Australia: The Land Where Time Began

A biography of the Australian continent 

The Cryosphere - Influence on Circulation of the Atmosphere

Dense, high-pressure ("continental polar" ) air masses, which penetrate to midlatitudes as cold fronts, are formed in winter under the influence of cooling  snow-covered surfaces, that help to promote their formation. As continental polar air masses set up over a region there are strong winds and snowfall, which is followed by cold, clear weather, as well as temperature inversions.  During winter these air masses form over sea ice and land masses at high latitudes, that are snow-covered, as a result of long-wave radiative cooling, with low insolation and the cold surface area.

Cold fronts in the Northern Hemisphere are associated with upper-air troughs and displacement of the polar front to the south over the continents, which drives Roseby Wave structure and meridional mixing (baroclinic instabilities). In the Northern Hemisphere the thermal contrast between land that is snow-covered and open ocean in the midlatitudes promotes strong temperature gradients that help in driving this mixing. This leads to a mobile polar front and alternation between boreal air masses, that are cold and dry, and southwesterly or westerly air masses that are mild and wet, in winter over much of the Northern Hemisphere landmasses.

Over Antarctica similar cooling of air masses occurs throughout the year, resulting in the driving of very strong katabatic winds that descend from the continent. There is a land-sea configuration that is relatively simple in the high latitudes of the Southern Hemisphere, unlike the situation in the Northern Hemisphere, which results in a strongly zonal atmospheric and oceanic circulation. The Antarctic Vortex is strengthened with the help of the deep cooling influence of the high-elevation continent that is perennially covered with snow, increases the isolation of the continent from midlatitude air masses that are warm.

Katabatic winds are also produced in Greenland, and glacier winds descend from most mountain glaciers and icefields, that are on a smaller scale than those in Antarctica. In Greenland these glacier winds are of 2 types, true katabatic winds that are driven by gravity, associated with air masses that are cold and dense that were formed in the upper catchment, and a second type of glacier wind that result more from topographic funneling of regional winds, which is particularly the case with valley glaciers. In summer a strong cooling influence is exerted by glacier winds, whatever their dynamical origin, when the melting surface of a glacier at 0oC cools the air in the glacier boundary layer. The surface of the glacier and the adjacent downwind region both influence the cooling.

The climate is influenced by ice sheets as large as those in Antarctica and Greenland by the topographic obstruction presented to circulation in the troposphere. As the air flows around these obstacles, as it does in mountain belts, the result is that for much of the time they generate high-pressure (anti-cyclonic) circulation patterns that are persistent. Ice sheets over North America and Eurasia disrupted the midlatitude westerlies and the prevailing stationary wave pattern during the glaciations of the Pleistocene which led to the polar jet stream being split over North America and increased precipitation to the south of the ice sheets. During the glacial period the stationary wave pattern and associated storm tracks were also modified by the expansion of sea ice cover in the North Pacific and the North Atlantic as a result of the displacement to the south of Aleutian and Icelandic Lows.

Interannual variability of seasonal snow and ice are suggested by the author1 to possibly play a role in variability patterns in atmospheric circulation at high latitudes, such as the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO), as a result of their effect on pressure patterns of snow and ice, though this is not fully understood.

Sources & Further reading

  1. Marshall, Shawn J., 2012, The Cryosphere, Princeton University Press.
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 29/04/2013

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