Coupled ice/ocean modeling of Baffin Bay and the formation of the North Water polynyas.

摘要

Northern Baffin Bay is the location of a number of recurrent polynyas (areas of reduced ice cover) together called the North Water. In aggregate, the North Water is the largest recurrent ice anomaly in the Northern Hemisphere and has important local and regional climatic effects. The mechanism by which the polynyas are created has been a subject of debate. The primary role of wind is recognized but observational evidence suggests that upwelled warm water from a subsurface Atlantic layer inhibits ice formation, enlarging the anomaly.; In this study, a coupled ice/ocean model was applied to Baffin Bay to (1) evaluate existing theories about the North Water, (2) examine the sensitivity of the polynyas to changes in forcing, and (3) investigate the relationship between the polynyas and deep and bottom water formation. The model consists of a 3D sigma-coordinate primitive equation ocean model with turbulence closure and a 2D ice dynamics/thermodynamics model. Coupling between the ice and ocean components permits the transfer of momentum, heat, water, and salt. Realistic fields for seafloor topography, ocean temperature, salinity, and currents, snow depth, and ice concentration and thickness were used as initial and boundary conditions.; In a control simulation, with atmospheric forcing typical of wintertime polynya events, the behavior of the ice and ocean corresponded well to observation. Areas of reduced ice cover were produced in the locations and with the sizes of the real examples. Strong upwelling occurred in the polynyas, bringing warm water to the surface. Surface heat fluxes were within the observed limits. An experiment in which the upper ocean layers were made isothermal at freezing temperature showed that the warm water upwelling resulted in a polynya approximately twice as large. Unrealistically low surface heat fluxes in this experiment indicated that the North Water cannot be purely wind-driven. Life cycle simulations with time-variable wind forcing showed that the warm water helps maintain the ice anomaly for a longer time. Sensitivity studies confirmed the critical role of wind forcing and demonstrated that the availability of oceanic heat is extremely important to the size of the polynyas.