domain essentially stretched across the eastern Gulf of Mexico to assess the oceanic impact on Michael’s intensity changes. The APEX-EM floats released ahead of Michael measured more than 650 hourly profiles of evolving temperature, salinity, current structure between the near-surface layer and 300 m, and the drifters measured surface ocean conditions including directional surface wave spectra. These measurements were acquired across a frontal regime where a cool pool of water was located between a retracting Loop Current and a warm eddy previously shed. Ocean structure revealed a barrier layer where strong salinity gradient was evident within the ocean mixed layer between 30 to 40 m depth. As Michael encountered these regimes, minimal SST cooling was observed during a rapid intensity event, as the near-inertial current shears across the mixed layer base were insufficient to lower the bulk Richardson numbers to below critical values. That is, the resultant density gradient, and hence the buoyancy frequency, precluded significant SST cooling, allowing for a more sustained enthalpy flux to the tropical cyclone boundary layer. A second rapid intensity change was observed prior to landfall as Michael reached category five status over a warm filament where SSTs exceeded 30^oC that extended over the shelf. These SSTs were approximately 2 to 2.5^oC warmer than observed in 2017 in the northern Gulf. These types of measurements are crucial to evaluating and improving coupled model forecasts for landfalling tropical cyclones.