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Ross Ice Shelf – Basal Melting from the Absorption of Solar Heat in an Ice Front Polynya

The bases of Antarctic ice shelves ice-ocean interactions are only rarely observed, and yet they have a profound influence on the evolution and stability of ice sheets. Ice sheet models are highly sensitive to basal melt rates of ice shelves that are assumed; however, there are not many direct observations of basal melting or of the oceanic processes driving it and as a consequence understanding of these interactions have remained limited. In this study Stewart et al. used in situ observations from the Ross Ice Shelf in order to examine the oceanic processes that drive basal ablation of the largest ice shelf in the world. They found that beneath a thin, though structurally important part of the ice shelf, basal melt rates are an order of magnitude higher than the average for the entire shelf. A seasonal inflow of surface water, which is heated by solar radiation, from the adjacent Ross Sea Polynya that downwells into the ice shelf cavity, which almost triples the basal melt rates during summer, has a strong influence on this melting. It is predicted that the melting that is driven by this process, that is frequently overlooked, is expected to increase as the surface water is warmed. Stewart et al. infer that solar heat that is absorbed by ice front polynyas can make an important contribution to the mass balance of ice shelves at the present, and potentially impact their stability in the future.

The ice shelves fringing Antarctica interact with the Southern Ocean across a basal surface of 1.56 x 10⁶ km² (Rignot et al., 2013). The single largest cause of loss of mass from the Antarctic Ice Sheet (Rignot et al., 2013; Depoorter et al., 2013) is melting of this vast surface. Thinning that is induced by ice shelf basal melting can also influence the ice flow from inland areas by reducing the stabilising effect of sills, shoals and sidewalls (Arthern & Williams, 2017; Reece et al., 2018;), in some cases driving instantaneous dynamic response  as far as 900 km inland (Reece et al., 2018). There are still relatively few direct observations of basal melting and oceanographic conditions within cavities of ice shelves (Jenkins, Nicholls & Corr, 2010), and theory and model development is hampered by this paucity of data, though these processes provide a primary control on the evolution into the future of the ice sheet (Arthern & Williams, 2017; Pattyn, 2017).

There are 3 main water masses that are believed to influence ice shelves (Jacobs et al., 1992);

1)    Circumpolar Deep Water (CDW), a relatively warm water mass surrounding Antarctica at intermediate depth;

2)    High and low salinity shelf water (HSSW and LSSW), which is formed as the surface of the ocean freezes during winter;

3)    And the Antarctic Surface Water (AASW), a water mass that is relatively fresh and buoyant, influenced by solar heating and melting of sea ice during summer (Orsi & Wiederwohl, 2009).

These water masses have contrasting impacts on ice shelves. Over recent decades Circumpolar Deep Water in the Amundsen Sea have caused thinning of the ice shelves in the region (Jenkins et al., 2010; Paolo, Fricker & Padman, 2015) driving mass loss from the interior ice sheet (Velicogna, Sutterley & Van Den Broeke, 2014; Shepherd et al., 2018). Contrasting with this, the vast Ross Ice Shelf and the Filchner-Ronne Ice Shelf appear to be near equilibrium (Shepherd et al., 2010; Pritchard et al., 2012), as a result of the presence of cold shelf waters thereby limiting their exposure to Cold Deep Water (Orsi & Wiederwohl, 2009; Jacobs et al., 1970; Nicholls et al., 2009). Something that is less clear and not often considered is the Influence of Antarctic Surface Water on ice shelves. Observations have been only recently been made of AASW beneath ice shelves (Hattermann et al., 2012; Stern et al., 2013), with few studies examining this process in detail, though buoyant, AASW can enter cavities in ice shelves due to wind (Sverdrup, 1954; Zhao et al., 2014) and tidal forcing (Gammelsrod & Slotsvik, 1981; Jenkins & Doake, 1991; Makinson & Nicholls, 1999).

