Recent decades have seen pronounced Arctic warming accompanied by significant reductions in sea ice volume and a dramatic increase in summer open water area. The resulting combination of increased ice-free area and more mobile ice cover has led to dramatic shifts in the processes that govern atmosphere–ice–ocean interactions, with profound impacts on upper ocean structure and sea ice evolution. The summer sea ice retreat and resulting emergence of a seasonal marginal ice zone (MIZ) in the Beaufort Sea exemplifies these changes and provides an excellent laboratory for studying the underlying physics.
The Office of Naval Research MIZ initiative employs an integrated program of observations and numerical simulations to investigate ice–ocean–atmosphere dynamics in and around the marginal ice zone in the Beaufort Sea. The measurement program exploits a novel mix of autonomous technologies (ice-based instrumentation, floats, drifters, and gliders) to characterize the processes that govern MIZ evolution from initial breakup and MIZ formation through the course of the summertime sea ice retreat. The flexible nature and extended endurance of ice-mounted and mobile, autonomous oceanographic platforms allows the array to follow the MIZ as it retreats northward, sampling from fully ice-covered waters, through the difficult MIZ and into the open water to the south. The nested array of drifting and mobile autonomous platforms resolves a broad range of spatial and temporal scales. By remaining focused on the MIZ as it retreats, the array resolves changes in the physics associated with increasing open water extent.
One of the more obvious impacts of the changes in ocean–ice–atmosphere interaction in the Beaufort and Chukchi seas region has been the expansion of a Marginal Ice Zone (MIZ); a long-standing feature in the Bering and Chukchi seas, but a relatively new phenomena in the deep Beaufort Sea. In the western Arctic, the northward retreat of the sea ice edge increases the open water area, allowing direct momentum transfer from the atmosphere to the ocean surface through wind-driven waves. The resulting fragmented ice field has different surface roughness features, and because the smaller floes are significantly more mobile, sea ice can absorb more atmospheric surface stress through deformation and transfer it to the ocean surface.
More efficient atmosphere–ocean coupling in regions of partial ice cover and open water can amplify upper ocean mixing far beyond levels observed under full ice cover. As in the open ocean, strong winds acting on ice-free regions of the Arctic will drive turbulent mixing that deepens the surface mixed layer, entraining waters from below. Because the sea ice cover moderates atmosphere–ocean fluxes and the ocean affects the ice cover through fracturing, divergence, and melting, the ice–ocean system is strongly coupled within the MIZ.
Key upper ocean processes that contribute to strong coupling:
- Propagation and attenuation of ocean surface waves
- Absorption and storage of incoming solar radiation and its subsequent lateral transport
- Vertical mixing within and at the base of the ocean mixed layer
Atmosphere, ice, and upper ocean processes in the MIZ. Ice cover modulates penetration of solar radiation and isolates the upper ocean from direct wind forcing, but increasing open water within the MIZ, and the proximity of large expanses of open water immediately to the south, permits more direct connection with the atmosphere. Strong open water swell and surface wave activity attenuates as it enters the MIZ. Likewise, internal waves, submesoscale eddies, and mixing weaken with increasing ice cover. Small-scale windstress curl associated with ice to open water transitions and variations in ice roughness may induce intense secondary circulations that drive rapid vertical exchange. Enhanced mixing and vertical exchange can entrain heat stored below the mixed layer, increasing basal melting of sea ice within the MIZ. In ice-covered regions, local radiative solar warming leads to direct ablation of sea ice and some bottom melt from the radiation penetrating weakly into the ice-covered upper ocean. Open water regions within and south of the MIZ allow increased radiative upper ocean warming and, through lateral advection, accelerated ice melt. These processes are expected to amplify variance at short spatial and temporal scales across the MIZ.