The application of new technologies has allowed oceanographers and meteorologists to study the ocean beneath typhoons in detail. Recent studies in the western Pacific Ocean reveal new insights into the influence of the ocean on typhoon intensity.

Three autonomous profiling Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats were air deployed one day in advance of the passage of Hurricane Frances (2004) as part of the Coupled Boundary Layer Air-Sea Transfer (CBLAST)-High field experiment. The floats were deliberately deployed at locations on the hurricane track, 55 km to the right of the track, and 110 km to the right of the track. These floats provided profile measurements between 30 and 200 m of in situ temperature, salinity, and horizontal velocity every half hour during the hurricane passage and for several weeks afterward. Some aspects of the observed response were similar at the three locations - the dominance of near-inertial horizontal currents and the phase of these currents - whereas other aspects were different. The largest-amplitude inertial currents were observed at the 55-km site, where SST cooled the most, by about 2.2C, as the surface mixed layer deepened by about 80 m. Based on the time-depth evolution of the Richardson number and comparisons with a numerical ocean model, it is concluded that SST cooled primarily because of shear-induced vertical mixing that served to bring deeper, cooler water into the surface layer. Surface gravity waves, estimated from the observed high-frequency velocity, reached an estimated 12-m significant wave height at the 55-km site. Along the track, there was lesser amplitude inertial motion and SST cooling, only about 1.2C, though there was greater upwelling, about 25-m amplitude, and inertial pumping, also about 25-m amplitude. Previously reported numerical simulations of the upper-ocean response are in reasonable agreement with these EM-APEX observations provided that a high wind speed-saturated drag coefficient is used to estimate the wind stress. A direct inference of the drag coefficient CD is drawn from the momentum budget. For wind speeds of 32-47 m s^-1, CD ~ 1.4 x 10^-3.

An array of instruments air-deployed ahead of Hurricane Frances measured the three-dimensional, time dependent response of the ocean to this strong (60 m s^-1) storm. Sea surface temperature cooled by up to 2.2°C with the greatest cooling occurring in a 50-km-wide band centered 60–85 km to the right of the track. The cooling was almost entirely due to vertical mixing, not air-sea heat fluxes. Currents of up to 1.6 m s^-1 and thermocline displacements of up to 50 m dispersed as near-inertial internal waves. The heat in excess of 26°C, decreased behind the storm due primarily to horizontal advection of heat away from the storm track, with a small contribution from mixing across the 26°C isotherm. SST cooling under the storm core (0.4°C) produced a 16% decrease in air-sea heat flux implying an approximately 5 m s^-1 reduction in peak winds

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.