Relatively low shelf-wide mean basal melt rates of 0.07 to 0.11 m/year (Rignot et al., Depoorter et al., 2013; 2013; Moholdt, Padman & Fricker, 2014) were suggested by recent satellite observations of the Ross Ice Shelf  (RIC), with an area of 500,809 km², which comprises 32% of the total area of ice shelf. These studies also indicate, however, rates above 1 m/year in the northwestern sector of the shelf (Rignot et al., 2013; Moholdt, Padman & Fricker, 2014). Though there are uncertainties of 100% in remote sensing estimates, rapid melting in the northwestern Ross Ice Shelf is also indicated by earlier glaciological observations (Crary, 1962; Moholdt, Padman & Fricker, 2014; Bamber & Bentley, 1994) and oceanographic models (Stern et al., 2013; Moholdt, Padman & Fricker, 2014; Assmann, Hellmer & Beckmann, 2003; Holland, Jacobs & Jenkins, 2003; Dinniman, Klinck & Smith, 2007; Arzeno et al., 2014). It is suggested by these models that active circulation of frontal water into the cavity in summer and variability of low frequency flow may influence this region. This picture is supported by observation from beneath the ice shelf (Stern et al., 2013; Arzeno et al., 2014), though the details of these processes and their impact on the ice shelf has remained unclear.

In this paper Stewart et al. present in situ observations of basal melting and oceanic conditions beneath the ice shelf from the northwestern Ross Ice Shelf. There are 2 aims of the study: to quantify and map basal melting in the region around Ross Island, and to examine the role of surface water in driving this process.

Surface Ocean Heat

Crucial questions are raised by the identification of warm surface water inflow driving rapid basal melting: what is the origin of this heat and could this process influence other ice shelves? They examined summer Sea Surface Temperature and observations of sea ice concentration from coastal Antarctica in order to address these questions.

At the largest scale, summer Sea Surface Temperature correlates inversely with the concentration of sea ice and typically the coldest waters are found near the coastline. Higher temperatures are observed, however, where the sea ice is absent, which includes the coastal polynyas near the Ross Ice Shelf and the Amery Ice Shelf (Oshima, Nihashi & Iwamoto, 2016).

Variability of Sea Surface Temperature within the Ross Sea is dominated by a warm surface anomaly, which was previously identified in CTD observations (Smethie & Jacobs, 2005), which matches closely the Ross Sea Polynya. January mean sea surface temperature reaches ~0.5^oC. This pattern of warming is consistent with atmospheric modelling which indicates that solar heat is absorbed rapidly during summer in Antarctic polynyas (Renfrew, King & Markus, 2002), and has been attributed previously to summer insolation in the Ross Sea Polynya (Stern et al., 2013; Jacobs & Comiso, 1989).

In order to assess whether the warm surface pool could supply the energy that is required for elevated melting in the region of the survey, Stewart et al. calculated the thermal energy that is available within its surface waters during January. When considering the region within the 0^oC Sea Surface Temperature isotherm, and assuming a surface mixed layer of 10 m depth, provides a sensible heat content of 8.3 x 10⁸ J, which is sufficient to melt 22 Gt of the ice shelf. Within the survey region, this is approximately twice the observed mass loss. Surface waters in the Ross Sea clearly represent a heat reservoir in summer that is glaciologically important, in spite of uncertainty in the depth of the mixed layer.

Coastal SSTs above -0.5^oC beyond the Ross Sea are seen only in the northwestern Antarctic Peninsula, where there is a low concentration of sea ice, and in the polynya adjacent to the eastern Amery Ice Shelf. As a consequence, this process does not appear to be widespread at present, though these regions may be affected by heat in the surface layer.

