Seasonal Cycle of the Confluence of the Tsugaru Warm, Oyashio, and Kuroshio Currents East of Japan. Issue 8 (10th August 2022)

• Record Type: Journal Article
• Authors: Itoh, S.; Tsutsumi, E.; Masunaga, E.; Sakamoto, T. T.; Ishikawa, K.; Yanagimoto, D.; Hoshiba, Y.; Kaneko, H.; Hasegawa, D.; Tanaka, K.; Fukuda, H.; Nagata, T.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-10
• Subjects: Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC018556
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: The Sanriku Confluence east of Japan is the region of confluence of the Tsugaru Warm Current from the Sea of Japan, the Oyashio Current from the western subarctic gyre and the Sea of Okhotsk, and Kuroshio water that has detached from the subtropical gyre. It is a field of vigorous stirring driven by variability in current systems, but transition processes between water masses have yet to be clarified. High‐resolution underwater conductivity‐temperature‐depth observations were undertaken during each of the four seasons to test a hypothesis that the water‐mass gradient in the Sanriku Confluence is set without full stirring. Analyses in isopycnal coordinates indicate the seasonal occurrence of prominent fronts of both salinity and displacement, related to the emergence of various water masses. After fitting error functions to frontal features, the seasonally emerging isopycnal salinity fronts often became extremely sharp with widths typically narrower than 3 km, supporting the hypothesis. Disturbances distinguished from frontal structures indicate variance peaks with ranges similar to or slightly greater than the baroclinic Rossby radius, likely stirring the large‐scale tracer gradient at this scale. Elevated variances in isopycnal salinity were often observed around the fronts, and variances caused by isolated patches with salinity anomalies were also large. By determining the horizontal scales at which the effects of internal wave heaving becomes dominant, we were able to calculate the potential vorticity (PV) of the balanced flow field. As the PV becomes negative or near zero around the fronts, symmetric instability might have developed. Plain Language Summary: The Sanriku Confluence east of Japan is an area where three currents meet: the Tsugaru Warm Current from the Sea of Japan, the Oyashio Current from the subarctic Pacific and the Sea of Okhotsk, and Kuroshio water that detaches from the subtropical region. High‐resolution field surveys were undertaken during the four seasons to clarify the processes of interaction between these water masses. We captured seasonal encounters between the various water masses that led to the formation of very sharp fronts between warm/salty and cold/fresh water masses. By removing the signals of frontal structures from the original data, we identified that active stirring of water masses occurred at spatial scales of 5–15 km, similar to the scale of eddies. Such stirring activities were intensified not only around the fronts, but also in areas distant from the fronts, as isolated patches of high‐salinity water. The density gradient at the fronts becomes so high that an instability process might occur, resulting in small‐scale fluctuations that stir the water masses. Key Points: High‐resolution in situ observations captured seasonal formations of sharp fronts in the Sanriku Confluence east of Japan Disturbances after subtracting the frontal structure had a scale similar to or slightly greater than the baroclinic Rossby radius Ertel's potential vorticity around the fronts decreased to near zero, suggesting the occurrence of symmetric instability