As a diagnostic of the ocean response, the time history of the density anomaly near the coast is plotted in Figure 3. This is the spatial average along the channel and within 4 km of the coast. For about 23 days, the density anomaly increases nearly as the time integral of the buoyancy flux. By day 25, equilibrium has been reached with a quasi-steady density anomaly of about 09 kg m-3.
To demonstrate the similarity of this response to that of a constant surface buoyancy flux, the same calculation was repeated but using the approximate average buoyancy flux (dashed curve in Figure 2). The response is shown in Figure 3 (dashed curve). Clearly, the overall behavior is the same, although the details differ slightly. Both the time scale to reach equilibrium and the equilibrium density anomaly are the same.
4. CONCLUSIONS
The ocean responds to time-dependent polynya forcing on a much longer time scale than the polynya variations. As a result, the ocean essentially responds to an average of the polynya forcing over along time. This suggests that the simple ideas of Chapman and Gawarkiewicz (1997), based on steady forcing, may be used to estimate properties of dense water formed beneath Arctic coastal polynyas using a seasonal average of the forcing. The Pease model shows that the surface buoyancy flux and the polynya size are closely coupled and should not be treated as independent variables. In particular, the maximum density anomaly achievable and the flux of dense water from a coastal polynya depend primarily on the offshore wind speed and not the air temperature. Results suggest that most dense water is formed in relatively small polynyas (10-20 km wide), which may be missed in estimates based on satellite images. Variable polynya width acts like a forcing decay region, indicating the importance of the feature and the likelihood that the new scales derived by Chapman and Gawarkiewicz (1997) are appropriate. Finally, the results suggest that it is difficult to find reasonable combinations of parameters hat produce density anomalies greater than 1 kg m-3.
5. REFERENCES
Aagaard, K., L. K. Coachman, and E.Carmack, 1981: On the halocline of the Arctic Ocean. Deep-Sea Res., 28A, 529-545.
Cavalieri, D.J. and S. Martin, 1994: The contributions of Alaskan, Siberian, and Canadian coastal polynyas to the cold halocline layer of the Arctic Ocean, J. Goophys. Ros., 99, 18343-18362.
Chapman, D.C., 1998: Setting the scales of the ocean response to isolated convection, J, Phys, 0coanogr., in press.
Chapman, D.C, and G. Gawarkiewicz, 1995:Offshore transport of dense shelf water in the presence of a submarine canyon. J, Geophys. Res., 100, 13373-13387.
Chapman, D.C. and G. Gawarkiewicz, 1997: Shallow convection and buoyancy equilibration in an idealized coastal polynya, J, Phys. 0coanogr.,27, 555-566.
Gawarkiewicz, G. and D,C. Chapman, 1995:A numerical study of dense water formation and transport on a shallow, sloping continental shelf.J Geophys. Res,,100,4489-4507.
Haidvogel, D., J. Wilkin and R. Young, 1991: A semi-spectral primitive equation ocean circulation model using vertical sigma and orthogonal curvilinear horizontal coordinates, J Comp. Phys., 94, 151-185.
Markus, T. and B.A. Burns, 1995:A method to estimate subpixel-scale coastal polynyas with satellite passive microwave data, J. Goophys, Res.,100,4473-4487.
Pease, C. H., 1987: The size of wind-driven coastal polynyas,,J Geophys, Res., 92, 7049-7059 .
Visbeck, M., J. Marshall and H. Jones, 1998: Dynamics of isolated convective regions in the ocean,J. Phys. Ocoanogr,,26, 1721-1734.
