Australia: The Land Where Time Began

A biography of the Australian continent 

Antarctica – Larsen C Ice Shelf – Impact on Basal Melting of Tide-Topography Interactions

Antarctic ice shelf basal melting contributes to the formation of Antarctic Bottom Water, and by altering the offshore flow of grounded ice streams and glaciers can affect global sea level. Basal melt rates (w_b) of ice shelves is influenced by tides, as tides contribute to ocean mixing and mean circulation, as well as thermohaline exchanges with the ice shelf. To investigate the relationship between topography, tides, and w_b for the Larsen C Ice Shelf (LCIS) in the northwestern part of the Weddell Sea in Antarctica Muller et al. used a 3D model that was thermodynamically coupled to a nonevolving ice shelf. Muller et al. found by using their best estimates of the thickness of the ice and the topography of the sea bed that the largest modelled LCIS melt rates occurred in the northeast, and it was there that their model predicted there were strong diurnal tidal currents (~0.4 m/sec). This distribution differs significantly from models that do not include tidal forcing, which predict that the largest melt rates occur along the deep grounding lines. In order to explore melt rate sensitivity to geometry, initial ocean potential temperature (θ₀), thermodynamic parameterisations of heat and freshwater ice-ocean exchange, and tidal forcing, Muller et al. compared several runs of their model. The range of LCIS-averaged w_b is ~0.11 – 0.44 m/a. The spatial distribution of –w_b was found to be very sensitive to model geometry and thermodynamic parameterisation while θ₀ influenced the overall magnitude of w_b. A need for high-resolution maps of ice draft and topography below the ice shelf, together with measurements of ocean temperature at the ice shelf front in order to improve the representation of ice shelves in coupled climate system models is reinforced by these sensitivities in w_b predictions.

Around Antarctica the oceans interact with the continental ice sheet at the floating ice shelves which occupy about 50 % of the coastline of Antarctica (Drewry et al., 1982). Global ocean properties are influenced by melting at the base of an ice shelf by producing cold melt-water plumes of low-salinity that carry freshwater mass away from the continent and preconditioning the surrounding waters of the continental shelf for formation of Antarctic Bottom Water (e.g. Jacobs, 2004). An ice shelf can also be weakened by basal melting, which increases the likelihood of calving events or disintegration (Vieli et al., 2007). The total mass balance of an ice shelf is determined by the balance between ice gain, by advective input of grounded ice, accumulation of snow, and the accretion of marine ice, and ice loss, primarily basal melting and calving. Negative mass balance can destabilise an ice shelf to the extent that it can be compromised. The stresses that impede the offshore flow of the ice shelves can be reduced by mass loss which leads to increased rapidity of the seaward movement in the inflowing glaciers and ice streams (Rignot et al., 2004; Scambos et al., 2004). The ocean can affect the overall mass balance of the Antarctic ice sheet, as well as associated global sea level on decadal time scales, by these processes.

Though these is a general understanding of processes causing basal melting of ice shelves (Lewis & Perkins, 1986; MacAyeal, 1984; Hellmer & Olbers, 1989; Jacobs et al., 1992; Holland & Jenkins, 1999), the ability to model accurately the spatial distribution of basal melt rate (w_b) and the associated net loss of ice mass is limited by several factors which include: ice shelf and seabed geometry is poorly known, a paucity of hydrographic data which defines the nature of oceanic inflow to the cavity beneath the ice shelf, and neglect of specific processes for the purpose of making computation simpler and more efficient. Models which attempt to project land ice contribution to the changes of sea level over long time scales (e.g. Pollard & DeConto, 2009) will be subject to errors which will potentially be very large until these deficiencies in the model and data deficiencies are resolved.

The tides are a forcing that is usually excluded from numerical models of basal melting of ice shelves. It has been postulated (MacAyeal, 1984) that there is a relationship between tides and basal melting, noting that the ice shelf isolates the cavity beneath the ice shelf from direct forcing by the wind, and, therefore, increases the importance of tidal currents as a source of oceanic kinetic energy for conversion to mixing. It has been demonstrated by more recent studies that tides can be a significant factor in interactions between ocean and ice shelves close to the ice shelf boundaries (Makinson & Nicholls, 1999; Makinson, 2002; Joughin & Padman, 2003; Holland, 2008; Robinson et al., 2010). It was predicted (Makinson et al., 2011) that tides contribute about half of net loss of mass from the Filchner-Ronne Ice Shelf (FRIS) in the southern Weddell Sea.

In this study Mueller et al. focussed on understanding the sensitivity to errors in initial boundary conditions for ice shelves where tidal forcing is significant. With this aim they report on studies of sensitivity of the effects of adding tides to an ocean model that is coupled thermodynamically to the Larsen C Ice Shelf (LCIS) in the northwestern part of the Weddell Sea, Antarctica. This ice shelf is subject to significant tidal variability (King et al., 2011), and is ventilated by cold, High Salinity Shelf Water (HSSW) (Nicholls et al., 2004); in some respects it is therefore similar to the much larger FRIS studied by Makinson et al., 2011). When studying the influence of tides under these conditions it is advantageous to use the Larsen C Ice Shelf because of its smaller size as it allows a much finer model grid resolution. As the Larsen C Ice Shelf is in a more northerly location it is likely to respond to climate change earlier than the Filchner-Ronne Ice Shelf. The view that the Larsen C Ice Shelf is undergoing changes that may lead to weakening of the ice shelf is supported by recent surface lowering of the Larsen C Ice Shelf (Shepard et al., 2003; Fricker & Padman, 2012).

Important roles in ice shelf mass and elevation variability are played by atmospheric forcing and open ocean circulation (Fricker & Padman, 2012), though in this study their model is forced only with tides. The approach taken by Mueller et al. allowed them to focus on the factors that contributed to uncertainty in w_b. Relevant sources of uncertainty with regard to the Larsen C Ice Shelf include the thickness of the water column (wct), the temperature of the ocean water that flowed into the cavity beneath the ice shelf, and the heat parameterisation and exchange of freshwater at the ice-ocean interface. They were particularly interested in the effect of errors in the water column thickness. On small spatial scales tidal currents can be very sensitive to water column thickness errors, and as a result there is significant uncertainty in the contribution of tides to w_b. According to Mueller et al. this study complements the application of a plume model to the LCIS (Holland et al., 2009), which provides a valuable comparison for their simulations.

Sources & Further reading

1. Mueller, R. D., L. Padman, M. S. Dinniman, S. Y. Erofeeva, H. A. Fricker and M. A. King (2012). "Impact of tide-topography interactions on basal melting of Larsen C Ice Shelf, Antarctica." Journal of Geophysical Research: Oceans 117(C5): C05005.