Concentration gradients and growth/decay characteristics of the seasonal sea ice cover

Abstract

The growth and decay characteristics of sea ice in the northern and southern hemispheres are presented quantitatively by using brightness temperature data from the Nimbus 5 Electrically Scanning Microwave Radiometer. The areal extent of >15% ice cover follows a basically symmetrical seasonal cycle in the northern hemisphere, ranging from 8×106 km2 in summer to 16×106 km2 in winter. In contrast the seasonal cycle in the southern hemisphere is asymmetrical, with longer periods of growth than decay and areal extent ranging from 4×106 km2 in summer to 20×106 km2 in winter. At different longitudes the occurrences of the maximum rates of advance of the ice edge differ by up to 2 months, and similarly for retreat. The distribution of the rate of change in the latitudinal position of the ice edge, using as a reference point the 50% ice concentration line near the ice‐ocean boundary, follows a quasi‐Gaussian function with peak at about 60 km/month and with a standard deviation of about ±120 km/month. However, for some months, rates as large as a 940‐km/month change in the latitudinal position are observed when the movement of the ice edge tends to be in the eastward or westward direction as, for example, in the Weddell Sea. The gradient of ice concentration near the ice edge has considerable variability, following a Poisson distribution with peaks at about 0.89% ice concentration per kilometer as derived from the 3‐day averages and 0.62% ice concentration per kilometer from the monthly averages. During the growth cycle, the dominant change in ice cover is the advance of consolidated ice behind the advancing ice edge, while during decay the retreat of the ice edge is accompanied by massive reduction of ice concentration within the ice pack. The monthly changes in ice edge position in the southern hemisphere are also shown to have good correlation with the rates of advance and retreat of the 271 K surface temperature isotherms inferred from climatological temperature data.