Near‐Surface Arresting of Wind‐Driven Cooling in the Gulf of Mexico Facilitates the Rapid Intensification of Major Hurricane Michael (2018)

Abstract Hurricanes require large moisture flux from warm waters to intensify—hurricane intensity affects and is affected by water cooling underneath. Historically, Gulf of Mexico hurricanes preferentially experienced rapid intensification (RI) and major development over localized deep warm waters; recently, unusually warming surface waters facilitated broad RI events in an increasing number of major hurricanes until landfall. Unprecedented air‐sea interaction observations in Hurricane Michael (2018) including EM‐APEX floats were acquired to elucidate on this pressing issue. These indicate that inner‐core sea surface temperature >28°C consistently sustained sea‐to‐air moisture flux >350 W m −2 during Michael's RI from category‐1 to category‐5 landfall. Air‐sea moisture disequilibrium led to peak surface moisture flux during RI over the Loop Current's deep warm thermal structure; peak near‐surface wind speeds enhanced evaporation during RI over warm shelf waters, but air parcels were drier. Outside localized deep warm waters, tight vertical temperature gradients in the upper ocean and saltier near‐surface North Atlantic Subtropical Underwater (NASUW) caused strong stability (13–18 cycles per hour). Direct observations and predictions from an ocean mixed layer model indicate that NASUW reduced sea surface cooling by arresting near‐inertial shear, vertical mixing, and dissipation and heat exchange between surface and thermocline waters—a pattern that is expected to exacerbate in a warmer Gulf. This study sets a benchmark for better understanding this threat. Including realistic mesoscale variability in near‐surface salinity in coupled forecast systems can yield important progress in predictions of sea surface cooling and RI of tropical cyclones over tropical evaporative oceans.