Geometric and oceanographic controls on melting beneath Pine Island Glacier

Abstract

Observations beneath the floating section of Pine Island Glacier have revealed the presence of a subglacial ridge which rises up to 300 m above the surrounding bathymetry. This topographic feature probably served as a steady grounding line position until sometime before the 1970s, when an ongoing phase of rapid grounding line retreat was initiated. As a result, a large ocean cavity has formed behind the ridge, strongly controlling the ocean circulation beneath the ice shelf and modulating the ocean water properties that cause ice melting in the vicinity of the grounding line. In order to understand how melt rates have changed during the various phases of cavity formation, we use a high-resolution ocean model to simulate the cavity circulation for a series of synthetic geometries. We show that the height of the ridge and the gap between the ridge and ice shelf strongly control the inflow of warm bottom waters into the cavity, and hence the melt rates. Model results suggest a rapid geometrically controlled increase of meltwater production at the onset of ice thinning, but a weak sensitivity to geometry once the gap between the ridge and ice shelf has passed a threshold value of about 200 m. This provides evidence for a new, coupled, ice-ocean feedback acting to enhance the initial retreat of an ice stream from a bedrock high. The present gap is over 200 m, and our results suggest that observed variability in melt rates is now controlled by other factors, such as the depth of the thermocline.

Ocean‐Forced Ice‐Shelf Thinning in a Synchronously Coupled Ice‐Ocean Model

“…These two water masses are separated by a linearly varying pycnocline of 400 m thickness, starting at 300 m depth. These temperature and salinity profiles are consistent with previous work on Pine Island Glacier (PIG) (De Rydt et al, ). Sensitivity studies have been carried out around this baseline by varying the depth of the pycnocline by ±100 m and 200 m in both directions, but maintaining its thickness of 400 m. This gives us five different forcings, henceforth referred to by the depth of the upper limit of the pycnocline (100, 200, 300 (baseline), 400, 500).…”

Bathymetric Controls on Calving Processes at Pine Island Glacier

“…This reorientation of the calving front was reconfirmed by the 2017 event. Pinning point loss results in 5 reduced back stress to the ice shelf through reduction of basal drag (Favier et al, 2014) and the topography of an ice-shelf cavity is important for internal water exchange (De Rydt et al, 2014). Therefore, in the following we evaluate how the bathymetric ridge and the two highs A and B affected the development of the PIG ice shelf.…”

Ocean mixing beneath Pine Island Glacier ice shelf, West Antarctica

“…Retreat of the grounding line has since resulted in the formation of a large ocean cavity behind the ridge. This transverse ridge separates the PIG cavity into two gyres and interaction of these gyres modulates the ocean water properties near the grounding line [ De Rydt et al ., ; Dutrieux et al ., ].…”