Drivers and impacts of surface water impacts

Though the surface waters have been considered for some time (Jacobs et al., 1992; Hattermann et al., 2012; Stern et al., 2013; Moholdt, Padman & Fricker, 2014) to be a potential driver of basal melting of ice shelves, the observations presented in this paper provide detailed evidenced of the process. It is suggested by these data that surface water that is solar heated contributes substantially to the basal mass balance of the Ross Ice Shelf, and that a larger role is played by surface water in the mass balance of ice shelves than has previously been assumed.

The impact of surface water in the northwestern Ross Sea can be attributed to 2 processes; solar heating of surface ocean during summer, that is localised, and the transport of this energy into the cavity is by seasonal inflow. It seems that surface heating is linked closely to the consistent expansion of the Ross sea polynya in spring, which is driven by the wind (Stern et al., 2013; Jacobs & Comiso, 1989). Sustained southerly winds during this period, which were guided by the Transarctic Mountains, export preferentially sea ice from the western ice front (Bromwich et al., 1993; Comiso et al., 2011). As the temperature of the atmosphere and the insolation increases throughout November and December, there is rapid expansion of the polynya, as is illustrated by the distribution of sea ice over this period of time. The absorption of solar energy in the surface layer increases by this process.

The drivers of late summer inflow are less obvious. According to Stewart et al. due to the buoyancy of the surface layer, however, it appears likely that the external forcing is required. In contrast to the downwelling observed elsewhere (Zhao et al., 2014) that is wind-driven, the inflow that has been observed here is not associated with winds that are favourable to downwelling. It was suggested by modelling near and beneath the ice shelf that the circulation is influenced strongly by density gradients resulting from the seasonal release of brine in the polynya (Jendersie et al., 2018) and that the variability of the seasonal flow near Ross Island (Assmann, Hellmer & Beckmann, 2003) are influenced by these circulation patterns. When these factors were considered by Stewart et al. they that concluded that the elevated melt rates in the northwestern Ross Ice Shelf are linked to the location of the Ross Sea Polynya, and ultimately to the mean winds and orography of the region.

There are several important implications of the identification of surface layer heat as a driver of basal melting of the Ross Ice Shelf.

1)    Within the polynya the absorption of heat is controlled by atmospheric processes (Renfrew, King & Markus, 2002), basal mass balance in the frontal zone of the ice shelf is likely to vary with atmospheric and surface ocean conditions near the ice front on seasonal, interannual and long term timescales. Considering that sea ice concentrations in summer in the Ross Sea are projected to decrease by 56% by 2050 (Smith et al., 2014), and it is also expected that the ice free period will increase (Dinniman et al., 2019), it seems likely that the basal melting of the ice shelf will increase rapidly in this region. If surface warming and there is widespread loss of sea ice, this process may also become more widespread.

2)    A mode of basal ablation that is distinct from that of denser water masses is driven by AASW, and these differences have implications for the stability of the ice shelf. E.g. whereas in shallower regions meltwater that is derived from HSSW can refreeze, thereby potentially stabilising the ice shelves (Jansen et al., 2013), it is not likely that meltwater formed from AASW will be redeposited as a result of its relative warmth. Also, the influence of surface water is at its greatest in frontal regions. Some frontal regions contain critical pinning points that maintain the location of the front (Doake et al., 1998; Fürst et al., 2016), though others are not important to the stability of the ice shelves. It seems that Ross Island is one of these pinning points, and it has been shown by recent modelling that rapid melting that is identified here influences a region that is structurally critical in which changes in ice thickness can influence the speed of flow of the entire ice shelf (Pattyn, 2017).

Exposure to surface ocean heat of this sensitive part of the ice shelf implies that the grounding line flux of the entire ice shelf may be modulated at seasonal to interannual timescales by the inflow of surface water. A frequently overlooked, though potentially important, factor in the mass balance of a regional ice shelf and should be considered in future assessments of ice shelf stability.

Sources & Further reading

Stewart, C. L., et al. (2019). "Basal melting of Ross Ice Shelf from solar heat absorption in an ice-front polynya." Nature Geoscience.