Enhancement of Eddy Heat Transport due to the Anticyclonic Submesoscale Eddies around Ryukyu Islands near Kuroshio in East China Sea

Abstract:

A synoptic, regional downscaling experiment of Kuroshio off Ryukyu Islands, Japan, exhibits the evident predominance of submesoscale anticyclonic eddies over cyclones in the narrow strip between Kuroshio and the islands (Uchiyama et al., 2013). In the present study, the mechanism and impacts of the anticyclone dominance are examined with a detailed oceanic downscaling model in a double nested ROMS configuration at the horizontal resolution of 3km (ROMS-L1) and 1km (ROMS-L2), forced by the assimilative JCOPE2 oceanic reanalysis and the JMA GPV-MSM atmospheric hindcast. The model results are extensively validated against a variety of data including shipboard hydrography and satellite altimetry and temperature data to show a good agreement. An alternative ROMS-L2 experiment is also conducted to examine topographic effects on the anticyclones around the Ryukyu Islands by eliminating all the island topography above z > -1000 m, while the other configurations are held unchanged. If the islands are removed, the submesoscale negative vortices on the eastern side of the Kuroshio become much weaker than those of the original case with the islands. The experiment clearly demonstrates that dominance of the negative vorticity between Kuroshio and the Ryukyu Islands is caused by enhanced lateral shear due to the concentrated Kuroshio mean current associated with appropriate formation of the eastern branch, the northward-drifting Ryuku Current, and resultant eddy shedding in the narrow channel between the continental shelf of the East China Sea and the Okinawan ridge. A diagnostic eddy heat flux analysis illustrates that the submesoscale anticyclonic eddies play a crucial role in enhancing the eddy heat transport and thus the lateral mixing between Kuroshio and the islands as compared to those in the coarser resolution models (L1 and JCOPE2), resulting in promoting regional larval and material transport from Kuroshio to the islands.