The Kuroshio Extension Observatory (KEO) is a highly instrumented moored reference station located at 32.3°N, 144.5°E in the recirculation gyre south of the Kuroshio Extension. On 19 September 2009, the eye of Typhoon Choi-Wan (International designation: 0914) passed –40 km to the southeast of the KEO surface mooring. Hourly meteorological and physical oceanographic measurements together with 3 hourly air-sea carbon dioxide observations telemetered from KEO in near real time show the evolution of the upper ocean and its associated air-sea fluxes during the passage of this storm and its aftermath. During the approach of the storm, the mixed layer freshened because of intense rainfall. This was followed by a large outgassing of CO_2, rapid cooling, and an increase in salinity. Although these changes in mixed layer properties imply substantial entrainment, they were accompanied by upwelling and ultimately a temporary –20 m shoaling of the mixed layer. This upwelling, which was observed at all depths, including the deepest sensor near 500 m, was coincident with the onset of near-inertial oscillations in the mixed layer currents. After the typhoon passed, inertial pumping caused –15-20 m amplitude vertical displacements throughout the top 500 m that continued for at least 6 days. A large oceanic response was observed in this case even though the eye of Choi-Wan passed to the right of KEO, resulting in winds rotating cyclonically with time, in opposition to the anticyclonic-rotating near-inertial currents.
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