Australia: The Land Where Time Began

A biography of the Australian continent 

The Cryosphere - Interaction between Ocean and Ice  

Ocean temperatures are moderated by the melting and freezing of sea ice, as well as affecting the salinity budget in polar seas, the ramification of which affects the circulation of the oceans and water mass formation. As sea ice forms, brine is expelled which then descends to the seafloor as dense, saline water that forms the bottom water. Salinity is the main factor determining the density of the polar ocean waters that are at near-freezing temperatures. Adjacent to the coast of Antarctica polynya, that are maintained katabolically, are the "sea-ice factories" associated with high rates of production of bottom water. A location where this is particularly effective is wide parts of the continental shelf in the Weddell Sea where high-salinity shelf water is formed, mixing with highly saline circumpolar deep water that is present just off the shelf, creating the Antarctic Bottom Water which is found in the deep abyss of the world ocean. It is believed that beneath the ice shelves melting and circulation probably plays some role in this, as meltwater from ice shelves can be exceptionally cold and it contributes to the deep water mass outflow from Antarctica.

Sea ice formation in the Arctic Ocean has a similar effect, though saline deep waters are largely confined to the Arctic Basin. In the Labrador Sea and the Scandinavian Seas formation of sea-ice contributes to the formation of intermediate and deep water formation in the North Atlantic.

The melting of sea ice has the opposite effect, with the surface waters being freshened where it melts. Strong stratification of surface waters in the Atlantic is contributed to in the Arctic by high volumes of river runoff in spring and summer as well as the freshening of the surface waters by melting ice. Arctic surface waters are fresher than any others in the world, with the freshening being added to by glacial meltwater, in particular the runoff from the Greenland ice sheet. The magnitude of the annual meltwater discharge from Greenland is similar to that of the major Arctic river basins draining Canada and Russia, about 350 km3 w.e. per year, most of which is concentrated in the summer months. A significant export of freshwater, which is capable of tilting the salinity budget of the North Atlantic, occurs where water masses and sea ice are advected out of the Arctic basin, via Fram Strait and the Canadian Archipelago. In the past, major freshwater advection events have been observed, one such being the "Great Salinity Anomaly" that began in the late 1960s. In 1967 strong export of ice through Fram Strait was identified as a freshwater pool in southeast Greenland in 1968 which was transported to the Labrador Sea in the subpolar gyre, where it stalled until 1972, at which time it was caught up in the North Atlantic Drift which advected it back into the northeastern Atlantic. This low-density surface water anomaly, that was tracked until 1982, was a long-lived feature that disrupted convective mixing and the formation of intermediate water in the Labrador Sea, which resulted in sea surface temperatures that were lower than usual. Though strong, this event doesn't seem to be unique, as several anomalies, both multiyear high- and low-salinity, have been tracked in the North Atlantic region that were linked to variations in the export of freshwater from the Arctic.

The waters in which icebergs issuing from the polar ice sheets, ice caps and coastal tidewater glaciers, melt are cooled and freshened. The melting at depth of iceberg keels, that can be hundreds of metres deep, lead to freshwater plumes that promote mixing and ventilation. The nutrient delivery that accompanies these plumes lead to algal blooms that have been observed. The stratification of surface waters and strengthening of the estuarine circulation are helped in tidewater fjords and estuaries where most icebergs ground and melt locally. Isolation of a basin, with suppression of ventilation and the development of anoxia in the deeper layers in embayments that have shallow sills, and where mixing with offshore waters is limited, can result from the strong stratification that is associated with the runoff of freshwater and the melting of icebergs.

During the glaciations of the Pleistocene the Atlantic Meridional Overturning Circulation (AMOC) in the North Atlantic was intermittently disrupted by freshwater perturbations resulting from the large numbers of icebergs that were produced by the North American and Eurasian Ice Sheets. During the last glacial cycle several Heinrich Events, episodic iceberg fluxes from Hudson Strait, resulted in the North Atlantic region being flooded, the cold, fresh surface waters forming a cap that contributed to the persistence of the icebergs long enough for them to advect from the Labrador Sea as far south as Portugal. The worldwide climate effects that resulted  from these events, were telegraphed through the atmosphere and the disruptions of the formation of North Atlantic deep water. In the early stages of deglaciation during the glacial period, 8,200 years ago, runoff of meltwater from the Pleistocene ice sheets and the catastrophic release of water from massive glacier-dammed lakes, resulted in similar disruptions to surface stratification of the ocean and its circulation.

According to the author1 the oceans appear to have begun to perturb the polar ice sheets in the present interglacial. In Antarctica and Greenland ice shelves and marine-based outlet glaciers melt and calve off at the interface between the ocean and the ice, and ablation at this interface is exceptionally sensitive to the temperature of the oceans, which also occurs at other, smaller ice caps in the Arctic, where ice extends to the sea. Destabilisation of ice shelves and marine-based outlets can be triggered by warming of the ocean and wind-driven changes in the circulation of the ocean that bring warm water masses, such as circumpolar deep water or North Atlantic water, into contact with the ice. According to the author1 in a classically known "tidewater glacier" instability this destabilisation is propagated inland through retreat of the grounding line, thinning, and the acceleration of the rate of ice flow.

The length of time this can continue is unknown at the present, and how dramatic it will be found to be for major sectors of the polar ice sheets that are marine-based, though the process is well understood in tidewater outlet glaciers in places such as Alaska. In the Amundsen Sea sector of West Antarctica and Jakobshavn Isbrae in Greenland rapid changes that are ongoing in the 2,000s are linked to the advection of warm water to each region.

The author1 suggests large-scale perturbation of ocean circulation is unlikely to result from the melting of the ice sheets in Antarctica and Greenland,  as fluctuations of ice sheets did in the glaciations of the Pleistocene. In the Northern Hemisphere the landscape was much different during the Pleistocene, with much of the midlatitude land mass in that hemisphere being covered by permafrost, ice sheet lobes and proglacial lake systems. Compared with the present the area of glaciers that contributed to runoff was large, with greater energy being available for the melting of ice at low latitudes. The author1 suggests the quantity of meltwater that was characteristic of the glacial period is not conceivable from the ice sheets of Antarctica and Greenland at the present. The fluxes of freshwater associated with increased precipitation in the midlatitudes, Arctic export, and sea ice changes are of greater magnitude under most scenarios of climate warming for the coming century. An exception to this could be provided by dynamical destabilisation of a major sector of the Antarctic or Greenland ice sheet. Mechanical destabilisation as occurred to the Larsen Ice Shelf, or a major surge event, that is similar to that which is believed to have occurred in Heinrich Events, are included among the ice sheet processes that could deliver large fluxes of freshwater to the oceans.

 

Sources & Further reading

  1. Marshall, Shawn J., 2012, The Cryosphere, Princeton University Press.
 
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 28/04/2013

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading