Dense Water Formation Beneath a Time-dependent Coastal Polynya
David C. Chapman
Clark 316B, MS#21 Woods Hole Oceanographic Institution
Woods Hole, MA 02543
Tel: 1-508-289-2792
Fax: 1-508-457-2181
dchapman@whoi.edu
Dense water formed beneath Arctic coastal polynyas is generally acknowledged to play a leading role in providing cold, saline water to maintain the upper halocline of the deep Arctic basins. Recent modelling studies of the ocean response to idealized coastal polynyas have simple analytical expressions for the maximum density anomaly achievable beneath a polynya as a function of the polynya size and the buoyancy flux imposed at the ocean surface. These models have assumed a steady buoyancy flux and a fixed polynya geometry. Furthermore the polynya size and the surface buoyancy flux have been treated as 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 the model of Pease (1987, J. Geophys. Res.-Oceans, 92, 7049-7059). Though highly idealized, the Pease model produces polynyas which open and close on reasonably realistic time scales, and it dynamically couples the polynya size and buoyancy flux.
Results reveal several interesting and useful features of the ocean response to time-dependent polynya forcing. First, under reasonable atmospheric conditions, both the volume flux of dense water formed and the maximum density anomaly achievable are nearly independent of atmospheric temperature (and, therefore, surface buoyancy flux) but strongly dependent on the magnitude of the wind that forces the ice offshore. Second, variations in polynya size produce horizontal gradients in surface buoyancy flux which are important in setting the scales of the ocean response, as suggested by Chapman and Gawarkiewicz (1997, J. Phys. Oceanogr., 27, 555-566). Third, time scales of the ocean response (>20 days) are typically longer than time scales associated with polynya openings and closings (a few days). Therefore, the ocean response to time-dependent polynya size and buoyancy flux is nearly the same as if the polynya size and 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.
