Actividades propuestas para fomentar el cambio y el compromiso institucional.

The formation of sea ice near the Antarctic coast is significantly influenced by the existence of ice shelves and presence of polynyas. Almost three quarters of the grounded ice boundary of the Antarctic ice sheet abuts floating ice shelves (Bindschadler et al., 2011) and it is estimated from model simulations that, close to larger ice shelves, approximately 10-25 % of total sea ice thickness is attributed to the outflow of cold water from the ice shelf cavities (Hellmer, 2004). The sea ice cover in the Ross Sea sector is of particular interest as it has experienced a significant increase in extent during the satellite observation period of 5.0 ± 0.6 % per decade (Comiso et al., 2011). However, information on sea ice thickness over this period is very limited. Investigations that are spatially sparse and exhibit extended periods of temporal separation provide some insight indicating that sea ice thickness and extent do not co-vary (DeLiberty et al., 2011) and that the Ross Sea has thicker sea ice in comparison to other sectors, but is highly influenced by the opening of the Ross Sea polynya in spring and the advection of thicker ice from adjacent sea areas (Worby et al., 2008). A recent study based on satellite derived freeboard suggests a small decreasing trend in thickness of 0.01 m from 2003 to 2008, but due to the observed increasing extent, an increasing trend in volume over the whole Ross Sea sector for the period (Kurtz and Markus, 2012).

A major limitation of satellite based sea ice thickness studies in general is the temporal and spatial scarcity of freeboard, snow depth and thickness measured in situ for validation. McMurdo Sound, located in the south-western Ross Sea, provides a unique opportunity to address this issue. The sea ice formation process is comparatively well studied in McMurdo Sound and its suitability for assessment by satellite is bolstered by a developed understanding of local processes from in situ investigations (Gough et al., 2012; Smith et al., 2012; Dempsey et al., 2010; Leonard et al., 2006; Jeffries et al., 1993). In our study we concentrate on temporal and spatial trends of sea ice freeboard in McMurdo Sound from satellite laser altimetry and draw some conclusions on sea ice thickness in 2009 with the use of surface and airborne investigations.

43

The quantification of sea ice mass balance requires information on both its extent and thickness (Haas, 2010). The latter is particularly difficult to assess from satellites as it is indirectly determined from sea ice freeboard, which itself is estimated from an altimetric measurement of sea ice elevation. Freeboard is strictly the elevation of the sea ice surface above the ocean surface. However, accumulation of snow on top of the sea ice inhibits the direct measurement of the sea ice surface from laser altimetry methods. Therefore, a total freeboard (ice-plus-snow) is obtained, from here referred to as freeboard.

Our study is based on data from NASA’s Geoscience Laser Altimeter System (GLAS) onboard of the Ice, Cloud, and land Elevation Satellite (ICESat), which has been used for freeboard retrieval in the Antarctic (Kurtz and Markus, 2012) and Arctic (Kwok and Rothrock, 2009; Farrell et al., 2009; Kwok et al., 2004). As Antarctic sea ice is largely young and thin and therefore its freeboard relatively small (Worby et al., 2008), the required accuracy in the altimeter measurement is very high. Any errors in the initial freeboard estimation will be amplified in the later thickness calculation (Kwok and Cunningham, 2008).

To derive freeboard the local sea surface height must be identified as a reference surface. This task is complicated by natural undulations in this reference surface. Our current inability to represent this variation to the required accuracy hinders freeboard estimation. Inaccuracies are dominated by erroneous heights of the geoid and the tides provided by models. Unless the sea surface height can be established in time and space by independent information, open water must be available nadir to ICESat to produce a local sea surface height for the relative estimation of freeboard. The accuracy of this freeboard measurement will decrease as a function of distance from the sea surface height measurement until another open water area is available for referencing. McMurdo Sound harbors areas of fast ice creating an immediate challenge. The geoid dominates the trend in surface elevation and even over short distances of 10 km poor geoid knowledge can cause errors in estimated freeboard.

Taking account of buoyancy principles, converting freeboard to sea ice thickness involves inclusion of density information and snow depth. This is a complex relationship as density values and snow depth exhibit large temporal and spatial variability. If snow depth

44

information is not available, no information can be gained for sea ice mass balance alone. Zwally et al. (2008) suggest that interannual changes in estimated thickness are mainly representative of snow depth changes. Efforts to correct for snow depth may be hindered by the difficulty of achieving temporal coincidence of the altimetry and snow depth information and their differing spatial resolution (Weissling and Ackley, 2011). This is especially the case for smaller scale assessments in coastal areas where low resolution passive microwave techniques provide inadequate information on snow depth. Further errors introduced in the estimation of thickness by inclusion of nominal information on snow, ice and water density can also be quite significant (Yi et al., 2011). Other studies have made use of in situ information to support satellite data (Markus et al., 2011; Worby et al., 2011; Xie et al., 2011) allowing assessment of method accuracy.

For our study we derive the sea ice freeboard in an area where good temporal (Gough et al., 2012; Purdie et al., 2006) and spatial (Dempsey et al., 2010) in situ information is available and sea ice conditions are well understood. During the investigation from 2003 to 2009 the area hosted three, approximately level, sea ice types: multiyear (MY) sea ice persisted in the southern portion of the Sound, held fast by the McMurdo Ice Shelf in the south, Hut Point Peninsula in the east and the Victoria Land coast in the west (Figure 3.1). The MY sea ice was partly bordered by seasonal first-year (FY) landfast sea ice. The remainder of the study area was covered by variable FY pack ice.

We present two freeboard retrieval methods, Method-1 (M-1) utilizing ICESat information alone, variants of which have been well documented in the literature (Yi et al., 2011; Zwally et al., 2008). Our results show that we do not require more sophisticated approaches as described in Farrell et al. (2009) and Kwok et al. (2006) as our investigation area, characteristic of level thermodynamically grown sea ice, is void of leads, with a single open water area in the north. Method-2 (M-2) is a novel approach involving the development of sea surface height from a mean sea surface grid, tidal heights and atmospheric pressure data. The satellite analysis is validated by in situ and airborne measurements in 2009. These include near-coincident drill-hole measurement of freeboard and ice thickness and helicopter-borne electromagnetic induction sounding thickness (EM) and helicopter lidar (HL) freeboard measurements.

45

Figure 3.1. Location of the study area and satellite map (Envisat ASAR, 12 October 2004) with all used ICESat measurement points (orange) and track numbers mentioned in the text. Sectors of multiyear and first-year sea ice are outlined in green and red, respectively. The first-year-multiyear transition was variable throughout the study period but the study area remained fixed within the displayed bounds. VL: Victoria Land, MIS: McMurdo Ice Shelf, HPP: Hut Point Peninsula, large icebergs: B-15A, B-15K and C-16.

46

This research chapter is organised as follows: section 3.2 provides an overview of sea ice conditions in the study area. Section 3.3 explains M-1 with data quality and method quality checks provided separately in section 3.1. Section 3.4 outlines M-2 freeboard retrieval. Section 3.5 provides ICESat recorded freeboard information and section 3.6 describes the EM induction sounding and HL investigations and comparison of remote sensing techniques with in situ information. Sections 3.7 and 3.8 follow with discussion and conclusions.