Ice‐Shelf Melt Response to Changing Winds and Glacier Dynamics in the Amundsen Sea Sector, Antarctica

Abstract

It has been suggested that the coastal Southern Ocean subsurface may warm over the 21st century in response to strengthening and poleward shifting winds, with potential adverse effects on West Antarctic glaciers. However, using a 1/12° ocean regional model that includes ice‐shelf cavities, we find a more complex response to changing winds in the Amundsen Sea. Simulated offshore subsurface waters get colder under strengthened and poleward shifted winds representative of the SAM projected trend. The buoyancy‐driven circulation induced by ice‐shelf melt transports this cold offshore anomaly onto the continental shelf, leading to cooling and decreased melt below 450 m. In the vicinity of ice‐shelf fronts, Ekman pumping contributes to raise the isotherms in response to changing winds. This effect overwhelms the horizontal transport of colder offshore waters at intermediate depths (between 200 and 450 m), and therefore increases melt rates in the upper part of the ice‐shelf cavities, which reinforces the buoyancy‐driven circulation and further contributes to raise the isotherms. Then, prescribing an extreme grounding line retreat projected for 2100, the total melt rates simulated underneath Thwaites and Pine Island are multiplied by 2.5. Such increase is explained by a larger ocean/ice interface exposed to CDW, which is then amplified by a stronger melt‐induced circulation along the ice draft. Our main conclusions are that (1) outputs from ocean models that do not represent ice shelf cavities (e.g., CMIP5 models) should not be directly used to predict the thermal forcing of future ice shelf cavities; (2) coupled ocean/ice sheet models with a velocity‐dependent melt formulation are needed for future projections of glaciers experiencing a significant grounding line retreat.

Ocean-forced evolution of the Amundsen Sea catchment, West Antarctica, by 2100

“…For the optimum set of parameters obtained in the initialization procedure, we project a 2.0-4.5 cm SLE contribution in response to CMIP5 RCP8.5 projections of ocean temperature on-shelf in the ASE. The greater the magnitude of the temperature anomaly over the 21st century, the more extensive the grounding line retreat and projected mass loss from the ASE, which is consistent with findings from modelling studies and observations (Favier et al, 2014). Recent literature has established a close coupling between the basal melting of ice shelves and exacerbated grounding line retreat (Arthern and Williams, 2017;Christianson et al, 2016;Gladstone et al, 2012;Pritchard et al, 2012;Ritz et al, 2015;Seroussi et al, 2014).…”

“…The Amundsen Sea Embayment (ASE) sector, West Antarctica, has been identified as a focal region for mass loss (McMillan et al, 2014;Shepherd et al, 2012Shepherd et al, , 2018, draining one third of the West Antarctic Ice Sheet . Both observational Smith et al, 2017) and modelling studies (Favier et al, 2014;Gladstone et al, 2012;Golledge et al, 2019;Ritz et al, 2015) have inferred that the region is susceptible to rapid and widespread retreat through marine ice sheet instability (MISI), given that the ASE ice streams are grounded on retrograde bedrock below sea level (Schoof, 2010;Weertman, 1974). Ocean-forced sub-ice-shelf basal melting acts to reduce the buttressing effect of ice shelves in the ASE, altering the longitudinal stress balance and causing a speed-up of flow (Gudmundsson, 2013).…”

Assessment of Sub-Shelf Melting Parameterisations Using theOcean-Ice Sheet Coupled Model NEMO(v3.6)-Elmer/Ice(v8.3)

“…25 The linear formulation with a constant exchange velocity assumes a circulation in the ice-shelf cavity that is independent from the ocean temperature. This assumption is neither supported by modelling (Holland et al, 2008;Donat-Magnin et al, 2017) nor by observational studies that suggest a more vigorous circulation in response to a warmer ocean, subsequently increasing melt rates. 30 The quadratic, local dependency to thermal forcing accounts for this positive feedback between the sub-shelf melting and the circulation in the cavity (Holland et al, 2008), using a heat exchange velocity linearly depending on local thermal forcing.…”

The Antarctic Slope Current in a Changing Climate

“…Increased melt rates at the base of floating ice shelves, retreat of ice sheet grounding lines, and increased ice sheet discharge are all positively correlated with increasing ocean temperatures on the shelf (Figure 7a; Paolo et al, ; Pritchard et al, ; Schmidtko et al, ). There is a growing list of oceanographic processes identified as capable of changing the amount of warm CDW water brought onto the Antarctic continental shelf, including (i) changing Southern Ocean westerly and polar easterly winds (Spence et al, ; ); (ii) feedbacks between the glacial meltwater and the ocean circulation (Donat‐Magnin et al, ); (iii) mesoscale turbulence (Stewart et al, ); (iv) tide‐topography interactions (Flexas et al, ); and (v) ocean boundary layer processes (Fer et al, ). Understanding the circulation and cross‐shelf exchange of CDW around the Antarctic coastline is central to understanding the increased Antarctic glacial melt rates.…”

“…Understanding if an initial perturbation will be amplified or weakened, and the physical processes behind this feedback, is fundamental to climate change projections. Interactions between a multitude of processes including dense shelf water formation, on‐shelf transports of CDW, sea ice growth and melt, glacial ice cavity shapes, glacial ice freezing, and melting are important avenues of research that will improve global climate projections (e.g., Donat‐Magnin et al, ; Silvano et al, ).…”