The polar regions as components of the global climate system
WOR 6 The Arctic and Antarctic - Extreme, Climatically Crucial and In Crisis | 2019

Why it is so cold in the polar regions

Warum es in den Polarregionen so kalt wird © [M] mare, Foto © Jacob Aue Sobol/Magnum Photos/Agentur Focus

Why it is so cold in the polar regions

> The climate in the polar regions is the result of a self-reinforcing process. Because so little solar energy is received, the water freezes to ice, which then, like a mirror, reflects the small amount of radiation that does arrive. A multi-layered, complex wind system, which plays a decisive part in the weather and climate on our planet, is driven by differences in temperature and pressure between the warm and icy regions.

It doesn’t get any colder

According to the World Meteorological Organization (WMO), the coldest place in the world is the Russian Antarctic research station Vostok. It was established in 1957 in the middle of the East Antarctic Ice Sheet, where it lies at an elevation of 3488 metres above sea level. From the station building it is about 1300 kilometres to the geographic South Pole. On 21 July 1983, at the standard measurement height of two metres above the ice, the station meteorologist measured a low temperature of minus 89.2 degrees Celsius – officially the coldest temperature ever directly measured on the Earth.
But at a height of just a few centimetres above the surface of the East Antarctic Ice Sheet the air temperature drops even further. According to satellite data obtained between 2004 and 2016, in a region of the ice sheet further to the south with a higher elevation, near-surface air temperatures can fall to minus 98 degrees Celsius.
2.1 > Elongated ridges of snow on the East Antarctic Ice Sheet. These are called sastrugi, and they are formed when the wind sweeps over the surface in places with somewhat harder snow, carrying loose snow crystals with it and carving aerodynamic elevations or ­furrows in the previously higher snow cover.
fig. 2.1 © Ted Scambos, NSID

Extra Info The levels of the atmosphere

The thermal engine of the Earth’s climate

The singular interplay between sun, ice, humidity and wind is the key to the extremely cold climate in the polar regions. The sun is the primary driving force of weather and climate on the planet. Its radiation warms the continents, the oceans and the atmosphere. The intensity with which the sun’s rays impinge upon the outer boundary of the Earth’s atmosphere has remained fairly constant since satellite measurements began in 2000. But because of the spherical shape of the Earth, not all locations on its surface receive the same amount of solar radiation. Where the rays intersect with the atmosphere at right angles, the light energy has a strength of 1361 watts per square metre (solar constant). Where the solar radiation strikes the Earth’s atmosphere at a much lower angle, as in the polar regions, the incoming solar energy per unit of area is substantially reduced. Moreover, the radiation always falls only on the side of the Earth that is facing towards the sun. Accordingly, the global average solar energy arriving at the upper margin of the atmosphere can be calculated as approximately 340 watts per square metre. The much smaller amount of heat that reaches the polar regions can be illustrated by a simple example: If sunlight falls on the Antarctic continent at an angle of 30 degrees on a cloudless summer day, only half as much energy will arrive there as will fall on the surface near the equator at an angle of 90 degrees.
The major reason for the differences in heat input during the year is the fact that the Earth is spinning like a top in space, and its axis of rotation is not exactly perpendicular to its plane of orbit around the Sun. Instead, it is presently tilted at an angle of 23.4 degrees. If the Earth’s axis were at right angles to its orbit there would be no seasons. Due to its inclination, however, the northern hemisphere faces toward the sun during the northern summer and receives more sunlight, but in the northern winter it is tilted away from the sun and thus receives less radiation. In the southern hemisphere the exact opposite is the case.
2.3 > Ice and snow surfaces in the polar regions reflect up to 90 per cent of the incoming solar radiation back into space, which results in a cooling of the Earth.
fig. 2.3 © NASA/GSFC/Science Photo Library

Weather and climate
For meteorologists, “weather” means the current conditions ­within the lower atmosphere (troposphere) as well as short-term changes at a particular time and place. They use measurements of the temperature, precipitation, wind direction and other parameters to describe these conditions. The term “climate”, on the other hand, refers to the statistics of weather. It uses statistical mean and extreme values to describe the weather patterns at some location over a period of 30 years.

