DENSE WATER FORMATION BENEATH A TIME-DEPENDENT COASTAL POLYNYA

　

David C. Chapman*

Woods Hole Oceanographic Institution

Woods Hole, Massachusetts

　

1. INTRODUCTION

　

It is generally accepted that dense water formed on Arctic continental shelves must play a leading role in the maintenance of the cold upper halocline in the deep Arctic basins, as first proposed by Aagaard et al. (1981), although the dynamics of the maintenance process largely remain a mystery. The required dense shelf water is believed to be produced primarily in regions of rapid and persistent ice growth, e.g. coastal polynyas, in which brine rejection during ice formation acts as a negative buoyancy flux into the waters beneath. The amount of dense water produced in coastal polynyas is uncertain, partly because of our lack of understanding of the ocean response beneath a coastal polynya.

Several recent modeling studies have begun to examine the ocean response beneath a shallow coastal polynya and the subsequent offshore transport of dense water (Gawarkiewicz and Chapman, 1995; Chapman and Gawarkiewicz, 1995, 1997;Chapman, 1998). Results show that the dense shelf water is carried rapidly offshore in a complicated field of small-scale (15-25 km)eddies. Scaling arguments based on he approach of Visbeck et al. (1996) have been used to derive simple algebraic expressions for he maximum density anomaly achievable beneath the polynya and the time required to read this density anomaly, each in terms of the prescribed surface buoyancy flux and the polynya geometry. These studies have all considered the response beneath a polynya in which the polynya geometry and surface buoyancy flux are held constant in time and are treated as in-dependent parameters. To relax these constrains, the idealized polynya model developed by Pease (1987)is used (i)to introduce time-dependence in the surface buoyancy flux and polynya size and (ii) to dynamically couple the two parameters.

　

2. THE POLYNYA MODEL

　

Pease (1987) proposed a simple dynamical model of a latent heat coastal polynya in which the polynya size is determined by a competition between ice formation tending to close the polynya and wind tending to open the polynya by blowing the ice offshore. The offshore width of the polynya r₀ is given by

where B₀ is the surface buoyancy flux, V_i is the offshore speed of the ice which is taken to the 30% of the offshore wind speed V_a and κ =0.0165 m²s^-2 is a constant that accounts for the increase in density from salt rejection and the 'collection depth' of the newly formed frazil ice (taken here to be 0.1 m based on Markus and Burns, 1995). The surface buoyancy flux is determined by the ice production rate and can be estimated from the surface heat flux, making the same assumptions as Pease (1987). For Arctic conditions, the most important contribution comes from the sensible heat flux which is linearly proportional to the magnitude of the wind speed ｜V_a｜ and the air temperature T_a. So, T_a and V_a are the independent variables that drive the Pease model.

If T_a and V_a are constant, then the solution to (1) is

which describes an exponential opening of a polynya, from r₀=0 to the maximum width of r_0max =0.03κV_a/B₀. For reasonable wind speeds and air temperatures, the maximum polynya width is 10/20 km. The time scale over which the polynya opens, κ/B₀ is typically less than one day, suggesting that the ocean probably cannot respond as rapidly as the polynya, The ocean may, therefore, be expected to respond to some average of the polynya forcing.

It is interesting to note from (2) that B₀ and r₀ are closely coupled and should not be considered independent as in previous studies. In fact, the maximum possible in crease in density beneath a coastal polynya and the resultant seaward flux of dense water depend on the total amount of salt released during ice production which is proportional to B₀r₀. After many days of forcing (i.e. large t),(2) yields B₀r₀ = 0.03κV_a which is independent of air temperature! So, the rate at which ice moves offshore determines how dense the water can become, not the air temperature or the polynya width. Thus, the presence of a very large polynya or very cold air temperature does not necessarily imply that unusually dense water is being formed. The basic reason is that very cold air temperatures lead to rapid ice production which both rejects a lot of salt and limits the polynya size. These two responses have opposite

　

*Corresponding author address: Dr. David C. Chapman, MS#21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543;e-mail:dchapman@whoi.edu

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