Impact of the sea ice reduction in the Sea of Okhotsk on the North Pacific

K. I. Ohshima
Institute of Low Temperature Science, Hokkaido University


The Sea of Okhotsk is the southern limit of sea ice in the Northern Hemisphere. This is because the cold pole in the Northern Hemisphere is located in the upwind region of the Sea of Okhotsk.  When sea ice is formed, most of the salt content is rejected from the ice and thus cold, saline and dense water is released into the ocean below. Since large amounts of sea ice are formed in the Sea of Okhotsk, the densest water on the surface of the North Pacific is produced there. Sinking of this dense water creates the vertical circulation (overturning) down to the intermediate depths (approx. 200 to 800 m deep) in the North Pacific. The Okhotsk thus plays a role as the pump of the North Pacific. We found that the water temperature in the intermediate layer of the Okhotsk has increased over the past five decades while the oxygen has decreased. This means that sinking of cold oxygen-rich surface water into the intermediate layer has decreased. These signals have spread to the North Pacific along the pathway of the seawater flowing out of the Okhotsk. The Okhotsk is sensitive to global warming: over the past 50 years, sea ice production has decreased by about 20% and dense water sinking has thus declined, thereby weakening the overturning in the North Pacific. The weakened overturning possibly affects the iron circulation, since the iron in the western North Pacific presumably originates from the dense (intermediate) water from the Okhotsk, further from the Amur River. Recent studies suggest that iron is a substantial factor in determining biological productivity. Current global warming, through sea ice reduction, might decrease the iron supply in the North Pacific as well as in the Okhotsk, thus reducing levels of biological productivity and fishery resources.