Inter-comparison and Assessment of

Sea Ice Thickness Estimates Obtained from Satellites

Using Submarines and Other In Situ Observations

PIs: Ignatius Rigor, APL

Mark Wensnahan, APL

Ron Kwok, NASA/JPL

Jay Zwally, NASA/GSFC

Collaborators: Christian Haas, Jenny Hutchings, Jackie Richter-Menge, Arctic Submarine Lab, National Ice Center, and many others.

Introduction

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 the 1993–1997 with data from 1958–1976 (Rothrock, et al. 1999). The annual average extent of sea ice has decreased by 8%, and these decreases are larger during summer, 15–20% 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 1990’s (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?

The Problem

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?

Figure 1. ICESat elevations after removal of updated ArcGP geoid, tidal and inverted barometer effects for 2 surveys (a) February/March 2004; and (b) February/March 2005. Map samples are median elevations on a 10 km grid. The black line on (b) shows the cruise track of the submarine in November 2005.

The Plan

Through this grant, we plan to compare 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?

Acknowledgements

This research is primarily funded by the National Aeronautics and Space Administration. The Investigators are also funded by The U.S. National Science Foundation, National Oceanic and Atmospheric Administration, National Ice Center, and Office of Naval Research. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of our funders.

Figures

Figure 2. In Situ observations of sea ice thickness.

§ The ice draft measurements obtained from the 2005 submarine cruise are shown as green lines, while the planned Ice Camp in 2007 is shown as a blue and cyan rosette, while the subsequent transect to the pole in 2007 is shown as a blue line. A second cruise for 2005 traversed the Arctic approximately following these lines, but at the time of this grant the exact dates and times of this cruise has not been declassified.

§ The moorings deployed by WHOI in 2003 (yellow triangles) and 2004 (green triangle) have Upward Looking Sonars that measure the draft of sea ice as it drifts by.

§ The drift of Ice Mass Balance buoys deployed by the IABP are shown as black lines, with colors dots marking the location of these buoys during 2003 (yellow), 2004 (red) and 2005 (green), when the ICESat laser altimeter was on. The blue dots show the planned deployment positions of IMB buoys in 2006 & 2007.

§ The ElectroMagnetic (EM) measurements by Christian Haas at the AWI are shown in the Lincoln Sea. Dr. Haas plans to make similar measurements in the Lincoln Sea and in the Transpolar Drift Stream south of the pole through IPY, and at the Ice Camp in 2007.

§ Proposed observations which we plan to use (if funded) are noted with cyan. E.g. for the Ice Camp, the submarine cruises are “planned”, however, the additional measurements by Jenny Hutchings have only been “proposed” at this time, thus the blue and cyan rosette. And Jackie Richter-Menge has proposed to deploy IMB buoys with the DAMOCLES ocean buoys.

§ The gray circle around the North Pole marks the area which is not visible to ICESat.

Figure 3. Digitizing an analog chart. Top panel is an analog paper chart of ice draft recorded by a submarine. Bottom panel shows the data corrected to rectilinear and scaled to time and depth. Grey dots are the pixel data for the draft trace from the first panel. The final draft profile is shown by the red line.

Figure 4. Observations from Ice Mass Balance (IMB) buoy 24290 and JAMSTEC Compact Arctic Drifter (JCAD) 7, which were deployed together on the drifting Arctic sea ice in 2003 (Fig. 1). These buoys measure sea level pressure (SLP), surface air temperature (SAT), ice thickness and temperatures, snow depth, and ocean temperatures and salinity. From these measurements, we can also estimate a number of other geophysical quantities such as ocean surface heat flux and heat storage. The gray, dotted boxes denote periods during which the ICESat laser altimeter was on.