Icebergs/Polynyas

Icebergs that break free from the continent are subjected to persistent off-shore winds that create regions of open water between the continent and the seaward-drifting ice mass.(Figure 11a)* Strong and persistent off-shore winds are created due to continuous cooling over the Antarctic continent that leads to downslope flow from the high latitudes on the continental interior. Although the upper surface of the ice may cool to very low temperatures, the water between icebergs, known as polynyas, will remain near 4 degrees C since further cooling will increase its density and cause it to sink (Figure 12). These regions of open water represent relative heat sources for the atmosphere, since they may be several degrees warmer than the ice surfaces of the surrounding icebergs. Water near the freezing point is not normally considered to be a source of heat, but compared to ice at -20 degrees C, it is comparatively "hot". This relatively warm water encourages evaporation, leaving behind water whose density is rapidly increasing due to both cooling and increased salt concentration during evaporation. This process creates downwelling plumes of cold, highly saline water that leads to production of deep water as discussed in the lecture on ocean structure and circulation and is shown in Figure 13.

Figure 14 shows the mass balance for Greenland. Note for this ice sheet that calving and melting/runoff make nearly equal contributions to the loss of mass and their sum equals the accumulation rate to give a steady state to the total glacier mass. The following image gives results of earlier estimates of the Greenland ice mass balance showing some discrepancy in estimates among different observers. Data given in Figure 15 should be considered more recent and reliable estimates.

*Figure 11a is theb0481, from the Ship Collection of the NOAA Photo Library

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