The seasons are more pronounced the further one moves away from the equator, but these are not the only phenomena that can be attributed to the planet’s inclination. It is also the reason why the sun does not set at all in the polar regions at certain times of the year (polar day) and why it remains hidden below the ­horizon (polar night) at others. At the geographic North and South Poles, the polar night lasts for almost a half year. If one moves from the Pole towards the Polar Circle, the length of time during which the sun does not rise ­above the horizon decreases steadily until it is only 24 hours precisely at the Polar Circle.
During its polar night, each polar region is therefore completely cut off from the solar source of heat energy. But even during the polar day, the period of continuous sunlight, only a relatively small amount of solar energy reaches the Arctic or Antarctic regions due to the low angle of incoming rays. These two phenomena thus form the basis for sustained cold conditions in the ­northern and southern polar regions. There are, however, two other important factors: albedo and water vapour.


White reflects

One of these reinforcing or amplifying factors is the reflective capacity of the Earth’s surface, called the albedo effect. It determines how much of the incident solar ­radiation is reflected by the Earth’s surface. As a basic rule of thumb, the darker or rougher a surface is, the less ­radiation it reflects. A freshly ploughed field may reflect about ten per cent of the sun’s energy; green meadows and pastures can account for about 25 per cent. Light-coloured surfaces such as desert sands have an albedo of about 40 per cent, but still do not begin to approach the reflection values of snow and ice. Freshly fallen snow, for example, will reflect up to 90 per cent of the incident solar energy. Depending on its age and surface structure, sea ice can have an albedo of 50 to 70 per cent. This rather large range is partially due to the deposition of dust and soot particles on the ice surface over time, which change its colour, particularly in the Arctic. Ice floes from which the wind has blown away the snow layer also have a different surface texture than floes with a hardened snow surface. Snow-free glacier ice, for example, reflects up to 60 per cent of the radiant energy, but with a fresh snow cover the albedo is greater.
Thus, the general situation for the Arctic and Ant­arc-tic is as follows: In both regions a relatively small amount of solar energy reaches the Earth’s surface for long ­periods of time because of the spherical shape of the Earth and the tilt of its axis. A large proportion of the energy that does arrive impinges on white ice or snow surfaces and is mostly reflected away. As a result, it is not stored as thermal energy in the ground or ocean, and ­therefore does not ­contribute to the warming of the air layers near the land surface or sea. In this way, the high reflectivity of the snow and ice surfaces reinforces cooling in the polar ­regions. This means that more sea ice is formed in response to the increasing cold, which in turn increases the total albedo levels. This then results in even more solar radiation being reflected. Climate researchers refer to such self-amplifying processes as positive feedback.
2.4 > Ocean surface temperatures reflect the strong contrast between the warm equatorial regions and cold polar ­regions. Where the sea and air are warm, greater amounts of water evaporate, and clouds are formed. In colder areas, however, evaporation rates are low.
fig. 2.4 © after NASA/Goddard Space Flight ­Center, Scientific Visualization Studio

