Ice front blocking of ocean heat transport to an Antarctic ice shelf

Abstract

Mass loss from the Antarctic Ice Sheet to the ocean has increased in recent decades, largely because the thinning of its floating ice shelves has allowed the outflow of grounded ice to accelerate 1,2. Enhanced basal melting of the ice shelves is thought to be the ultimate driver of change 2,3 , motivating a recent focus on the processes that control ocean heat transport onto and across the seabed of the Antarctic continental shelf towards the ice 4-6. However, the shoreward heat flux typically far exceeds that required to match observed melt rates 2,7,8 , suggesting other critical controls. Here we show that the depth-independent (barotropic) component of the flow towards an ice shelf is blocked by the dramatic step shape of the ice front, and that only the depth-varying (baroclinic) component, typically much smaller, can enter the sub-ice cavity. Our results arise from direct observations of the Getz Ice Shelf system and laboratory experiments on a rotating platform. A similar blocking of the barotropic component may occur in other areas with comparable ice-bathymetry configurations, which may explain why changes in the density structure of the water column have been found to be a better indicator of basal melt rate variability than the heat transported onto the continental shelf 9. Representing the step topography of the ice front accurately in models is thus important for simulating the ocean heat fluxes and induced melt rates. Main text: The fate of the Antarctic Ice Sheet is the greatest remaining uncertainty when predicting future sea level 10. Estimates of its contribution to global sea-level rise range from none to a catastrophic > 5 cm/year 10-12 (4 m by the year 2100). The ice sheet drains into the ocean where it terminates in floating ice shelves, overlying vast sub-ice cavities. These buttress the flow of the ice sheet, regulating the speed at which it flows into the ocean 13. Rapid thinning of ice shelves in coastal regions with warm ocean water on the continental shelf is accelerating the outflow from the ice sheet 1,2. The perceived reason-although rarely observed directly 14is that ocean currents deliver more warm water to the ice shelf cavities, causing increased basal melt. These currents originate in a reservoir of warm and salty water, known as Circumpolar Deep Water (CDW) 15 , residing at 300-1000 m depth in the Southern Ocean. Substantial amounts of dense CDW are carried onto the continental shelf by various mechanisms 4-7,16 , but only a fraction of this is needed to explain observed basal melt rates 17. The CDW flows southward in deep troughs that crosscut the continental shelf 4,18-21. The currents are steered by the bathymetry and move with shallower water to the left of the flow direction 22-24 so southward transport occurs along the eastern, and northward on western,

Bathymetric Influences on Antarctic Ice‐Shelf Melt Rates

“…The negative sensitivities downslope and positive sensitivities upslope imply that a steepening of the trough margin would amplify the geostrophically driven flow of warm water to the ice shelf, and thus increase melting. This result is corroborated by recent observational and experimental work which highlights the critical role of topography in steering heat to Antarctic ice shelves (Wählin et al., 2020).…”

“…These results provide a complementary perspective to the observations and experimental results shown in Wählin et al. (2020).…”

“…Specifically, we show that steepening the trough in front of the Dotson Ice Shelf would increase melting as a result of increasing the geostrophic inflow. These results provide a complementary perspective to the observations and experimental results shown in Wählin et al (2020).…”

“…The bathymetry of the ocean exerts a leading order influence on ocean circulation, both at global and regional scales (e.g., Gille et al., 2004; Hughes & Killworth, 1995; Marshall, 1995; Roberts & Wood, 1997). It plays a key role in regulating exchanges between the Antarctic continental shelf and the deep ocean (e.g., Graham et al., 2016; Thoma et al., 2008; Thompson et al., 2018; Walker et al., 2013) and in setting circulation patterns on the continental shelf (e.g., Arneborg et al., 2012; Cochran & Bell, 2012; De Rydt et al., 2014; Jacobs et al., 2011; Padman et al., 2010; Rosier et al., 2018; Wählin et al., 2020). Its role in ice sheet‐ocean interactions is accentuated by the fact that a large part of the Antarctic ice sheet rests well below sea level (Bentley et al., 1960), with a sizable portion of its margins terminating in large floating ice shelves.…”

Coupling framework (1.0) for the ice sheet model PISM (1.1.1) and the ocean model MOM5 (5.1.0) via the ice-shelf cavity module PICO

“…In the coupling framework, ocean input for PICO is averaged over the entire basin, not taking into account horizontal differences such as cavity in-and outflow regions and possible modifications of water masses on the continental shelf. Furthermore, complex processes determine what water masses make their way from the open ocean onto the continental shelf and to the grounding lines (Nakayama et al, 2018;Wåhlin et al, 2020). However, in our coarse grid setup of MOM5, bathymetry and circulation on the continental shelf are only partly represented (see also Fig.…”