Dense Water Formation beneath a Time-Dependent Coastal Polynya*

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Abstract
Recent modeling studies of dense water formation beneath an idealized steady coastal polynya have provided simple analytical expressions for the maximum density anomaly achievable as a function of the polynya geometry and the imposed surface buoyancy flux. These studies have assumed that the buoyancy flux and polynya geometry are both constant and independent parameters. To relax these assumptions, dense water formation is examined beneath a coastal polynya whose size and surface buoyancy flux are computed from atmospheric temperature and wind velocity according to a polynya model developed by Pease. Though highly idealized, the Pease model produces polynyas that open and close on reasonably realistic timescales, and it thermodynamically couples the polynya size and buoyancy flux. Results reveal several interesting and potentially useful features of the ocean response to time-dependent polynya forcing. First, under reasonable atmospheric conditions, both the maximum density anomaly achievable and the volume flux of dense water formed are nearly independent of polynya width and atmospheric temperature (and, therefore, surface buoyancy flux), but they are strongly dependent on the magnitude of the wind that pushes the ice offshore. Second, variations in polynya size produce horizontal gradients in surface buoyancy flux that are important in setting the scales of the ocean response. Third, timescales of the ocean response (>10 days) are typically longer than timescales associated with polynya openings and closings (a few days). Therefore, the ocean response to time-dependent polynya size and surface buoyancy flux is nearly the same as if the polynya size and surface buoyancy flux were fixed at the time average of the forcing (over 30–60 days). This suggests that reasonable estimates of dense water formed beneath Arctic polynyas may be possible by applying the simple expressions based on steady forcing, but using the seasonal averages of the parameters. Finally, it is difficult to find realistic combinations of atmospheric conditions that produce large quantities of water with density anomaly greater than about 1 kg m−3.

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