Extra Info The Earth’s heat and radiation balance

Water vapour – invisible regulator of heat

A third factor relating to the origin of cold climate at the poles is water vapour. Water is an extremely versatile element of our climate system. It can evaporate, condense and freeze, and it occurs in nature in three ­physical states: as a liquid (water), frozen (ice), and as a gas (water vapour).
This odourless and invisible gas is formed when liquid water evaporates. The Earth’s atmosphere contains around 13 trillion cubic metres of water. This amount represents about 0.001 per cent of the accessible water on the Earth, whereby the largest proportion of water in the atmosphere is in the gaseous state. If all of the water vapour in the atmosphere were to condense and fall to the surface as rain, it would cover the entire globe with a layer of water about 25 millimetres thick. Still, the proportion of water vapour in the air by mass is on average only 0.25 per cent.
However, this average value is misleading because water vapour is distributed very unevenly throughout the atmosphere. Its concentration decreases rapidly with increasing elevation, due in part to the fact that warm air can hold more water vapour than cold air. Accordingly, large amounts of water can be converted into water vapour in warm regions and less in colder regions. In the polar regions, because of the low temperatures, evaporation and water-vapour content in the atmosphere are very low in winter. The water-vapour capacity of the atmosphere increases with every degree Celsius of air temperature. As an example, one cubic metre of air at a temperature of minus 20 degrees Celsius can hold at most 1.1 grams of water vapour. However, if this volume is heated to plus 20 degrees Celsius, it can contain a maximum of 17.2 grams of water vapour.
2.5 > Over the oceans large amounts of ­water are constantly evaporating, especially in the warmer ocean regions. The moisture does not remain in the atmosphere for long, however. Within ten days it falls to the Earth again as precipitation.
fig. 2.5 © after Buchal et al.
The amount of water vapour present in the atmosphere at a given time is commonly referred to as “humidity”. When meteorologists report a condition of high humidity, this means that the air contains a large amount of water vapour. The most common measure used is relative humidity in per cent. Because a given volume of air at a given temperature and pressure can only hold a certain maximum amount of water vapour, we refer to a relative humidity of 100 per cent when this maximum amount is reached.
As a general rule, when water evaporates over the sea or on land, no more than ten days will pass before the water vapour leaves the atmosphere again in the form of precipitation. In contrast to carbon dioxide, which may be retained for several centuries, water vapour leaves the atmosphere rather quickly and it is thus referred to as short-lived. Nevertheless, water vapour is regarded as the most important natural greenhouse gas. Firstly, this is because it occurs in higher concentrations in the atmosphere than carbon dioxide, methane or nitrous oxide (laughing gas). Secondly, it contributes two to three times more to the natural greenhouse effect than does carbon dioxide.
The Earth’s climate, and particularly the climate of the polar regions, is strongly influenced by the presence or absence of water vapour. The atmosphere has to contain water vapour before fog or clouds can form. However, the water vapour only condenses when the air is supersaturated with the gas, i.e. when it contains more water vapour than it can physically retain. This supersaturation occurs when warm humid air masses rise and are cooled, and thus lose their capacity to absorb more water vapour. The gas condenses into small droplets or, in certain circumstances, into small ice particles that waft freely in the air and commonly become visible from the ground as clouds or fog.
There are two ways in which clouds are important for the global climate. The billions of water droplets they contain refract sunlight from above, preventing these rays from striking the Earth’s surface directly. Instead, they are deflected in many different directions. A certain portion even escapes back into space. Ultimately, therefore, less solar radiation reaches the ground than it would if there were no cloud cover. As a consequence, the cloud cover effectively cools the Earth. On the other hand, however, clouds also block the long-wave heat radiation rising from the Earth. They absorb a large portion of this heat radia­tion and release the heat again in all different directions. In this way clouds can also contribute to warming in the atmosphere. Which of the two features is dominant depends upon the type of cloud. Clouds are most commonly differentiated based on their altitude and form. Visibly thick, low-hanging clouds primarily reflect the incoming sunlight and cool the Earth. High thin clouds, on the other hand, let the solar radiation through. They subsequently block the outgoing heat radiation from the Earth and absorb a large portion of the thermal energy. The day-night effect also plays a role. Obviously, a cloudless sky usually means warmer temperatures during the day because the sun’s rays are unobstructed. But it becomes cooler at night with no clouds because the Earth’s absorbed heat energy can be radiated outward again unhindered.

