Sea Ice Thickness Estimates Obtained from Satellites Using Submarines and Other In Situ Observations

Dramatic changes in Arctic sea ice have been documented during the last few decades. Sea ice thickness has decreased by over 40% as revealed in a comparison of submarine ice draft data from 1993 through 1997 with data from 1958 through 1976 (Rothrock, et al.1999). The annual average extent of sea ice has decreased by 8%, and these decreases are larger during summer, 15 to 20% over the past 30 years (ACIA, 2004). Taken together, these studies imply a precipitous decline in total volume of sea ice on the Arctic Ocean. The decline of Arctic sea ice has been attributed to global warming and the flushing of most of the older, thicker sea ice from the Arctic Ocean during the extreme high Arctic Oscillation (AO) period in the early 1990s (e.g. Lindsay and Zhang, 2005; Rigor and Wallace, 2004); given the return to more moderate AO conditions, has the decline of Arctic sea ice continued? Why?

Research on the variability of Arctic sea ice requires reliable observational data sets of sea ice extent and thickness. While sea ice extent can be readily obtained from satellites, we are only now developing our capability to remotely monitor sea ice thickness. The sea ice thickness estimates obtained from satellite (freeboard) require careful validation (e.g. Fig. 1). While in situ observations of sea ice thickness e.g. by submarines (Fig. 2 & 3) and drifting buoys (Figs. 2 & 4) are more accurate, these observations are sparse in space and time. How do these very different observations relate to each other?

Mark Wensnahan and Ignatius Rigor are comparinge the observations of sea ice thickness estimates from satellites (ICESat and RADARSAT, e.g. Fig. 1; Kwok et al. 2004 and Kwok et al. 2006), with in situ observations (Fig. 2) collected by submarine cruises and moorings under the sea ice, by direct measurement during field camps, by electromagnetic instruments flown over the sea ice, and by buoys drifting with the sea ice in order to provide a careful assessment of our capabilities to monitor the thickness of sea ice.

Through this grant, we plan to:

1.      Extend the public record of sea-ice draft measurements from submarines through 2007 (currently only available to 2000);

2.      Compare in situ observations with satellite derived sea-ice freeboard and thickness obtained from ICESat to assess sources and degree of uncertainty in the satellite estimate;

3.      Assess methods of improving the satellite estimate of thickness;

4.      Study the variability of Arctic sea-ice thickness.

Some questions we hope to answer:

 1.      How do the 2005 estimates of ice thickness, for what may be the warmest year on record, compare to the longer record? How do the submarine estimates of sea ice draft for 2005 and 2007 compare to the measurements taken prior to 2000?

2.      Given the return to moderate/low AO conditions during the past few years, has the thickness of Arctic sea ice recovered?