Numerical process studies of the impact of mesoscale dynamics on primary production in the Arctic Marginal Ice Zone

The effect of mesoscale eddies on the primary production in the Arctic MIZ is investigated numerically by using a dynamical model utilizing an isopycnic layer formulation coupled to an ice model and a 5 compartment NPZD ecosystem model.

Idealized experiments are carried out to investigate the response of the ice-ocean system and the ecosystem for various wind scenarios acting on an ice edge. A novel feature is the employment of a simple parameterization making the wind stress dependent on the ice state yielding typically maximum stress on the ocean surface for intermediate ice concentrations. The only specification is the wind vector on top of the atmospheric boundary layer.

The results agree with classical theory in that up-welling ocurs near the ice edge when a wind is blowing with the ice to the right -- ITR --  (in the northern hemisphere). Growth is increased both due to light abundance and entrainment of nutrient from below. A more pronounced additional effect is the formation of an ice-edge jet which whithin a few days disintegrates into eddies. The energy transfer feeding the process is relatively modest, because the ice edge is compacted and the region of maximum stress is narrowed. The resulting weak cyclones (O(5km)) are very effective in transporting nutrient from the ice covered to the open water, enhancing biological growth within a 5 kilometers wide band along the ice edge several days after the initial burst.

In the ITL case (opposite wind) the wind expands the MIZ, widening the region with high stress, such that energy is comparably more efficiently transferred to the ocean. Eddies are anticyclonic, larger (15 km) and more energetic. Biological growth is enhanced over a 20 km wide stripe, while some inhibition due to shading of ice bands extending off-ice takes place. In an experiment examining the effect of a wind turning 360 degrees (ITR-ITL-ITR), the resulting response yielded a qualitatively similar pattern as the ITR case, but with a stronger response due to the energizing during the ITL period.

A common qualitative feature for all cases, independent of wind history,  is the petsistence of eddies along the ice edge transporting nutrient from the unaffected ice-covered portion, to the open water which is void of nutrient. As such it behaves as a horizontal analogue of the deep chlorophyll maximum in the vertical. The small scales involved imply that there may be large sampling errors for the biogeochemical fields in reality.