Saildrone-observed atmospheric boundary layer response to winter mesoscale warm spot along the Kuroshio south of Japan

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.