Australia: The Land Where Time Began

A biography of the Australian continent 

Pine Island Bay, West Antarctica – Export and Circulation of Cavity Water

Large sectors of the Antarctic ice sheet have been found to be vulnerable to melting at the bases of fringing ice shelves, the rates of melting being dependent on temperatures of the ocean and circulations in the cavities beneath the ice. In this paper Thurnherr et al. present the results of their analysis of an oceanographic data set that was obtained in Pine Island Bay in the southern summer of 2009, which is bounded to the east by the calving front of the fast moving Pine Island Ice Shelf in the Amundsen Sea. The velocity field in the upper ocean in the ice-free part of the bay was found to be dominated by a gyre that was 700 m deep and 50 km wide circulating 1.5 Sv (Sverdrup) of water clockwise around the bay. It was observed that Ice Cavity Water (ICW) was present in a surface-intensified and southwards-intensified boundary current moving along the ice front, as well as in a small ice cove at the end of the southern shear margin of the ice shelf. Persistent ice cavity water was revealed by repeat measurements in the cove being exported at a rate of ≈ 0.25 Sv (Sverdrup units) during a 10 day sampling period. In the cove above the ice draft vertical velocities were dominated by buoyancy-frequency oscillations that had amplitudes of several cm per second with no significant upwelling. This observation, combined with the seawater properties, indicates that much of the upwelling occurs within fractured ice near the cove, which potentially contributes to weakening of the ice shelf shear margin.

The Antarctic Ice Sheet has been found to be vulnerable to ocean forcing acting on its fringing ice shelves (Joughin et al., 2012). Grounded ice sheets are dynamic entities that gain mass mainly from precipitation and lose mass to the sea as embedded glaciers, aka ice streams. The ice streams become floating ice shelves when they lose contact with the seabed, and these ice shelves terminate in iceberg calving fronts. Overturning circulations that are fed by Circumpolar Deep Water (CDW), that is comparatively warm, are known to be hosted by some cavities beneath the floating ice shelves, which flow towards the back of the cavities at depth. Rapid melting near grounding lines, where cooler, fresher water and mixtures of Circumpolar Deep Water and meltwater are produced, that is less dense, drives the overturning. Towards the calving fronts the buoyant waters rise and sculpt networks of channels into the ice (e.g., Stanton et al., 2013; Dutrieux et al., 2013). In regions where the Ice Cavity Water (ICW) that is exported retains sufficient buoyancy at the calving fronts it rises towards the surface of the ocean, and sometimes reaches the surface (e.g. Mankoff et al., 2012).

As the ice sheet provides storage on land of immense amounts of water that are enough to affect global sea level, the mass balance of Antarctic glaciers and ice streams is important (Church et al., 2001). Evidence has been mounting of links between stronger ocean circulation coupled with increased subsurface temperatures, increased ice shelf melting, and the thinning and acceleration of their incoming ice streams, though the mass balance of any given glacier is complex (Joughin et al., 2012). Much of the West Antarctic Ice Sheet rests on a bed grounded below sea level, which deepens in a landward direction, so the West Antarctic Ice Sheet is of particular interest, because this setup can lead to runaway retreat of the ice sheet as the buttressing provided by the ice shelves is weakened by melting (Schoof, 2007). Researchers have directed considerable attention to the Pine Island Glacier (PIG) in West Antarctica, this glacier being a fast-moving ice stream that evolves into the Pine Island glacier Ice Shelf (PIIS), which terminates in Pine Island Bay (PIB) in the southeast Amundsen Sea (Jacobs et al., 1996; Rignot, 2008; Jenkins et al., 2010; Joughin et al., 2010; Bindschadler et al., 2011). Changes of about 50 % in the production of meltwater have been inferred (Jacobs et al., 2011; Dutrieux et al., 2014), based primarily on ocean measurements from 1994 to 2009 and along the calving front of the Pine Island glacier Ice Shelf. Also, a striking cyclonic gyre was observed in the summer of 2009 that occupied a large fraction of the surface area of the ice-free Pine Island Bay (Jacobs et al., 2011; Mankoff et al., 2012; Tortell et al., 2012).

Thurnherr et al., say the objective of the study reported in this paper was to obtain a better understanding of seawater flows to and from the cavity beneath the Pine Island Ice Shelf by also including many of the 2009 CTD and ADCP measurements (section 2) that were not used in the earlier studies. The horizontal circulation in Pine Island Bay away from the immediate vicinity of the calving front of the Pine Island Glacier was dominated by a geostrophic gyre recirculating about 1.5 Sv (1.5 Sverdrup) of water around the bay (Section 3). The recirculating water in the gyre, near the southeast corner of Pine Island Bay, was joined by Ice Cavity Water, that had recently formed, flowing out of a small cove near the southern end of the calving front (section 4) and by a southwards coastal boundary current that carried the flux from the remainder of the front (section 5). The manuscript concludes with a discussion of the main results.

Sources & Further reading

1. Thurnherr, A. M., S. S. Jacobs, P. Dutrieux and C. F. Giulivi (2014). "Export and circulation of ice cavity water in Pine Island Bay, West Antarctica." Journal of Geophysical Research: Oceans 119(3): 1754-1764.