An engine of ocean circulation
The Sea of Okhotsk is an anomaly. Located at latitudes stretching from roughly 44°N to 62°N—comparable to France and Scotland—it defies its temperate position by producing vast amounts of seasonal sea ice every winter. This occurs because of a specific combination of atmospheric and oceanographic conditions. The primary driver is the Siberian High, a massive semi-permanent atmospheric pressure system that funnels intensely cold, dry air from the continent eastward over the sea during winter.
This frigid wind is exceptionally effective at cooling the sea surface. A second critical factor is the sea's unique water structure. A massive influx of freshwater from the Amur River creates a low-salinity layer on the surface, about 50 meters deep. This less-saline water has a higher freezing point and is less dense than the saltier water below it. When winter winds cool the surface, this stratified layer prevents mixing with warmer, deeper water, allowing the surface to reach its freezing point of approximately -1.8°C much faster. As ice forms, it expels salt in a process called brine rejection. This creates incredibly cold and dense water that sinks to the continental shelf. This Dense Shelf Water (DSW) is so dense it cascades into the deeper ocean, becoming a primary source for North Pacific Intermediate Water, a component of global ocean circulation. The Sea of Okhotsk pumps cold water for the entire North Pacific.
An ice-dependent ecosystem
The ice begins forming in the northwestern part of the sea in November and is carried south by winds and the East Sakhalin Current. By late February or early March, the ice cover reaches its maximum extent, covering 80% to 90% of the sea's 1.58 million square kilometer surface. The average ice thickness is about one meter. Within this vast ice pack, areas of persistent open water called polynyas form, typically maintained by strong offshore winds. These polynyas are sites of intense ice production, where cumulative ice formation can reach 11 meters over a single winter.
These polynyas and the marginal ice edge are important biological hotspots. The constant ice formation and brine rejection super-oxygenate the water, supporting a diverse ecosystem. The ice itself provides a platform for marine mammals, particularly for the ribbon seal (Histriophoca fasciata) and spotted seals, which use the floes for resting and raising their pups. The edge of the ice is the primary wintering and feeding ground for one of the world's largest raptors, the Steller's sea eagle (Haliaeetus pelagicus). Thousands of these birds migrate south to Hokkaido, Japan, following the drift ice to hunt for fish like cod and pollock in the open leads between floes. The entire ecosystem is tightly coupled to the annual pulse of sea ice, which begins to retreat in March and typically melts completely by June.