Altimetry peaks at sea ice

Image of the Month - July 2011

Sea ice cover deduced from altimetry echo shapes for the austral winter of 2009 and 2010 (yellow sea ice cover for both winters, red only 2009, blue only for 2010). For comparison an indication of scatterometer measurements is given for 2009, which is matching quite well with the altimetry-deduced sea ice cover for the same year. (Credits ISRO/SAC)

Antarctica is a frozen continent. However, sea ice around it mostly forms and melts each year, since the Southern Ocean that surrounds it is open (contrary to the Arctic Ocean, enclosed by lands). Variations occur from one year to the other, with impacts on iceberg sheddings, marine animals... However, the Southern Ocean is not a place where a ship can safely navigate and easily take measurements, even when the ocean is ice-free. It is next to impossible in the core of the Austral winter. So satellites are the only way of really monitoring ice cover around Antarctica.

Altimetry can observe sea ice in different ways. We already wrote about some of them. Today is still another one. Altimeter radar waves do not bounce back in the same way over water or ice. When the waves are bouncing on ice, the echoes are much "peakier" than over water. A parameter can be included in altimetry data that measures this "peakiness'. Thus when such peaky echoes are encountered over ocean, it is a sign that there is sea ice rather than water.

Jason-2 has been measuring the sea surface height, and incidentally a number of other parameters, for three years now. With the future Saral mission and the AltiKa altimeter onboard, other information will be added, and the time series continued.

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