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Strong Sensitivity of Pine Island Ice-Shelf Melting to Climatic Variability

Pine Island Glacier has thinned continuously during past decades driven by an acceleration in its flow. The acceleration is thought to be caused by thinning of the floating ice shelf created as the glacier slides into the sea. This has also released the glacier from friction caused by contact with the ground. Understanding the processes driving ice-shelf thinning and the glacier’s response is key to assessing how much it will contribute to rising sea levels. It’s known that much of the thinning is due to a deep oceanic inflow of Circumpolar Deep Water (CDW) on the continental shelf neighbouring the glacier. This warmer water then makes its way into a cavity beneath the ice shelf melting it from below. The passage of this warmer water was made easier by the unpinning of the ice shelf from an underwater ridge. The ridge had, in effect, acted as a wall preventing warmer water from getting to the thickest part of the shelf. This ungrounding event was one of the major driving forces behind the glacier’s rapid change.

In 2009, a higher CDW volume and temperature in Pine Island Bay contributed to an increase in ice-shelf melting compared to the last time measurements were taken in 1994. But observations made in January 2012, and reported now in Science, show that ocean melting of the glacier was the lowest ever recorded. The top of the thermocline (the layer separating cold surface water and warm deep waters) was found to be about 250m deeper compared with any other year for which measurements exist.
This lowered thermocline reduces the amount of heat flowing over the ridge. High-resolution simulations of the ocean circulation in the ice-shelf cavity demonstrate that the ridge blocks the deepest ocean waters from reaching the thickest ice. So its presence enhances the ice shelf’s sensitivity to climate variability since any changes in the thermocline can alter the amount of heat filtering through.

The fluctuations in temperature recorded by the team may be explained by particular climatic conditions. In January 2012 the dramatic cooling of the ocean around the glacier is believed to be due to an increase in easterly winds caused by a strong La Ninã event in the tropical Pacific Ocean. The observations suggest there is a complex interplay between geological, oceanographic and climatic processes. The study stresses the importance of both local geology and climate variability in ocean melting in this region.

Link to the full paper in the NERC Open Research Archive

Authors

Pierre Dutrieux, Jan De Rydt, Adrian Jenkins, Paul R. Holland, Ho Kyung Ha, Sang Hoon Lee, Eric J. Steig, Qinghua Ding, E. Povl Abrahamsen, Michael Schröder (2014)

Publication

Science, 343 (6167). 174-178