Upper ocean response to Typhoon Choi-Wan as measured by the Kuroshio Extension Observatory mooring

Abstract

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 CO2, 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.

Formation and erosion of the seasonal thermocline in the Kuroshio Extension Recirculation Gyre

“…In particular, the amplitude of shear oscillating at the inertial frequency found through complex demodulation (Fig. 9) is large throughout most the year, in all likelihood due to the passage of winter storms (D'Asaro, 1985) and summer tropical cyclones (Bond et al, 2011;Morozov and Velarde, 2008;Price, 1983). The large shears near the inertial frequency and at longer timescales are likely responsible for the enhanced mixing and diffusivity values found at KEO.…”

“…However if the mode water thermal properties are modified, then the memory of the previous winter is impacted. An example of mode water modification at KEO is shown by Bond et al (2011) in a case study of the oceanic response to the passage of a tropical cyclone.…”

“…3); the net surface heat flux warms the ocean. The more-or-less steady warming by the sun in summer is punctuated by short periods of thick and persistent clouds, sometimes accompanied by high winds associated with the northward passage of tropical cyclones (typhoons) Bond et al, 2011;Tomita et al, 2010). During these periods, typically lasting a few days, the net shortwave fluxes can be on the order of 200 W/m 2 less than usual and sustained dailyaveraged winds can be more than 12 m/s.…”

Validation of AMSR2 Sea Surface Wind and Temperature over the Kuroshio Extension Region

“…Standard geophysical products of AMSR2 have three accuracy mainly observed by KEO and are associated with typhoons (Tomita et al 2010b;Bond et al 2010Bond et al , 2011. However, large variance is observed at JKEO as well (Fig.…”

Numerical Study on the Effect of the Ocean on Tropical-Cyclone Intensity and Structural Change

“…The discussion in this subsection focuses on the mature phase of Typhoon Choi-Wan in 2009, which passed near the moored buoy of the NOAA Kuroshio Extension Observatory (KEO) located at 32.3ºN, 144.5ºE in the North Pacific recirculation gyre south of the Kuroshio Extension (Bond et al, 2011). The JMA global spectral model with a grid spacing of 20 km predicted the intensity of Choi-Wan to be much stronger in its mature phase than indicated by JMA best-track analysis, although the JMA track prediction was accurate.…”