Projects - Sea ice mass balance
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As part of the Arctic Observing Network we have developed and deployed  autonomous ice mass balance buoys (IMB). Since 1997 more than 40 of these buoys have been deployed in Arctic, while 5 have been deployed in the Antarctic. The buoys deployed in the Arctic are always collocated with other autonomous instruments that are monitoring the atmosphere or ocean. Each year IMBs are deployed at the North Pole Environmental Observatory and the Beaufort Gyre Exploration Project. The IMBs monitor the amount of snow accumulation and ice growth in winter and the amount of surface and bottom melt during summer.This provides the key feature of the IMBs; the ability to attribute the changes in the Arctic sea ice cover that are observed by satellite.

The buoys (see photo to right) consist of a Campbell scientific datalogger, an Argos transmitter, a thermistor string, and above ice and below ice acoustic sounders measuring the positions of the surface and within 5 mm. The buoys also have a barometer and an air temperature sensor. Other sensors can easily be incorporated into the buoy.


IMB results from the 2008 summer melt season are presented below. The plot shows the the approximate summer position (white dot), the total amount of summer surface melt (red bar), and the total bottom melt (yellow bar) for the summer of 2008. The top number above the bars was the thickness before melt and the bottom number was the thickness after melt. All of the buoys were installed in undeformed multiyear ice, with end of winter ice thicknesses  ranging from 1.83 m to 3.17 m. The map in the background was the September 2008 ice concentration courtesy of NSIDC and NASA. There was considerable regional variability in the amount of surface, bottom, and total melting. The minimum amount of melting was 0.4 m at a location north of Greenland, where 0.3 m of surface melt and 0.1 m of bottom melt resulted in a total thinning of only 0.4 m. The maximum melting was in the Beaufort Sea, where 3.2-m-thick ice completely melted by 23 August 2008, with more than 0.9 m of surface melt and 1.8 m of bottom melt. One of the sites, denoted by an asterisk was in a melt pond and had the second largest amount of surface ablation, even though it was the farthest north site. Four sites had more bottom melting than surface melting, while the remaining three had more surface melting. The averages for the seven sites were 0.53 of surface melting and 0.66 m of bottom melting. Four of the sites had more bottom melting than surface melting.

Some historical perspective on the 2008 summer melting can be obtained from observations made 50 years ago during the International Geophysical Year. Sea ice mass balance measurements were made on drifting ice camps in 1957, 1958, and 1959 using ablation stakes and thickness gauges (Untersteiner, 1961; Hanson, 1964). The positions of these results are denoted by a light gray diamond, the surface melt by a dark gray bar, and the bottom melt by a magenta bar. The observed surface and bottom melting were between 0.1 and 0.4 m and average values for the 1957 – 1959 data were 0.28 m of surface melting and 0.20 m of bottom melting, much less than the averages for the summer of 2008. Interestingly, the 2008 site north of Greenland had melt amounts similar to the 1950s values. This is a region that has not exhibited any recent reduction in ice extent, ice concentration, or ice thickness.

The background map in Figure 1 displays ice extent and concentration in September 2008. A casual examination shows a tendency towards greater bottom ablation in regions of lower ice concentration. The largest observed bottom melting was at the site at the ice edge in the Beaufort Sea, while the smallest was north of Greenland where the ice concentration was large throughout the summer. In the summer of 2007, there was also large amounts of bottom melting in the Beaufort Sea, which were caused in part by enhanced solar heating of the upper ocean.
 
 
For more detailed information plus results from all the IMBs visit the web site Monitoring the mass balance, motion, and thickness of Arctic sea ice.

POC: Jackie Richter-Menge, Don Perovich



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