SURFACE COOLED CONVECTION SIMULATIONS WITH APPLICATION TO ICE-COVEREDSEAS

Some idealized numerical experiments were carried out to study certain basic aspects of convective motions likely to occur during cooling an d ice formation in the Nordic seas. The studies were restricted to two -dimensional motions in shallow water up to 300 m deep. The freezing p oint was not assumed to depend on salinity. Cooling rates of 200-500 W m(-2) (mass flux rates of (4-10) x 10(-5) kg m(-2) s(-1)), and brine discharge rates of (4-8) x 10(-5) kg m(-2) s(-1) were assumed, the lat ter corresponding roughly to a 1 cm h(-1) ice growth. The initial flui d state was assumed to be homogeneous, having been thoroughly stirred by the preceding strong wind events common to those regions. It was as sumed that upon reaching the freezing temperature, all further cooling of the surface goes into ice formation, thus losing the buoyancy forc ing owing to the cooling-associated mass flux. Under these conditions, the results showed that the occurrence of freezing regions and ice fo rmation can either inhibit or enhance convection in the underlying wat er masses, depending on the ratio of the cooling-associated mass flux loss (owing to ice formation) to the brine release mass flux gain. Ano ther important dependence was found on diffusivity, which can enhance (or diminish) the heat advected from the ambient ocean below, The augm entation of the upward heat flux by the higher diffusivity delayed bot h the onset of freezing and the growth of the freezing area, thus dimi nishing the loss of buoyancy forcing owing to the (higher) cooling-ass ociated mass flux. For a cooling/brine mass flux ratio of five, the ki netic energy of the no-freezing system exceeded that of the freezing s ystem by a factor of three. For cooling rates of 476 W m(-2) the maxim um vertical velocities of the no-freeze simulation steadied at about 1 0 cm s(-1), and for the case of cooling-freezing with a mass flux rati o of about five, at about 5 cm s(-1). These values are within the rang e observed in penetrative ocean convection, or simulated by previous m odelers in convection experiments. In the case of insufficient cooling (238 vs. 476 W m(-2)), the freezing point was reached only briefly, w ith brine injection lasting for a period of about 3 h. Then the heat t ransported from below by the convective cells (mainly driven by coolin g-associated buoyancy) raised the surface temperature to above freezin g conditions. (C) 1997 Elsevier Science B.V.