Freeze-dried air

The Arctic and Antarctic are fundamentally different with regard to the influence of clouds. While dense fog and cloud cover are phenomena often observed during the summer in the Arctic – much to the dismay of polar explorers who usually plan their expeditions for the summer – in Antarctica they normally only form in coastal areas. The air above central Antarctica is simply too cold due to the limited amount of solar radiation, and therefore contains too little water vapour for condensation to form a thick cloud cover. Instead, with increasing cold, all of the residual moisture condenses into ice crystals and falls to the ground as a form called diamond dust. The air is thus essentially freeze-dried, which is why Antarctica is con­sidered to be the world’s driest continent.
For comparison: In Germany around 700 litres of precipitation per square metre fall each year. The same amount is also recorded at the weather station on the Antarctic Peninsula. In the coastal area of the Weddell Sea, i.e. near the German Antarctic research station Neumayer III, there are only 300 litres of precipitation per square metre, which is equivalent to a layer of snow about one metre thick. In central Antarctica, on the other hand, annual precipitation rates are less than 50 litres per square metre over vast areas because of the extremely dry air. Only under exceptional conditions have meteorologists reported a thin veil of clouds over the Antarctic Ice Sheet. However, these are not substantial enough to prevent the ice surface from radiating the small amount of incident heat back into space, which leads to further cooling of the air above Antarctica.
In the Arctic, on the other hand, water vapour, clouds and fog can promote warming, especially in summer. One reason for this is the shrinking of the sea-ice cover in the Arctic Ocean during the summer. The white ice floes, ­drifting in the winter and spring and reflecting a large portion of the sun’s radiation, are partially replaced in summer by the much darker sea surface. This absorbs up to 90 per cent of the sun’s energy, which causes a rise in the sea-surface temperature. Because this is accompanied by a corresponding warming of the air, the atmosphere can absorb more moisture. The humidity increases, so that only small soot, dust or salt particles in the air are required for the water vapour to condense and form clouds or fog.
In addition to the fact that clouds can be formed from it, water vapour possesses another property that is significant for the heat balance and weather patterns: It stores heat energy. This heat cannot be detected by a thermometer or felt by humans. Meteorologists therefore refer to it as latent heat. It is sometimes referred to as evaporation heat because its value corresponds precisely to the energy originally required to evaporate the water. What is special about the heat storage of water vapour, however, is that as soon as the vapour condenses into water droplets in the atmosphere, the stored heat from evaporation is released again as condensation energy and warms the surrounding air. In regions with high atmospheric water-vapour content, this effect causes additional warming. In areas with low humidity or little water vapour in the atmosphere, this effect is much less significant.
In some small depressions on the southern slope of the East Antarctic Ice Sheet, the paucity of water vapour is one of the reasons that it can get even colder than it does at the Vostok Research Station. In July and August, the air layer directly above the ice sheet becomes so cold that, according to scientific reckoning, it cannot become any colder. Minus 98 degrees Celsius seems to be the coldest temperature possible on the Earth under natural conditions.
For the air in the depressions to become this cold, a number of conditions must be met. Incoming solar radia­tion has to be absent for several weeks, which can only occur during the polar nights. Furthermore, the air above the snow-covered ice sheet may not contain any water vapour that could give off heat in the case of condensation, or could absorb radiation energy reflected from the snow and then be held in the atmosphere. According to researchers, the air in the region contains so little water vapour in winter that, considered as a water column, its height would be just 0.04 to 0.2 millimetres. Ideally, the water-vapour content has to be less than 0.1 millimetres. Addi­tionally, the wind must be extremely weak and the sky free of clouds for several days.
Under these conditions, the layer of air directly above the snow cools down stepwise. It becomes denser and heavier, slowly flows downslope, and collects in the depressions where researchers have been able to detect it from satellites.

fig. 2.6 © Babak Tafreshi/Science Photo Library

2.6 > In late Antarctic spring, the sun rises above the horizon again and marks the end of polar night in Antarctica. In most of the coastal regions of the southern continent this lasts about two months. The nearer one moves to the South Pole, however, the longer the period of darkness lasts.

