Highly resolved observations and simulations of the ocean response to a hurricane

An autonomous, profiling float called EM‐APEX was developed to provide a quantitative and comprehensive description of the ocean side of hurricane‐ocean interaction. EM‐APEX measures temperature, salinity and pressure to CTD quality and relative horizontal velocity with an electric field sensor. Three prototype floats were air‐deployed into the upper ocean ahead of Hurricane Frances (2004). All worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane. At a float launched 55 km to the right of the track, the hurricane generated large amplitude, inertially rotating velocity in the upper 120 m of the water column. Coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2.2°C. We find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind‐speed saturated drag coefficient.