Abstract
Direct current measurements by a shipboard and bottom-mounted acoustic Doppler current profiler and concurrent hydrographic observations with a CTD were conducted off southeastern Hokkaido, Japan, between January and May 2005 to reveal temporal variations in the current structure and volume transport of the Coastal Oyashio (CO). The CO, which has a baroclinic jet structure with southwestward speeds exceeding 90 cm s−1 and a width of 7–8 km, was associated with a surface-to-bottom density front and was formed on the offshore side of the shelf break. The volume transport of CO (T CO) was estimated by integrating the fluxes of lower-density water that was trapped against the coast along the density front represented by the 26.2 σ θ isopycnal line. This transport decreased monotonously from 0.79 Sv (1 Sv = 106 m3 s−1) in January to 0.21 Sv in March and subsequently to 0.12 Sv in May, possibly due to the decay of the East Sakhalin Current Water in the Okhotsk Sea. Accompanied by a decrease in T CO, the location of the jet structure associated with the density front moved toward the coast while the maximum speed of the jet decreased and the tilt of the front became more horizontal. Consequently, more saline offshore Oyashio water flowed into the deep part of the shelf area, and the current structure altered from relatively barotropic in winter to baroclinic in spring. This study is the first to estimate the observed volume transport of the CO from direct current measurements.
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Acknowledgments
We would like to acknowledge the assistance of the captain and crew of the R/V Hokko Maru of the Hokkaido National Fisheries Research Institute, Fisheries Research Agency for their help with the deployment and recovery of the bottom-mounted ADCP. We also greatly appreciate Dr. T. Tozuka and the anonymous reviewers for reviewing our manuscript and giving useful comments. This work was financially partly supported by the Ministry of Agriculture, Forestry and Fisheries, Japan through a research project entitled “Development of mitigation technologies to climate change in the agriculture sector”.
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Appendix: F-COW
Appendix: F-COW
In the time series for A01 (and P21), warmer and less saline water (T > 2 °C, S < 33) was observed around December 2004 and January 2005 (this period is denoted by thick arrows in Fig. 9). This water cannot be judged to be the COW because its temperature was higher than the upper limit (2 °C) of the COW temperature index. Such less saline and warmer water was also observed on the coastal side of A02 on January 11 (upper and middle panels in Fig. 3a). CTD data between 10 and 100 m at A01 and A02 in January and March were plotted on the T–S plane (Fig. 14). In this study, the less saline and warmer water observed at A01 and A02 on January 11 is called as the Forerunner COW (F-COW) because it was in the process of being modified to the COW. However, note that the F-COW and Surface COW (S-COW; Rosa et al. 2007) cannot be distinguished by a T–S analysis alone, because both water mass characteristics overlap in the T–S plane (Fig. 14).
A temperature–salinity diagram for data obtained at A01 (gray) and A02 (black) in January and March 2005. CTD data between 10 and 100 m at each station are plotted. COW, F-COW, S-COW, M-SWCW, and OW denote the regions of Coastal Oyashio water (T < 2 °C and S < 33; Ohtani 1971), Forerunner COW (T > 2 °C and S < 33), Surface COW (T > 2 °C and S < 33; Rosa et al. 2007), Modified Soya Warm Current Water (T > 7 °C and 33 < S < 33.7; Oguma et al. 2008), and Oyashio water (T < 7 °C, 33 < S < 33.7 and σ θ < 26.7; Hanawa and Mitsudera 1987), respectively. Curves denote isopycnals at 25.0, 26.0, and 26.7
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Kusaka, A., Shimizu, Y., Sato, T. et al. Temporal variations in the current structure and volume transport of the Coastal Oyashio revealed by direct current measurement. J Oceanogr 72, 601–615 (2016). https://doi.org/10.1007/s10872-016-0356-2
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DOI: https://doi.org/10.1007/s10872-016-0356-2