Ocean-atmosphere heat exchange occurs across the world ocean. When the air temperature drops below that of the surface ocean the water gives up heat to the atmosphere, leading to cooling of the sea surface and warming of the overlying air. As the warm air rises and cold air replaces it the ocean gives up more heat. This process eventually leads to sea ice formation in high latitudes.
The Early Stages of Sea Ice Formation
The cooling of surface waters leads to an increase in density and these waters sink, allowing upwelling of warmer waters that then lose their heat to the atmosphere. This overturning circulation typically continues until the entire mixed layer (the region stirred by waves, wind and overturning) is at freezing point, when ice begins to form. Because of its salt content, seawater freezes at a slightly lower temperature (roughly -1.86 °C) than fresh water.
For the entire mixed layer (typically a few metres to several tens of metres thick, depending on local conditions) to reach freezing point, the ocean must lose a large amount of heat. This is why seawater does not normally freeze in temperate regions, even if air temperatures drop below zero. In these regions, the temperature contrast between sea and air is not sufficiently large or sustained for the ocean to lose enough heat to freeze.
When the ocean loses heat to the air a layer fog can form over the surface of the sea as water evaporating from the sea surface cools rapidly. This phenomenon is known as ‘sea smoke’ and is often seen in the early stages of sea ice formation. The first ice crystals take the form of needles or platelets and are kept in suspension throughout the upper water column until a layer of surface ice has built up.
The surface ice reduces wind-induced mixing, and allows more ice to build up. Ice crystals clump together to form a slushy aggregate known as ‘frazil ice’. In calm conditions a thin, smooth layer of frazil, termed ‘nilas’, may form at the surface. A thicker layer of frazil on the ocean surface can give the sea an oily appearance and is termed ‘grease ice’.
Sea ice is slightly salty but much less so than ocean water due to the phenomenon of ‘brine rejection’. As the ice forms it expels much of the salt contained in the seawater and this salt is mixed back into the ocean. The increase in salt in the water leads to an increase in density, which helps to drive the overturning circulation that brings warm waters to the surface.
This mixing process means that newly formed ice crystals within the water column may be brought into contact with warmer water from depth and melt again. This is why the entire mixed layer must reach freezing point before the sea ice cover is able to form. The time taken to achieve this varies depending on local conditions such as wind mixing and heat stored in the deep ocean.
The Formation of Pancake, Pack and Fast Ice
Frazil may also accrete to form cm-sized discs which clump together to form ‘pancake ice’ – plates of ice several tens of centimetres in size. In turn, aggregations of pancake ice form ice floes and drift ice (a large mass of ice that drifts under the influence of wind and currents). Where floes have collided there can be an accumulation of broken ice fragments, termed ‘brash ice’.
Large regions of sea ice are known as ‘fast ice’ when anchored by a shoreline (i.e. not free to drift with ocean currents or winds) or ‘pack ice’ when not anchored by land. First year ice may grow as thick as 1.8 m in the Arctic and 0.7 m in the Antarctic. Ice is generally thinner in the Antarctic because the Southern Ocean holds more heat than the Arctic Ocean, which inhibits sea ice growth.
Polynyas and Multiyear Ice
Regions of particularly high oceanic heat fluxes can lead to thinning or even disappearance of the sea ice cover. Such ice-free regions are called polynyas and can persist throughout one or more winters. If sea ice survives the summer it can become multiyear ice, becoming thicker with each winter. Although more sea ice forms in the Antarctic each year, most of it melts so the Arctic actually holds more multiyear ice.
Ice Blink and Water Sky
Sea ice can be detected from far out in the ocean by ‘ice blink’ on the horizon, which is a white light caused by reflection off the ice field beyond. Conversely, when in the ice pack, ‘leads’ (open channels) or the open ocean can be detected by looking for ‘water sky’, which is a dark reflection on the overlying clouds. The observation of these phenomena is aided by the frequent cloud cover experienced in polar regions.
Ocean-atmosphere heat exchange is crucial in the formation of sea ice, while the process is strongly regulated by the flow of heat from the deep ocean to the surface. There are many different types of sea ice, and the type that forms at any one location depends on local conditions at the time of formation, with factors such as wind mixing playing an important role.
Sources
Eicken, H. 2003. Sea ice – an introduction to its physics, biology, chemistry and geology. D. Thomas & G.S. Dieckmann (eds), Blackwell Science, London.
National Snow and Ice Data Center, State of the Cryosphere, last updated 18 February 2010, accessed July 2010
Margaret Wallace - Hello, my name is Mags Wallace. I did an MESci in geophysics and Geology from Liverpool University, followed by a PhD in Physical ...
