A The Arctic

At 75°N, the sun is below the horizon for about ninety days, from early November until early February. Winter air temperatures over the Arctic Ocean average about -32°C, but they are usually 10-12°C higher some 1000 m above the surface as a result of the strong radiative temperature inversion. The winter season is generally stormy in the Eurasian sector, where low-pressure systems enter the Arctic Basin from the North Atlantic, whereas anticyclonic conditions predominate north of Alaska over the Beaufort and Chukchi seas. In spring, high pressure prevails, centred over the Canadian Arctic Archipelago-Beaufort Sea.

The average 3 to 4 m thickness of sea ice in the Arctic Ocean permits little heat loss to the atmosphere and largely decouples the ocean and atmosphere systems in winter and spring. The winter snow accumulation on the ice averages 0.25 to 0.30 m depth. Only when the ice fractures, forming a lead, or where persistent offshore winds and/or upwelling warm ocean water form an area of open water and new ice (called a polynya), is the insulating effect of sea ice disrupted. The ice in the western Arctic circulates clockwise in a gyre driven by the mean anticyclonic pressure field. Ice from the northern margin of this gyre, and ice from the Eurasian sector, moves across the North Pole in the Transpolar Drift Stream and exits the Arctic via Fram Strait and the East Greenland current (see Figure 10.35A). This export

MOSCOW MURMANSK TURUKHANSK CAPE CHELYUSKIN VERKHOYANSK OKHOTSK

MOSCOW MURMANSK TURUKHANSK CAPE CHELYUSKIN VERKHOYANSK OKHOTSK

Eurasian Arctic Weather

YALTA YEREVAN TASHKENT OMSK KRASNOYARSK VLADIVOSTOK

Figure 10.37 Months of maximum precipitation, annual regimes of mean monthly precipitation and annual regimes of mean monthly frequencies of five main weather types in the former USSR showing the climate severity of the Arctic coast.

YALTA YEREVAN TASHKENT OMSK KRASNOYARSK VLADIVOSTOK

Figure 10.37 Months of maximum precipitation, annual regimes of mean monthly precipitation and annual regimes of mean monthly frequencies of five main weather types in the former USSR showing the climate severity of the Arctic coast.

Source: Reprinted from P. E. Lydolph (1977), with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

largely balances the annual thermodynamic ice growth in the Arctic Basin. In late summer, the Eurasian shelf seas and the coastal section of the Beaufort Sea are mostly ice-free.

In summer, the Arctic Ocean has mostly overcast conditions with low stratus and fog. Snowmelt and extensive meltwater puddles on the ice keep air temperatures at around freezing. Low-pressure systems tend to predominate, entering the basin from either the North Atlantic or Eurasia. Precipitation may fall as rain or snow, with the largest monthly totals in late summer to early autumn. However, the mean annual net precipitation minus evaporation over the Arctic, based on atmospheric moisture transport calculations, is only about 180 mm.

On Arctic land areas there is a stable snow cover from mid-September until early June, when melt occurs within ten to fifteen days. As a result of the large decrease in surface albedo, the surface energy budget undergoes a dramatic change to large positive values (Figure 10.38). The tundra is generally wet and boggy as a result of the permafrost table only 0.5 to 1.0 m below the surface, which prevents drainage. Thus the net radiation is expended primarily for evapotranspiration. Permanently frozen ground is over 500-m thick in parts of Arctic North America and Siberia and extends under the adjacent Arctic coastal shelf areas. Much of the Queen Elizabeth Islands, the Northwest Territories of Canada and the Siberian Arctic Islands is cold, dry polar desert, with gravel or rock surfaces, or

Temperature Graph Tundra

Figure 10.38 The effect of tundra snow cover on the surface energy budget at Barrow, Alaska, during the spring melt. The lower graph shows the daily net radiation and energy terms.

Source: Weller and Holmgren (1974) From Journal of Applied Meteorology, by permission of the American Meteorological Society.

Figure 10.38 The effect of tundra snow cover on the surface energy budget at Barrow, Alaska, during the spring melt. The lower graph shows the daily net radiation and energy terms.

Source: Weller and Holmgren (1974) From Journal of Applied Meteorology, by permission of the American Meteorological Society.

ice-caps and glaciers. Nevertheless, 10 to 20 km inland from the Arctic coasts in summer, daytime heating disperses the stratiform cloud and afternoon temperatures may rise to 15 to 20°C.

