Transient sea-ice polynya forced by oceanic flow variability

In the Weddell Sea during the winters of 1974-1976 a significant opening in the sea-ice cover occurred in the vicinity of a large bathymetric feature - the Maud Rise seamount. The event is commonly referred to as the Weddell Polynya. Aside from such a large-scale, relatively persistent polynya in th e Weddell Sea, transient, small-scale polynya can also appear in the sea-ic e cover at various times throughout the winter and at various locations wit h respect to the Maud Rise. The underlying causes for the occurrence of suc h transient polynya have not been unambiguously identified. We hypothesize that variations in the mean ocean currents are one major contributor to suc h variability in the sea-ice cover. Analysis of the sea-ice equations with certain idealized patterns of ocean currents serving as forcing is shown to lead to Ekman transports of sea ice favorable to the initiation of transie nt polynya. Aside from the actual spatial pattern of the idealized ocean cu rrents, many other factors need also be taken into account when looking at such transient polynya. Two other such factors discussed are variations in the sea-ice thickness field and the treatment of the sea-ice theology. Simu lations of a sea-ice model coupled to a dynamical ocean model show that the interaction of (dynamical) oceanic currents with large-scale topographic f eatures, such as the Maud Rise, does lead to the formation of transient pol ynya, again through Ekman transport effects. This occurs because the seamou nt has a dynamic impact on the three-dimensional oceanic flow field all the way up through the water column, and hence on the near surface ocean curre nts that are in physical contact with the sea ice. Further simulations of a t sea-ice model coupled to a dynamic ocean model and forced with atmospheri c buoyancy fluxes show that transient polynya can be enhanced when atmosphe ric cooling provides a positive feedback mechanism allowing preferential op en-ocean convection to occur. The convection, which takes hold at sites whe re transient polynya ha ve been initiated by sea-ice-ocean stress interacti on, has an enhancing effect arising from the convective access to warmer, d eeper waters. To investigate all of these effects in a hierarchical manner we use a primitive equation coupled sea-ice-ocean numerical model configure d in a periodic channel domain with specified atmospheric conditions. We sh ow that oceanic flow variability can account for temporal variability in sm all-scale, transient polynya and thus point to a plausible mechanism for th e initiation of large-scale, sustained polynya such as the Weddell Polynya event of the mid 1970s. (C) 2001 Elsevier Science Ltd. All rights reserved.