high-latitude tropospheric circulation

high-latitude tropospheric circulation The high latitudes of the northern and the southern hemispheres are very different geographically. In the northern hemisphere the polar region is an ocean basin which is almost completely enclosed by surrounding major land masses. Much of the ocean basin is covered by floating sea ice. In the southern hemisphere the polar region is occupied by the Antarctic continent, much of which is at an altitude greater than 3000 m. This land mass is completely surrounded by the Southern Ocean and in middle latitudes of the southern hemisphere there is very little land. As a consequence of these very different geographical configurations, the tropospheric circulations at high latitudes in the two hemispheres are also very different.

Southern hemisphere

The continental landmass of Antarctica is almost entirely ice-covered and the ice surface is mostly at a high altitude. Air temperatures at the surface are very low throughout the year, but particularly so in the dark winter months. From the edge of the continent, northwards across the Southern Ocean to mid-latitudes, there is a very marked temperature gradient. This temperature gradient provides an environment which forces the continual development of mobile low-pressure systems in a band circling the continent. These generally move in a south-easterly direction, and each one typically has a life span of several days. Monthly mean surface pressure charts show a band of low pressure encircling Antarctica between 60° S and 70° S. Figure 1 is an example, showing the average mean sea-level pressure for July.

In the low-pressure belt circling Antarctica the weather is constantly very changeable and is often stormy. In many ways the weather patterns in this belt are similar to those in the northern fringes of the temperate latitudes in the North Atlantic and North Pacific Oceans. The main differences is that the blocking anticyclones which are common in the northern hemisphere are much less common in the southern hemisphere. When they do form they are normally short-lived. The reason for this is that the southern hemisphere has no substantial land masses in middle latitudes and the weather patterns are totally dominated by the mobile maritime low-pressure systems.

Polarwards of the belt of low pressure the average mean sea-level pressure is somewhat higher. The active low-pressure systems surrounding the continent do not normally extend their influence much into the continent itself. Over the Antarctic Plateau the tropospheric air is generally subsiding, and there is thus little cloud or precipitation. Indeed, the very small amounts of precipitation categorize Antarctica as a desert. Precipitation does occur around the coasts on the fringes of the low-pressure systems but inland, conventional precipitation is rare. There is occasional precipitation inland in the form of ice crystals. This occurs when relatively moist maritime air is carried inland at levels above the surface. This air cools significantly to become supersaturated with respect to ice, and some of the moisture precipitates out in the form of ice crystals. The great blizzards, for which Antarctica is notorious, are caused by blowing snow rather than by falling snow.

One very notable feature of the lower troposphere over the Antarctic Plateau is the very marked surface temperature inversion that exists throughout the year, although it is most pronounced in winter. This temperature inversion means that at heights of a few hundred metres above the surface the temperature is normally significantly higher than at the surface. In extreme conditions in winter the temperature difference has been known to exceed 30 °C. One effect of this marked temperature inversion is that winds at the lowest levels, below the inversion, are normally completely decoupled from the winds above the inversion. Above the inversion the winds follow the normal relationship between pressure gradient and wind. Below the inversion the winds are largely driven by gravity, with the cold air draining down the slopes. These winds are known as katabatic winds. In some localities the shape of the land can cause winds of gale force or stronger to blow for days or even weeks at a time. At the top of the inversion there can be very marked changes in wind direction and wind speed over only a very small vertical distance. This is a very important factor for aircraft pilots to take into consideration during take-off and landing or when engaged in low-level flying. A sudden decrease in head wind or increase in tail wind can cause an aircraft to stall.

Over the Antarctic Plateau there is normally very low humidity throughout the troposphere. This results in largely clear skies allowing maximum radiation cooling in the winter months. Winters are very long with constantly very low temperatures. The lowest temperature every recorded (−89 °C) on the surface of the Earth was at the Russian base, Vostok, which is at an altitude of 3400 m. Summers are short and temperatures are slow to rise to their midsummer maximum and quick to fall again subsequently.

