Submesoscale structures observed in the shadow zone of Nishinoshima volcanic island by uncrewed surface vehicle

We observed the atmosphere and ocean around the Nishinoshima volcanic island (27 $$\,^{\circ }$$ 15 $$^{\prime }$$ N, 140 $$\,^{\circ }$$ 53 $$^{\prime }$$ E) in Japan from May 16 to May 28, 2023, using the Bluebottle uncrewed surface vehicle. Approximately 20 km east and northeast of the island, the vehicle observed submesoscale (<20 km) sea surface anomalous waters with core temperatures and salinities lower than 22 $$^{\circ }$$ C and 34.4, respectively, which are significantly colder and fresher than the surrounding waters. From a climatological hydrographic profile, the anomalous water was inferred to be uplifted from 450 to 500 m depth or deeper layers to the sea surface. The meridionally elongated pattern of these anomalous waters are consistent with island wakes caused by mesoscale cyclonic eddy and tidal flows colliding with the island. The turbulent wakes behind the island exhibited very short time scale (<10 min) water temperature and salinity variations, resulted in the colder water near the island than the surrounding waters. The Nishinoshima volcano was weakly active and warmed the surrounding atmosphere during the observation. The combined effect yielded negative sea–air temperature difference near Nishinoshima Island, implying heat absorption by the ocean near the island, in contrast to off-island regions.