Autonomous Surface Vehicle Measurements of the Ocean's Response to Tropical Cyclone Freda

On December 31, 2012, an instrumented autonomous surface vehicle (ASV, Wave Glider) transiting across the Pacific from Hawaii to Australia as part of the PacX project came very close (46km) to the center of a category 3 tropical cyclone, TC Freda. The Wave Glider was instrumented for surface-ocean-lower-atmosphere (SOLA) measurements including atmospheric pressure, surface winds and temperature, sea surface temperature, salinity, dissolved oxygen, fluorescence (Chlorophyll-a and turbidity), and surface-wave directional spectra. Such measurements in close proximity to a tropical cyclone are rare. Winds of up to 37 m/s and significant wave heights up to 10m were measured as the wave glider passed near the eye of the tropical cyclone. We present novel observations of the ocean's response in three quadrants of TC Freda collected from the instrumented glider. Evolution of the sea surface temperature, the wind, the directional wave field, and the Stokes drift profile as Freda passed near the vehicle are examined. Results are discussed in the context of the recent coupled wind-wave modeling and LES modeling of the marine boundary layer in hurricane Frances (Sullivan et al. 2012). Processes by which cold nutrient-rich waters are entrained and mixed from below into the mixed layer as the TC passes near the Wave Glider are presented and discussed. The results of this encounter of an autonomous surface vehicle with TC Freda supports the use of ASVs for regular TC (hurricane) monitoring to complement remote sensing and "hurricane hunter" aircraft missions.