Northern hemisphere

Although the northern hemisphere polar region is an ocean basin mostly surrounded by land masses, much of the sea surface is ice-covered for most of the year. The main exceptions to this are the Norwegian Sea and the Barents Sea, which are ice-free in summer as far north as 80° N. Even in winter the ice edge is often north of 70° N, especially in the Norwegian Sea. The coastal fringes all around the Arctic Ocean have ice-free stretches in summer but freeze over in winter. Because of the extensive ice cover in winter the region has more of a land environment in that season than a marine environment. The great difference compared to the polar regions of the southern hemisphere is that the surface is mostly at or close to sea level. The only high elevations are found on the peripheral land masses, notably Greenland.

In the middle and upper levels of the troposphere the circulation is dominated by a cold-cored polar vortex throughout the year. This is at its most pronounced during the winter months. Strong westerly winds circle the globe in middle latitudes around this vortex. This basic westerly flow is modified by the land masses, notably over North America, where the Rocky Mountains cause a semi-permanent upper atmosphere ridge resulting in a corresponding upper atmosphere trough downstream over eastern North America.

Minor short-wave troughs moving in the westerly upper air flow provide the dynamics conducive to the development of surface low-pressure systems. Conditions are particularly suited to such cyclogenesis in winter off the east coasts of Asia and North America where horizontal temperature gradients are greatest. These lows typically track north-eastwards, reaching maximum intensity in the vicinity of the Aleutian Islands in the Pacific Ocean and in the vicinity of Iceland in the Atlantic Ocean. Figure 2 shows the average mean sea-level pressure for January. This shows clearly that the main low-pressure areas of middle latitudes in winter are to be found over the seas, while the land masses are dominated by areas of high pressure. This juxtaposition of high-pressure and low-pressure systems in similar latitudes causes much more meridional airflow than is normally experienced in similar latitudes in the southern hemisphere. As a result of this, in the northern hemisphere, polar air in the lower troposphere is frequently advected into middle latitudes and, occasionally, even as far as subtropical latitudes. These outbreaks of polar air can occur at any longitude but are particularly common and intense along the eastern fringes of Asia and North America. The outbreaks of polar air to middle latitudes are normally balanced by intrusions of mild maritime air from middle latitudes into the Arctic and polar regions. The preferred regions for this to happen are in the Bering Sea area and, particularly, through the gap between Greenland and Scandinavia.

It is not uncommon for the Atlantic and Pacific low-pressure systems to continue their north-easterly track during the decaying phase of their life cycle and move completely into the polar regions. By this stage they are normally relatively weak systems, having moved away from the regions of marked horizontal temperature gradient that caused their formation. The circulation is then entirely of cold polar air, and there is little precipitation associated with such lows when they reach the polar areas. The very low annual precipitation totals categorize the northern polar regions as a desert area: the very cold air cannot hold enough water vapour to permit substantial precipitation.

In contrast to situations dominated by low pressure, the northern polar regions also experience spells of very strong high-pressure domination in winter, often linked to major middle-latitude high-pressure systems over Asia or North America. In extreme cases the surface atmospheric pressure in the polar regions can reach or exceed 1050 millibars.

In the summer months the upper-air polar vortex is much weaker and the persistent cold high-pressure systems are absent from the middle-latitude land masses. As a result, there is much less interaction between the lower troposphere air masses of the polar regions and middle latitudes. During this season the polar vortex of the middle and upper troposphere is also reflected in the lower troposphere, with relatively low surface pressures being typical of the polar regions.

Norman Lynagh

Bibliography

King, J. C. and and Turner, J. (1997) Antarctic meteorology and climatology. Cambridge University Press.
Sater, J. E.,, Ronhovde, A. G.,, and and Van Allen, L. C. (1971) Arctic environment and resources. The Arctic Institute of North America, Washington, DC.