Winter mixed layer and its yearly variability under sea ice in the southwestern part of the Sea of Okhotsk

Hydrographic observations under sea ice were conducted in the southwestern part of the Sea of Okhotsk for 4 years from 1996 to 1999. Every year a cold mixed layer with near-freezing temperature was distributed from the sea surface to a depth of 150–300 m near the shelf break under sea ice. The thicknesses of the mixed layer and sea ice were largest in 1997. While the depth of the mixed layer was considerably deeper than that of dichothermal water, which is identified as a temperature minimum from spring to fall in this region, the density of water in the mixed layer was equal to or less than that of dichothermal water. It is shown that deepening of isopycnals due to the alongshore component of the wind stress is essential for thickening of the mixed layer. In 1997 the nearly northerly winds, which are usually directed offshore, were more parallel to the coast than those in the other years. Thickening of the mixed layer in 1997 is attributed to this wind condition. The air temperature and wind indicate that the sea-ice production rate was low in 1997, whereas the wind direction was favorable for rafting and ridging. Thus the change in wind direction is proposed as an important factor in determining the thickness of both the mixed layer and the sea ice in this region.