Formation Mechanism of Huge Coastal Polynyas and Its Application to Okhotsk Northwestern Polynya

Abstract

Abstract This paper investigates the formation mechanism of broad coastal polynyas beyond 100 km in offshore width. It is known that two regimes for wind-driven polynya opening exist: one is a convergent regime at the polynya edge in which inner frazil ice catches up with outer consolidated ice, whereas the other is a divergent regime in which the consolidated ice drifts offshore faster than the frazil ice at the edge. In this study, the authors focus on the latter, divergent polynya-edge regime. Because in the divergent regime the polynya possibly evolves without bound, they consider a thermal growth for inner frazil ice to find a finite solution of offshore width. Then, the authors investigate responses of the polynya opening for various wind angles ϕ from the offshore direction from the viewpoint of the polynya-edge regimes. At first, the authors estimate the deviation angle and wind factor for the frazil and consolidated ice based on each momentum balance, because sea ice motion driven by wind varies depending on the ice thickness due to relative effect of the Coriolis force. It was found that, when the surface wind deviates leftward about 10° or greater from offshore, the divergent regime at the polynya edge generates a great polynya evolution. Otherwise, the convergent regime takes place yielding small offshore extent. These theoretical results were verified for the Okhotsk northwestern polynya using thin-ice-thickness data derived from Advanced Microwave Scanning Radiometer for Earth Observing System (AMSR-E) and the Japanese 25-year Reanalysis (JRA-25) surface wind. The data show that the polynya development distinctively changes at ϕ ∼ 0°, and the largest width is shown in the leftward wind angle of ϕ = 0°–30°. This clear relationship between the surface wind angle and the offshore extent of polynya nicely supports the theory.