PIV analyses of ocean eddies in the southern part of the Sea of Okhotsk visualized with floating sea ice

Mesoscale eddies of various sizes, both cyclonic and anti-cyclonic, exist in sea surface. Recently, these eddies have been recognized with sea level anomalies measured by altimeter installed on satellites and their behavior is being unveiled with the progress of satellite altimetry. However, over limited area where its surface is covered with sea ice in all or certain period of the year, such eddies could be visualized, enabling to detect their existence on ground-based, airborne and satellite radar images or photographs. Especially, under suitable weather conditions when floating ice acts as passive tracer, PIV analyses of their successive images reveal detailed current distribution including that of ocean eddies. The Sea of Okhotsk, known as the southern margin of seasonal sea ice in the northern hemisphere, is exactly such an area. Here, we examine the velocity fields surrounding ocean eddies in the southern part of the sea of Okhotsk by visual imagery of geostationary satellites with PIV technique.
The area of focus is semi-enclosed by eastern coast of Sakhalin, northeastern coast of Hokkaido and Kuril islands, where the Eastern Sakhalin current from north and the Soya warm current from northwest collide to produce complex flow pattern including mesoscale eddies of both cyclonic and anti-cyclonic rotation. Most typical ones are cyclonic eddy streets formed along northeastern coast of Hokkaido (Wakatsuchi & Ohshima 1990), which is caused by barotropic instability due to cyclonic shear formed along northeastern end of the Soya warm current (Ohshima & Wakatsuchi 1990). The second group consists of anti-cyclonic eddies seen offshore over the Kuril basin, where anti-cyclonic circulation is dominant (Wakatsuchi & Martin 1991). Here, as the last group, we distinguish cyclonic eddies appeared along the northeastern coast of Hokkaido but offshore of the Shiretoko peninsula, the eastern end of the northeastern coast of Hokkaido, from the first group since they formed over the slope toward Kuril basin as an isolated eddy in contrast to the eddy streets, consisting of two or more eddies, emerge over shallow continental shelf.
In this study, we have focused on the cases of 8 March in 2002 and 27 January in 2003, and the PIV analyses were carried out based on GMS-5 visual images. As the result, it is revealed that the isolated eddies belonging to the third group are formed in relation to the East Sakhalin current alone so that they are completely different from the eddy streets of the first group which are caused by barotropic instability of the Soya warm current. The flow fields surrounding the anti-cyclonic eddies belonging to the second group are also introduced.