Simulations of sub-mesoscale oceanic convection and ice-ocean interactionsin the Greenland Sea

A high-resolution, rotational non-hydrostatic coupled ice-ocean convection model, defined in a vertical ocean slice, was applied to simulations of oce anic convection and its interaction with an ice cover. The model experiment s exemplify typical mixed layer situations observed in the stratified upper Greenland Sea. Results are discussed in relation to observed hydrography, and ice conditions. The focus is placed on the initial penetrative phase of convection that covers small (sub-meso) spatial and temporal scales. Short episodes of strong atmospheric forcing (outbreaks of cold polar air) are c onsidered. Simulations of convective erosion of both a shallow and a deep c old, low-saline surface 'freshwater' layer of the Greenland Sea highlight t he thermal feedback on ice growth and surface buoyancy flux caused by an en trainment of warmer underlying waters. The entrainment, and an erosion of t he stratification well below the actual penetration depth of convection, is caused by penetrative convective plumes. Plumes transmit fluctuations of n on-hydrostatic pressure across isopycnals. The model predicted a transition from haline to predominantly thermal convection whenever ice formation was involved. After a cold-air outbreak the freshwater layer is largely re-est ablished, but has increased salinity, and temperatures well above the freez ing point. A thin ice cover( < 10 cm) with a coverage between 60 and 90% re mains at the sea surface, thus impeding a deep penetration of convection fo r subsequent cold air outbreaks. With a moderate cooling ice formation and melting are in balance. The net buoyancy flux of the ice-ocean system attai ns zero values, and convection is shut off. Ice cover and ocean approach a state of stagnation which has often been observed. An attempt was made to h indcast an observed event of localised deep convection in the absence of se a ice. Forced by an oceanic heat loss of about 1000W/m(2) which lasted for 140h, the simulation reproduced the observed mixed-layer deepening from an initial 500 to 1200 m. These results and observations obtained with moored instruments suggest that deep reaching, penetrative convection in the Green land Sea is the result of thermal convection,favoured by an ice-free ocean surface, rather than of haline convection. The increased frequency of sight ings of a locally confined deep penetration of convection, in conjunction w ith the model hindcast, provide support for a hypothesised pre-conditioning of deep convection due to the mesoscale eigendynamics of the freshwater la yer. (C) 1999 Elsevier Science Ltd. All rights reserved.