Movement of sea ice | Previous Top Next

Sea ice is divided into two main types according to its mobility. One type is Pack ice, which is continually in motion under the action of wind and current stresses; the other is Fast ice, attached to the coast or islands, which does not move.
Wind stress in the pack ice casus the Floe s to move in an approximately downwind direction. The deflecting force due to the Earth's ration (Coriolis force) causes the floes to deviate about 30° to the right of the surface wind direction in the northern hemisphere and to the left in the southern hemisphere. Since the surface wind itself is deviated by a similar amount but in the opposite sense from the geostrophic wind (measured directly from isobars) the direction of movement of the ice floes, due to the wind drift alone, can be considered to be parallel to the isobars.
The rate of movement due to wind drift varies not only with the wind speed, but also with the concentration of the pack ice and the extent of deformation (see below). In very open pack ice (1/10-3/10) there is much more freedom to respond to the wind than in close pack ice (7/10- 8/10) where free space is very limited. Two per cent of the wind speed is a reasonable average for the rate of ice drift caused by the wind in close pack ice, but much higher rates of ice drift may be encountered in open pack ice. Since it is afloat, a force is exerted on pack ice by currents that are present in the upper layers of the water, whether these are tidal in nature or have a more consistent direction due to other forces. It is usually very difficult to differentiate between wind- and current-induced ice drift, but in any case where both are present the resultant motion is always the vector sum of the two. Wind stress normally predominates, particularly in offshore areas.