Movement of Sea Ice

Figure 1-36.—Various types of ice topography caused by pressure. Movement of Sea Ice As floes break free of the fast-ice sheet, they move under the effects of the winds and currents. Any movement of ice floes is called "drift." Drift refers to sea ice, as well as broken off portions (outer edges) of fast ice that moves along with wind, tides, and currents. Drift may, therefore, be more specifically defined by direction and speed of movement. Pack ice usually drifts to the right of the true wind in the Northern Hemisphere (left in the Southern Hemisphere). Observations show that the actual drift is about 30° from the wind direction, or very nearly parallel to the isobars on a weather map. The drift more closely follows the wind in winter than in summer. In summer, the tides play a bigger role in the movement of the ice. A close estimate of the speed of drifting pack ice is possible by using wind speed. On the average, the drift of ice in the Northern Hemisphere ranges from 1.4% of the wind speed in April to 2.4% of the wind speed in September. Although wind is the primary driving force, the presence or absence of open water in the direction of the drift greatly influences the speed of drift. Ice-free water in the direction of the drift, no matter how distant, permits the pack ice to drift freely in that direction. Ice- clogged water, on the other hand, slows the forward movement of ice. ICE OF LAND ORIGIN Ice of land origin is ice that forms on land, usually as glaciers, and moves to the sea. Composed by large accumulations of compacted (freshwater) snow on land areas, the weight of the accumulated snow/ice forces the ice sheets to move as glaciers. If a glacier is located along a coastline and reaches the sea, the leading edge of the glacier may break off (calve) and fall into the sea. This ice then drifts to sea as an iceberg. Since 86% of the world’s glaciers occur in Antarctica, most icebergs originate around that continent. Most of the remainder of the world’s glaciers are located in Greenland. Greenland is the main source of icebergs in the Northern Hemisphere (about 90%). Nearly 70% of Greenland’s icebergs originate along the western coast near 68°N. ICEBERG CLASSIFICATION Icebergs are classified by shape and by size. As classified by shape, icebergs are either pinnacled or tabular. Pinnacled bergs are generally cone-shaped, but may be very irregular. Tabular bergs are generally flat-topped and straight-sided. See figure 1-37. 1-53 Figure 1-37.—Types of icebergs.