Chapter 4 - External forcing and local response

Abstract

The Arctic Mediterranean is subject to several conflicting influences. The strong but seasonally varying heat loss to space cools and increases the density of the warm water advected from lower latitudes, while the atmospheric transport of water vapor, supplied to the Arctic Mediterranean as river runoff and net precipitation, increases the stability of the water column and limits the heat loss to a less dense upper layer. Ice formation rather than deep convection therefore takes place in most parts of the Arctic Mediterranean Sea. However, ice formation extracts freshwater and rejects saline brine, which increases the density of the underlying water column. If the water is shallow, as on the Arctic Ocean shelves, saline bottom waters are formed and accumulate during winter. These waters are dense enough to ventilate the deep Arctic basins, provided they reach the shelf break and sink down the continental slope. Open ocean deep convection only occurs in areas little influenced by freshwater input such as the Greenland Sea and the Labrador and Irminger seas. Both ice formation and deep convection contribute to the, albeit small, oceanic part of the heat balance of the Arctic. The heat loss to the atmosphere in winter is largely supplied by the release of latent heat of freezing, and in summer the created sea ice reflects the shortwave solar radiation and, by melting, keeps the surface temperature close to the melting point. Open ocean convection, by contrast, contributes oceanic heat and moisture to the atmosphere, drawing on the heat stored in the deeper layers. In summer the surface layer is heated by solar radiation and its temperature increases, and the absorbed heat is given up to the atmosphere in winter. This chapter deals first with sea ice, its structure, its formation, growth and melting, its thermodynamic and mechanical properties, and its interactions with the underlying water column. This leads to the second theme of the chapter, convection, both thermal and haline, how it is described observationally and theoretically, and how it has been modeled in the laboratory as well as numerically.