Abstract
Using an unmanned sailing vehicle, known as a Saildrone, we observed mesoscale and smaller scale structures of oceanic and atmospheric variables across the Kuroshio south of Japan during the winter of 2018/2019. From December 28 to December 29, 2018, the Saildrone crossed just north of the center of a very warm (∼23∘C) mesoscale spot in the Kuroshio centered around 31.5∘ N, 135.8∘ E. The northerly winter monsoon wind was intensified by ∼2ms−1 over the mesoscale warm spot (MWS) and accompanied by a submesoscale sea level pressure undulation of ∼1 hPa possibly due to two oppositely rotating ageostrophic vortices. At this time, the wind reached a maximum speed of greater than 12ms−1 and removed heat from the ocean at a rate of 1141Wm−2. Subsequently (January 3–5, 2019), the Saildrone observed weakening of wind and heat release to the atmosphere on the southern edge of the MWS, which was associated with the approaching low-pressure system over the Kuroshio. The observed submesoscale structures of atmospheric and oceanic variables near the center of the MWS suggest that the atmospheric boundary layer responded to the MWS through the pressure adjustment mechanism in the Kuroshio, where in situ high-resolution measurements have not been performed before.
Highlights
The Kuroshio, the western boundary current of the North Pacific subtropical gyre, originates east of the Philippines, flows northeastward through the East China Sea, proceeds south of Japan, and extends further eastward as the Kuroshio Extension (Nitani 1972)
We found that wind is intensified over the mesoscale warm spot (MWS) and that turbulent heat flux from the ocean to the atmosphere is large, likely resulting from the air-sea interaction over the spotty warm area of the Kuroshio
The Kuroshio south of Japan is illustrated by a sharp offshoreward increase in sea surface height (SSH) (Fig. 3a)
Summary
The Kuroshio, the western boundary current of the North Pacific subtropical gyre, originates east of the Philippines, flows northeastward through the East China Sea, proceeds south of Japan, and extends further eastward as the Kuroshio Extension (Nitani 1972). Atmospheric variables such as sea level pressure (SLP), air temperature, and wind speed and direction have been observed being affected by the high SST water in the Kuroshio region during the winter (e.g., Tokinaga et al 2006; Tomita et al 2013; Kawai et al 2014).
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