The Greenland ice sheet, 3 km thick and covering an area of 1.7 million km2, contains enough water to raise global sea-level by over 7 m if it were all melted. However, there is no melting above the equilibrium line altitude (where accumulation balances ablation), which is at about 2000 m (1000 m) elevation in the south (north) of Greenland. The ice sheet largely creates its own climate. It deflects cyclones moving from Newfoundland, either northward into Baffin Bay or northeastward towards Iceland. These storms give heavy snowfall in the south and on the western slope of the ice sheet. A persistent shallow inversion overlays the ice sheet with down-slope katabatic winds averaging 10 m s 1, except when storm systems cross the area.

b Antarctica

Except for protruding peaks in the Transantarctic Mountains and Antarctic Peninsula, and the dry valleys of Victoria Land (77°S, 160°E), over 97 per cent of Antarctica is covered by a vast continental ice sheet. The ice plateau averages 1800 m elevation in West Antarctica and 2600 m in East Antarctica, where it rises above 4000 m (82°S, 75°E). In September, sea ice averaging 0.5 to 1.0 m in thickness covers twenty million km2 of the Southern Ocean, but 80 per cent of this melts each summer (Figure 10.35B).

Over the ice sheet, temperatures are almost always well below freezing. The South Pole (2800-m elevation) has a mean summer temperature of -28°C and a winter temperature of -58°C. Vostok (3500 m) recorded -89°C in July 1983, a world record minimum. Mean monthly temperatures are consistently close to their winter value for the six months between equinoxes, creating a so-called 'coreless winter' (Figure 10.39). Atmospheric poleward energy transfer balances the radiative loss of energy. Nevertheless, there are considerable day-today temperature changes associated with cloud cover increasing downward long-wave radiation, or winds mixing warmer air from above the inversion down to the surface. Over the plateau, the inversion strength is about 20 to 25°C. Precipitation is almost impossible to measure, as a result of blowing and drifting snow. Snow pit studies indicate an annual accumulation varying from less than 50 mm over the high plateaux above 3000 m elevation to 500 to 800 mm in some coastal areas of the Bellingshausen Sea and parts of East Antarctica.

Lows in the southern westerlies have a tendency to spiral clockwise towards Antarctica, especially from south of Australia towards the Ross Sea, from the South Pacific towards the Weddell Sea, and from the western South Atlantic towards Kerguelen Island and East Antarctica (Figure 10.40). Over the adjacent Southern Ocean, cloudiness exceeds 80 per cent year-round at 60 to 65°S (see Figures 3.8 and 5.11) due to the frequent cyclones, but coastal Antarctica has more synoptic variability, associated with alternating lows and highs. Over the interior, cloud cover is generally less than 40 to 50 per cent and half of this amount in winter.

The poleward air circulation in the tropospheric polar vortex (see Figure 7.3) leads to subsiding air over the

Figure 10.39 Annual course of (A) mean monthly air temperature (°C) and (B) wind speed (m s-1) for 1980 to 1989 at Dome C (3280 m), 74.5°S, I23.0°E (plateau) and D-10, an automatic weather station at 240 m, 66.7°S, I39.8°E (coast).

Source: Stearns et al. (1993), by permission of the American Meteorological Society.

Source: Stearns et al. (1993), by permission of the American Meteorological Society.

Antarctic Polar Cyclone July
Figure 10.40 Southern hemisphere cyclone paths affecting Antarctica and major frontal zones in winter. I Polar front; 2 Antarctic front; 3 Cyclone trajectories.

Source: Carleton (1987), copyright © Chapman and Hall, New York. Reproduced by permission.

Antarctic Plateau and outward flow over the ice sheet surface. The winds represent a balance between gravitational acceleration, Coriolis force (acting to the left), friction and inversion strength. On the slopes of the ice sheet, there are stronger downslope katabatic flows, and extreme speeds are observed in some coastal locations. Cape Denison (67°S, 143°E), Adelie Land, recorded average daily wind speeds of >18 m s-1 on over 60 per cent of days in 1912 to 1913.

SUMMARY

Seasonal changes in the Icelandic low and the Azores high, together with variations in cyclone activity, control the climate of western Europe. The eastward penetration of maritime influences related to these atmospheric processes, and to the warm waters of the North Atlantic current, is illustrated by mild winters, the seasonality of precipitation regimes and indices of continentality. Topographic effects on precipitation, snowfall, length of growing seasons and local winds are particularly marked over the Scandinavian mountains, the Scottish Highlands and the Alps. Weather types in the British Isles may be described in terms of seven basic airflow patterns, the frequency and effects of which vary considerably with season. Recurrent weather spells about a particular date (singularities), such as the tendency for anticyclonic weather in mid-September, have been recognized in Britain, and major seasonal trends in occurrence of airflow regimes can be used to define five natural seasons. Abnormal weather conditions (synoptic anomalies) are associated particularly with blocking anticyclones, which are especially prevalent over Scandinavia and may give rise to cold, dry winters and warm, dry summers.

