Coastal Polynyas Enable Transitions Between High and Low West Antarctic Ice Shelf Melt Rates

Abstract

Melt rates of West Antarctic ice shelves in the Amundsen Sea track large decadal variations in the volume of warm water at their outlets. This variability is generally attributed to wind‐driven variations in warm water transport toward ice shelves. Inspired by conceptual representations of the global overturning circulation, we introduce a simple model for the evolution of the thermocline, which caps the warm water layer at the ice‐shelf front. This model demonstrates that interannual variations in coastal polynya buoyancy forcing can generate large decadal‐scale thermocline depth variations, even when the supply of warm water from the shelf‐break is fixed. The modeled variability involves transitions between bistable high and low melt regimes, enabled by feedbacks between basal melt rates and ice front stratification strength. Our simple model captures observed variations in near‐coast thermocline depth and stratification strength, and poses an alternative mechanism for warm water volume changes to wind‐driven theories.

A Predictive Theory for Heat Transport Into Ice Shelf Cavities

“…We might expect this assumption to fail on time scales shorter than the flushing time scale of the cavity (Holland, 2017), on which transient heat storage in the cavity and ice shelf boundary layer/plume dynamics more directly dictate the melt rate (Lazeroms et al., 2018). Our theory also predicts that the melt rate is entirely determined by the ice shelf geometry and the external hydrography, in contrast with previous studies showing that circulation within ice shelves can exhibit bi‐stable states (Caillet et al., 2023; Hellmer et al., 2017; Moorman et al., 2023). Future work is required to reconcile our theory with previous theories for bi‐stability of ice shelf cavity circulation and melt rates (Hazel & Stewart, 2020).…”

“…Wind stresses over the continental slope lead to cross‐slope Ekman transport that has been linked to variability of CDW heat fluxes across and along the shelf in observations (Assmann et al., 2013; Greene et al., 2017) and models (Dotto et al., 2020; Spence et al., 2014; Tamsitt et al., 2021; Thoma et al., 2008). Wind forcing over the continental shelf can also lead to vigorous deep mixing which erodes the thickness of CDW on the shelf (Caillet et al., 2023; Moorman et al., 2023). Surface buoyancy losses, for example, due to sea ice formation in coastal polynyas, are also able to erode the thickness of CDW across the shelf by deepening the mixed layer (Caillet et al., 2023; Webber et al., 2017).…”

Variability of Inflowing Current Into the Dotson Ice Shelf and Its Cause in the Amundsen Sea

“…In polynya, changes in surface density due to the melting and forming of sea ice and the cooling and heating of the ocean surface layer by heat exchange with the atmosphere affect thermocline depth. Therefore, buoyancy flux causes the thermocline to rise/fall and increase/decrease its density (Webber et al., 2017), which is one of the critical factors contributing to ice shelf melt (Moorman et al., 2023; St‐Laurent, et al., 2015). The buoyancy flux, estimated from air‐sea heat flux and freshwater flux obtained from SOSE (Southern Ocean State Estimate; Mazloff et al., 2010) data, was positive in autumn to winter (i.e., increases buoyancy flux from ocean to air), which coincided with the density decrease in April (Figure S3 in Supporting Information ).…”

Subsurface Warming of the West Antarctic Continental Shelf Linked to El Niño‐Southern Oscillation

“…At the same time, mixing of cooler water across the 100 m depth level is enhanced. This is likely caused by changes in the vertical stability under different signs of surface flux anomalies, by increased sea ice growth (Figure S9d in Supporting Information S1) and convection because the stronger easterlies have the potential to create larger polynyas (openings within the ice cover) that impact buoyancy fluxes and the flow of warm water onto the shelf (Moorman et al, 2023). This might also be a contributing factor why cooling during La Niña is larger than warming during El Niño.…”