As Ishikawa notes, Shiretoko’s drift ice is an “annual visitor from distant Siberia.” An extremely rare phenomenon, the sea conditions conducive to creating such frozen floes can be found in a mere handful of sites across the globe. In Japan, such floes can only be seen on the easternmost coast of Hokkaido, near the Sea of Okhotsk.
How does drift ice form? What compels its journey to Shiretoko? What effect does it have on Shiretoko’s marine habitat? Read on for a crash course on drift ice.
Drift ice originates in early November each year off the Siberian coast, in the northwesternmost reaches of the Sea of Okhotsk. Buffeted by seasonal northwesterly winds that sweep across the Siberian plains, ice begins to form, buoyed by the strong southernly East Sakhalin current. Come late January, these small patches of ice collide, clump, and cake over the course of their 1,000-kilometer journey across the Sea of Okhotsk, transforming into massive sheets by the time they reach Hokkaido. The ice makes its initial approach into Japan from Shiretoko’s western coast. By February, the sea is encrusted in a thick shelf of pure white. Once the ice has swelled to its maximum size in early March, the temperatures rise, and this annual visitor melts, receding back along the route from whence it came. Called umi-ake by the locals, this retreat signals the long-awaited “opening of the sea.” By mid-June, the ice disappears from Sea of Okhotsk without a trace.
The Sea of Okhotsk topographically resembles more of a “lake” than a “sea,” encircled by the Kamchatka Peninsula, the Kuril Islands, and Hokkaido. Shallower than the neighboring Pacific Ocean, its waters are uniquely disposed to freezing in the winter.
Whereas conventional oceans are cooled as a result of vertical convention currents, the Sea of Okhotsk is fed by a steady stream of freshwater from the Amur River, creating a 50-meter layer of relatively desalinized water at its surface.
This desalinization facilitates rapid freezing, while also resulting in two distinct sea layers: the saltier depths do not mix with the fresher surface waters, halting the vertical movement necessary for convention. When the frigid cold front roars in from Siberia, the sea’s surface cools abruptly, giving rise to the frozen ice floes in a stunningly short window of time.
Drift ice has a lower salinity that then surrounding sea water, and frees up a massive amount of salt as it freezes. As this heavy, briny water sinks, it churns the sea, acting as a pump that pushes nutrients up from the seafloor.
Moreover, swarms of phytoplankton (ice algae) are trapped underneath the frozen mantle. When the ice melts, these phytoplankton reproduce exponentially, attracting krill and fish, which in turn sustain an entire web of even larger fish, seals, and birds.
The drift ice’s encroachment locks off the Sea of Okhotsk, but the world underneath the icy surface is far from a standstill. On the contrary, the ice is an essential link in Shiretoko’s rich marine ecosystem.