The climate of North America is similarly affected by pressure systems that generate airmasses of varying seasonal frequency. In winter, the subtropical high-pressure cell extends north over the Great Basin with anticyclonic cP air to the north over Hudson Bay. Major depression belts occur at about 45 to 50°N, from the central USA to the St Lawrence, and along the east coast of Newfoundland. The Arctic front is located over northwest Canada, the polar front lies along the northeast coast of the United States, and between the two a maritime (Arctic) front may occur over Canada. In summer, the frontal zones move north, the Arctic front lying along the north coast of Alaska, Hudson Bay and the St Lawrence being the main locations of depression tracks. Three major North American singularities concern the advent of spring in early March, the midsummer northward displacement of the subtropical high-pressure cell, and the Indian summer of September to October. In western North America, the coast ranges inhibit the eastward spread of precipitation, which may vary greatly locally (e.g. in British Columbia), especially as regards snowfall. The strongly continental interior and east of the continent experiences some moderating effects of Hudson Bay and the Great Lakes in early winter, but with locally significant snow belts. The climate of the east coast is dominated by continental pressure influences. Cold spells are produced by winter outbreaks of high-latitude cA/cP air in the rear of cold fronts. Westerly airflow gives rise to chinook winds in the lee of the Rockies. The major moisture sources of the Gulf of Mexico and the North Pacific produce regions of differing seasonal regime: the winter maximum of the west coast is separated by a transitional intermontane region from the interior, with a general warm season maximum; the northeast has a relatively even seasonal distribution. Moisture gradients, which strongly influence vegetation and soil types, are predominantly east-west in central North America, in contrast to the north-south isotherm pattern.

The semi-arid southwestern United States comes under the complex influence of the Pacific and Bermudas high-pressure cells, having extreme rainfall variations, with winter and summer maxima due mainly to depression and local thunderstorms, respectively. The interior and east coast of the United States is dominated by westerlies in winter and southerly thundery airflows in summer.

The subtropical margin of Europe consists of the Mediterranean region, lying between the belts dominated by the westerlies and the Saharan-Azores high-pressure cells. The collapse of the Azores high-pressure cell in October allows depressions to move and form over the relatively warm Mediterranean Sea, giving well-marked orographic winds (e.g. mistral) and stormy, rainy winters. Spring is an unpredictable season marked by the collapse of the Eurasian high-pressure cell to the north and the strengthening of the Saharan-Azores anticyclone. In summer, the latter gives dry, hot conditions with strong local southerly airstreams (e.g. scirocco). The simple winter rainfall maximum is most characteristic of the eastern and southern Mediterranean, whereas in the north and west, autumn and spring rains become more important. North Africa is dominated by high-pressure conditions. Infrequent rainfall may occur in the north with extratropical systems and to the south with Saharan depressions.

Australian weather is determined largely by travelling anticyclone cells from the southern Indian Ocean and intervening low-pressure troughs and fronts. In the winter months, such frontal troughs give rains in the southeast. The climatic controls in New Zealand are similar to those in southern Australia, but South Island is greatly influenced by depressions in the southern westerlies. Rainfall amounts vary strongly with the relief.

The southern westerlies (30 to 40° to 60 to 70°S) dominate the weather of the Southern Ocean. The strong, mean zonal flow conceals great day-to-day synoptic variability and frequent frontal passages. The persistent low-pressure systems in the Antarctic trough produce the highest year-round zonally averaged global cloudiness.

The Arctic margins have six to nine months of snow cover and extensive areas of permanently frozen ground (permafrost) in the continental interiors, whereas the maritime regions of northern Europe and northern Canada-Alaska have cold, stormy winters and cloudy, milder summers influenced by the passage of depressions. Northeast Siberia has an extreme continental climate.

The Arctic and Antarctic differ markedly because of the types of surface - a perennially ice-covered Arctic Ocean surrounded by land areas and a high Antarctic ice plateau surrounded by the Southern Ocean and thin seasonal sea ice. The Arctic is affected by mid-latitude cyclones from the North Atlantic and in summer from northern Asia. A surface inversion dominates Arctic conditions in winter and year-round over Antarctica. In summer, stratiform cloud blankets the Arctic and temperatures are near 0°C. Subzero temperatures persist year-round on the Antarctic continent and katabatic winds dominate the surface climate. Precipitation amounts are low, except in a few coastal areas, in both polar regions.

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