fig 1. Average spring (March-April) Arctic sea ice thicknesses from 2012 to 2016 from satellite altimeter measurements of ice freeboard (height by which the ice extends above the water’s surface). Data are not available for the marginal ice zone, or above the ERS latitudinal limit of 81.58°N. Ice freeboard data are converted to thickness using fixed ice, snow and water densities and regional monthly snow depth. The mean thickness excludes thin ice (less than 0.5-1 m) and open water. (Credits University College London/Centre for Polar Observation and Modelling)
Sea ice is seawater that has frozen. It contains little salt as most of it is rejected as it forms. Sea ice covers the Arctic Ocean more or less permanently above the latitude of about 75°N. This permanent ice cap is composed of pack ice, which is kept in continuous motion by the wind, tides and ocean currents. It must be noted that, since sea ice is floating, if it were to melt it would not cause the sea level to rise directly. However, due to its high albedo, ice directly affects the global Earth energy budget by reflecting about 80% of incident sunlight back out to Space. Thus, once formed, ice tends to be maintained. However, if ice cover were to decrease, less solar radiation would be reflected away from the surface of the Earth – causing the ice to absorb more heat and consequently melt faster still. The thickness of Arctic sea ice also plays a central role in the polar climate as it moderates heat exchange by insulating the ocean from the cold polar atmosphere. Moreover, as sea ice forms, the salinity, and therefore the density, of the upper ocean increases. This density increase causes the surface waters to sink – in essence acting as a pump, driving cold, deep ocean currents from the polar regions towards the equator.
In the Arctic, sea ice typically covers about 14 to 16 million square kilometres at its maximum extent in late winter, and 7 to 9 million km2 at its minimum seasonal extent in late summer. In the Antarctic, sea ice at its maximum covers 17 to 20 million square kilometres and only about 3 to 4 million square kilometres at its minimum. Observations show that the mean Arctic ice extent is decreasing at a rate of about 3% per decade while Antarctic ice extent is quite stable. The maximal loss in the Arctic occurs in September, at the end of the summer, and can reach 8%.
Regional sea ice models have been successfully developed over the last decades. However, given the impact that sea ice has on the climate, it is essential to acquire more comprehensive data on sea ice thickness, in order to improve sea ice models for their implementation in general climate studies.
One method of computing sea ice thickness is based on the difference in height between sea and ice surfaces, allowing this parameter to be acquired from altimetry. A careful analysis of individual echoes can distinguish between those backscattered from the open ocean, new ice or multi-year ice. The difference between the elevation of the echoes from snow/sea ice and open water then gives the elevation of the ice above the ocean; the ice thickness can thus be deduced from this.
Since the launch of Seasat, the potential of altimeter data to estimate iceberg’s freeboard has been explored [McIntyre and Cudlip, 1987] and some examples of freeboard profiles have been published. However, the first generation of altimeters (Seasat, Geosat, Topex/Poseidon) used on-board trackers that frequently loose the surface during rapid transitions of elevation resulting in a several second long loss of data, which greatly hampered the possibility of iceberg freeboard measurement. Since the launch of Jason-1 and Envisat in 2002, the technological progress in altimetry allows to better cope with the rapid elevations changes occurring over a large iceberg or a coast [Gommenginger et al., 2011] opening a new opportunity
Example of waveforms over an Iceberg
fig 2. MODIS image of iceberg A43A on 2 October 2003 13:20 UT and ENVISAT RA2 ground track (fine black line) and freeboard profile (green line) on 1 October 2003 12:35 UT. The two red lines indicate the width of the altimeter swath and the magenta star the location of the iceberg in the BYU database. (Credits J. Tournade)
The waveforms corresponding to this pass, and the remapped waveforms using the tracker position are presented in the figures. As the altimeter approaches the iceberg from the north near 66º S, the tracker starts to move up mitigating the sea and iceberg surface elevations. As the tracker is not locked on the iceberg surface, the strong echo from the iceberg starts to appear in the first gate of the waveforms then moves toward the nominal track point (0) while the echo from the sea surface moves away from zero. Moving further, the tracker ‘‘overshoots’’ and continues to move up for a few tenth of seconds before locking on the surface. A symmetrical behavior occurs when the altimeter leaves the iceberg. The tracker starts to mitigate the iceberg and sea surface, and then slightly overshoots downward before relocking on the sea surface. In this particular case, it is worth noting that the altimeter ground track is almost perpendicular to the iceberg edge to the north, which gives a sharp elevation transition, while the track intersects the southern edge at a slanted angle resulting in a much longer transition during which the altimeter footprint contains both ocean and iceberg.
fig 3. Altimeter waveforms for the Envisat pass. The red line indicates the tracker position (a). Retracked waveforms using the tracker position, the red stars represent the iceT freeboard positions (b). Elevations from the MLE3 retracker (green line), the ICE2 retracker (black line), and iceT one (red line), and MODIS brightness (blue line)(c). Measured backscatter (d). The shaded area represents the zone over which only the iceberg surface is seen by the altimeter. (Credits J. Tournade)
Sea Ice Freeboard
Sea ice altimetry relies on being able to discriminate between waveforms originating from open water or freshly frozen ice in leads (cracks) in the sea ice and waveforms originating from the sea ice surface.
fig 4. Measuring Sea Ice Thickness Using Satellite Laser and Radar Altimeters (Credits NOAA)
Assuming hydrostatic equilibrium (i.e. the ice is floating), the freeboard can then be converted into an ice thickness estimate. By comparing the ocean elevation, as measured by the altimeter, with the mean geoid elevation, the sea ice altimeter processing provides information about the Arctic dynamic topography and also informs models of the Arctic Ocean geoid.
fig 5. CryoSat-2 power waveforms (top row) along with the corresponding phase (middle row) and coherence (bottom row) exhibiting a) purely diffuse behaviour (ice), b) purely specular behaviour (lead) and c) mixed behaviour. The vertical dashed line is the retracking point for the waveform and the pulse peakiness (PP) of each waveform is printed in the top right (Credits T. Armitage)
- Laxon, S., N. Peacock, and D. Smith, High interannual variability of sea ice thickness in the Arctic region, Nature, 425, 947-950, 2003Tournadre, J., N. Bouhier, F. Girard-Ardhuin, and F. R_emy (2015), Large icebergs characteristics from altimeter waveforms analysis, J. Geophys. Res.Oceans, 120, 1954–1974, doi:10.1002/2014JC010502.
- Armitage, T.W.K.; Davidson, M.W.J., ”Using the Interferometric Capabilities of the ESA CryoSat-2 Mission to Improve the Accuracy of Sea Ice Freeboard Retrievals,” IEEE Transactions on Geoscience and Remote Sensing, vol.52, no.1, pp.529,536, Jan. 2014, doi: 10.1109/TGRS.2013.2242082.
- L. Farrell, S. W. Laxon, D. C. McAdoo, D. Yi, and H. J. Zwally, “Five years of arctic sea ice freeboard measurements from the ice, cloud and land elevation satellite,” J. Geophys. Res., vol. 114, 2009.