The Dynamics of a Barotropic Current Impinging on an Ice Front

Abstract

The vertical front of ice shelves represents a topographic barrier for barotropic currents that transport a considerable amount of heat towards the ice shelves. The blocking effect of the ice front on barotropic currents has recently been observed to substantially reduce the heat transport into the cavity beneath the Getz Ice Shelf. We use an idealized numerical model to study the vorticity dynamics of an externally forced barotropic current at an ice front and the impact of ice shelf thickness, ice front steepness, and ocean stratification on the volume flux entering the cavity. Our simulations show that thicker ice shelves block a larger volume of the barotropic flow, in agreement with geostrophic theory. However, geostrophy breaks locally at the ice front, where relative vorticity and friction become essential for the flow to cross the discontinuity in water column thickness. The flow entering the cavity accelerates and induces high basal melt rates in the frontal region. Tilting the ice front, as undertaken in sigma-coordinate models, reduces this acceleration as the flow is more geostrophic. Viscous processes—typically exaggerated in low-resolution models—break the potential vorticity constraint and bring the flow deeper into the ice shelf cavity. The externally forced barotropic current can only enter the cavity if the stratification is weak, as strong vertical velocities are needed at the ice front to squeeze the water column beneath the ice shelf. If the stratification is strong, vertical velocities are suppressed and the barotropic flow is almost entirely blocked by the ice front.

Observed Seasonal Evolution of the Antarctic Slope Current System off the Coast of Dronning Maud Land, East Antarctica

“…There is an apparent link between seasonal variations in both the baroclinic and barotropic ASC strength and flow into the Fimbulisen cavity (Figure 9a). While not investigated here in detail, this link may include an intricate interplay between the local bathymetry at the sills (Eisermann et al., 2020; Nøst, 2004), bottom Ekman transport anomalies (Núñez‐Riboni & Fahrbach, 2009; Smedsrud et al., 2006) and potential vorticity constraints (Daae et al., 2017; Steiger et al., 2022; Wåhlin et al., 2020). Furthermore, a seasonal counter‐current at depth, neither observed at DML shallow nor at DML deep , but observed by Heywood et al.…”

Hydrography and circulation below Fimbulisen Ice Shelf, East Antarctica, from 12 years of moored observations

“…10a). This displacement is expected to vary seasonally, as the potential vorticity constraint depends on stratification (Steiger et al, 2022) and baroclinicity (Hattermann et al, 2014, estimated by velocity shear at M1, Fig. 10d).…”

“…We hypothesize that the seasonal interplay of the ASC with the ice shelf plays a role in setting up favorable conditions for WDW intrusions at times other than at the externally forced thermocline depth minimum. Recent observational and modeling studies have highlighted the role of conservation of potential vorticity for the interaction of the ASC with an ice shelf (Wåhlin et al, 2020;Steiger et al, 2022). As the ASC follows contours of water column thickness (Thompson et al, 2018), the presence of Trolltunga may displace the upstream ASC (Fig.…”

“…However, there was no evident increase in sub-ice-shelf temperatures during this period. Lauber et al (2023a) therefore suggested that additional processes like bottom Ekman transport (Smedsrud et al, 2006;Núñez-Riboni and Fahrbach, 2009) and potential vorticity dynamics at the ice front (Wåhlin et al, 2020;Steiger et al, 2022) may locally modulate the WDW access to the cavity.…”

Closing the loops on Southern Ocean dynamics: From the circumpolar current to ice shelves and from bottom mixing to surface waves

“…Rapid changes in water column thickness near ice shelf and glacier tongues modify local angular momentum balances (van Heijst, 1987). Strong potential vorticity gradients occur at ice-shelf fronts (Steiger et al, 2022), where the ice draft may be greater than half the local seabed depth. The ice creates a barrier against which water may pool and a strong along-front flow may develop (Malyarenko et al, 2019).…”