Sill-Influenced Exchange Flows in Ice Shelf Cavities

Abstract

Bathymetric sills are important features in the ocean-filled cavities beneath a few fast-retreating ice shelves in West Antarctica and northern Greenland. The sills can be high enough to obstruct the cavity circulation and thereby modulate glacial melt rates. This study focuses on the idealized problem of diabatically driven, sill-constrained overturning circulation in a cavity. The circulation beneath fast-melting ice shelves can generally be characterized by an inflow of relatively warm dense water (with temperatures of a few degrees Celsius above the local freezing point) at depth and cold, less-dense, outflowing water, which exhibits an approximately two-layer structure in observations. We use a two-layer isopycnal hydrostatic model to study the cross-sill exchange of these waters in ice shelf cavities wide enough to be rotationally dominated. A quasigeostrophic constraint is determined for the transport imposed by the stratification. Relative to this constraint, the key parameters controlling the transport and its variability are the sill height relative to the bottom layer thickness and the strength of the friction relative to the potential vorticity (PV) gradient imposed by the sill. By varying these two key parameters, we simulate a diversity of flow phenomena. For a given meridional pressure gradient, the cross-sill transport is controlled by sill height beyond a critical threshold in the eddy-permitting, low-friction regime, while it is insensitive to friction in both the low-friction and high-friction regimes. We present theoretical ideas to explain the flow characteristics: a Stommel boundary layer for the friction-dominated regime; mean–eddy PV balances and energy conversion in the low-friction, low-sill regime; and hydraulic control in the low-friction, high-sill regime, with various estimates for transport in each of these regimes.

Bistability of the Filchner‐Ronne Ice Shelf Cavity Circulation and Basal Melt

“…Our results therefore motivate further evaluation of ongoing and potential future changes in the katabatic winds (Bintanja et al, ; Hazel & Stewart, ). Further process‐oriented simulations will likely be required to provide a more general characterization of ice shelf cavities' dependence on local and global atmospheric/oceanic changes (Grosfeld et al, ; Grosfeld & Gerdes, ), as mediated by mesoscale/submesoscale eddy transports (Årthun et al, ; Stewart, ; Stewart & Thompson, ), tidally induced transport (Mueller et al, ), and the geometry of the cavity itself (de Rydt et al, ; Zhao et al, ).…”

Bathymetry constrains ocean heat supply to Greenland’s largest glacier tongue

“…S2). While hydraulic-controlled exchange flows for ice shelf cavities have not been measured before, they have been investigated through a recent modelling study 30 . A better understanding of AIW variability on the continental shelf will be critical in order to predict future changes in the ocean heat supply and submarine melt at the outlet glaciers of the NEGIS.…”

Acceleration and Overturning of the Antarctic Slope Current by Winds, Eddies, and Tides

“…The ASC and ASF therefore mediate exchanges of waters across the Antarctic continental slope and exert an outsized influence on local and global ocean circulation and climate (Rintoul 2018;Thompson et al 2018). For example, transfer of heat from the open ocean onto the continental shelf must cross the ASF (Heywood et al 2016;Goddard et al 2017;Palóczy et al 2018) and directly contributes to the melt of Antarctica's floating ice shelves (Pritchard et al 2012;Jenkins et al 2016;Zhao et al 2019). The production of dense shelf waters (DSW) supply the global volume of Antarctic Bottom Water Orsi et al 2001), which in turn ventilates the abyssal ocean (Sarmiento et al 1988;Hotinski et al 2001) and contributes to the baroclinicity and structure of the Antarctic Circumpolar Current (ACC) (Morrison and Hogg 2013;Stewart and Hogg 2017).…”