Estimation of Ice Shelf Melt Rate in the Presence of a Thermohaline Staircase

Abstract

Diffusive convection-favorable thermohaline staircases are observed directly beneath George VI Ice Shelf, Antarctica. A thermohaline staircase is one of the most pronounced manifestations of double-diffusive convection. Cooling and freshening of the ocean by melting ice produces cool, freshwater above the warmer, saltier water, the water mass distribution favorable to a type of double-diffusive convection known as diffusive convection. While the vertical distribution of water masses can be susceptible to diffusive convection, none of the observations beneath ice shelves so far have shown signals of this process and its effect on melting ice shelves is uncertain. The melt rate of ice shelves is commonly estimated using a parameterization based on a three-equation model, which assumes a fully developed, unstratified turbulent flow over hydraulically smooth surfaces. These prerequisites are clearly not met in the presence of a thermohaline staircase. The basal melt rate is estimated by applying an existing heat flux parameterization for diffusive convection in conjunction with the measurements of oceanic conditions at one site beneath George VI Ice Shelf. These estimates yield a possible range of melt rates between 0.1 and 1.3 m yr 21, where the observed melt rate of this site is ;1.4 m yr 21. Limitations of the formulation and implications of diffusive convection beneath ice shelves are discussed.

Increased ice flow in Western Palmer Land linked to ocean melting

“…Though significant, the dynamical imbalance is responsible for only a small proportion (35%) of the deflation that has occurred inland [Helm et al, 2014;McMillan et al, 2014;Wouters et al, 2015]. The pattern of increased ice flow coincides with the distribution of glaciers that are grounded more than 300 m below sea level, which corresponds to the depth at which warm circumpolar deep water resides within the neighboring ocean [Hofmann et al, 2009;Kimura et al, 2015]. A large fraction of Western Palmer Land is grounded well below sea level, and so there is a prospect that the ice dynamical imbalance could lead to further draw down of ice from the interior over time-as it has occurred in other sectors of Antarctica [Shepherd et al, 2002;Rignot, 2008;Payne et al, 2004;Joughin et al, 2014a].…”

“…This water is more than 3°C warmer than the local freezing temperature and has been recorded at depths below 200 to 300 m in the wider Bellingshausen Sea [Hofmann et al, 2009;Kimura et al, 2015] and at 340 m at the base of George VI ice shelf [Kimura et al, 2015]. Model simulations [e.g., Holland et al, 2010] suggests that it flushes much of the subshelf cavity, where it is estimated [Kimura et al, 2015] to generate …”

“…Warm circumpolar deep water (CDW) is present within the Bellingshausen Sea [Holland et al, 2010] and floods, periodically, through bathymetric depressions onto the continental shelf [Moffat et al, 2009] and into the ocean cavity beneath George VI ice shelf [Potter and Paren, 1985;Talbot, 1988;Jenkins and Jacobs, 2008]. This water is more than 3°C warmer than the local freezing temperature and has been recorded at depths below 200 to 300 m in the wider Bellingshausen Sea [Hofmann et al, 2009;Kimura et al, 2015] and at 340 m at the base of George VI ice shelf [Kimura et al, 2015]. Model simulations [e.g., Holland et al, 2010] suggests that it flushes much of the subshelf cavity, where it is estimated [Kimura et al, 2015] to generate …”

Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica

“…Observations beneath ice shelves surrounded by water that is near in situ freezing point, such as Filchner‐Ronne, Larsen C, and Ross ice shelves, suggest that the flow is in the high Reynolds number regime as a result of the large‐scale circulation, modulated by tidal motion [ Jacobs et al ., ; Nicholls and Jenkins , ]. In contrast, distinct signatures of thermohaline staircases (a stack of well‐mixed layers separated by sharp interfaces) were seen in the profiles beneath George VI Ice Shelf in the Bellingshausen Sea, Antarctica, indicating possible low Reynolds number flow [ Kimura et al ., ]. In the rest of this section, we will use the direct measurements of ϵ described in the previous section to estimate basal melt rate.…”

Explicit representation and parametrised impacts of under ice shelf seas in the <i>z</i><sup>∗</sup> coordinate ocean model NEMO 3.6

“…Recent observations beneath George VI ice shelf exhibit thermohaline staircases in the top 20 m below the melting ice shelf base, due to double-diffusive convection (Kimura et al, 2015). These observations raise a doubt about the applicability of the widely used three-equation model to predict the melt rate in regions where the flow beneath the ice shelf is weak.…”