Winds – the driving forces of weather

Looking at the polar regions through the eyes of a physi­cist, the Arctic and Antarctic are regions where the lack of solar radiation and the high proportion of heat reflection due to the albedo effect result in temperatures that are lower by far than in other regions of the world. Temperature differences are accompanied by density differences; cold air masses are denser and thus heavier than warmer ones. Cold air sinks and warm air rises. These density­differences and resulting air motions are generated by differences in the atmospheric pressure at different locations. Where air cools down and sinks, a high-pressure area develops near the ground, a phenomenon known in both the central Arctic and the Antarctic as a polar or cold high. In low-pressure areas like the tropics, by contrast, warm air rises.
These atmospheric temperature and pressure differences between the warm tropical and the cold polar regions are the true “weather generators” of the Earth. They drive the large wind and current systems of the Earth and thereby also global air circulation. All of the processes in the atmosphere are geared toward equalizing these temperature differences and pressure contrasts. This means that the warm air masses from the tropics migrate poleward at high altitudes, while the cold air masses from the polar regions flow towards the equator closer to the ground.
If the Earth were not rotating on its axis, the paths of the different air masses on a map might be seen as straight lines both near the ground and at higher altitudes. But because the Earth is turning, every air current travelling from a high-pressure to low-pressure area is diverted to the right in the northern hemisphere and to the left in the southern hemisphere. This effect is caused by the Coriolis force – an apparent force arising from the rotation of the Earth. It affects both air and ocean currents, increases with latitude, and is the reason why, for example, the ­trade winds in the northern hemisphere do not travel in a straight line directly southward towards the equator from the high-pressure area at 30 degrees North. Instead, they are deflected to the right with respect to their flow direction and therefore sweep across Africa and the Atlantic as northeast winds.
2.7 > Atmospheric circulation of the air masses surrounding the globe is so complex that researchers sometimes use this highly simplified model. It shows the six circulation cells and wind systems that are formed by deflected air-mass currents, and that are almost identical in the two hemispheres.
fig. 2.7 © after Hamburger Bildungsserver, Klimawandel
The Coriolis force is also the reason why air high above the equator moving towards the poles is more ­strongly deflected to the right with every metre that it ­travels. At about 30 degrees of latitude this deflection is so great that the air current flows parallel to the lines of ­latitude as high westerly winds, or the subtropical jet stream, and it can no longer perform its actual task of heat balance between the equator and the pole. Instead the air, which has become cooler by this time, descends and flows back toward the equator near the surface as trade winds. This is how the trade-wind circulation, also called Hadley circulation after the British hobby meteorologist George Hadley (1685–1768), is generated between the equator and 30 degrees latitude, both in the northern and southern hemispheres. In simple, idealized descriptions it is referred to as a closed cell.
A very similar circulation pattern, called the polar cell, forms above the northern and southern polar regions. Here, due to cooling, the cold heavy air in the centre descends (high-pressure area) and then flows near the surface toward the polar circle (low-pressure area). Again, the Coriolis force acts so that the air currents become polar easterly winds. On its way towards the Arctic Circle, however, the air warms up enough to rise and return to the pole at high altitude as a ounter-current.
Located between the two homologous systems of the Hadley and polar cells, there is room for a third system, called the Ferrel cell, named after the American meteorologist William Ferrel (1817–1891). In this, the air masses circulate in the opposite direction. This means that the near-surface air here is transported towards the pole and is deflected to the right (in the northern hemisphere), so that the winds blow from the west. This zone is therefore referred to as the prevailing westerlies. Unlike the polar cell or the Hadley cell, however, turbulence in the air masses of this zone produce low-pressure cells, which wander back and forth as waves and cause some degree of instability in the circulation. This instability is due to the large temperature contrast between the tropics and the polar regions, which cannot be balanced directly because of the strong Coriolis force. Instead, nature makes use of high- and low-pressure areas, which, like a paddle wheel, shovel warm air to the north and polar air to the south on the opposite side of the depression. For this reason, what meteorologists typically refer to as weather only occurs in the area of the Ferrel cell. In the other cells, the seasons rather than the weather tend to determine meteorological events.
In laboratory experiments scientists have tested how the atmospheric circulation would be different if the planet rotated faster on its axis. The results indicate that the increased Coriolis force would cause five cells to form on each hemisphere. If the Earth were to rotate much more slowly, there would be only one cell between the pole and equator, in which the air masses would then flow directly from the tropics to the poles and back. This kind of single-cell circulation is found, for example, on Venus.

The protective vortices

The wind and current patterns of atmospheric circulation are of great importance for the polar regions. So far, they have been very reliable in preventing warm air masses from reaching the centres of the Arctic or Antarctic ­regions. In order to understand how the winds protect the polar regions, we have to take a somewhat closer look at the atmosphere in the polar areas.
The air above the polar regions cools down drastically in the autumn and winter (polar night) and descends to the Earth’s surface. Whereas a high-pressure area, the polar high, forms near the surface, higher up at an altitude of eight to ten kilometres an area of depression is created. This can extend to an altitude of up to 50 kilometres, and is known as the stratospheric polar vortex. The air masses of this vortex are surrounded and held together by a strong westerly wind called the polar night jet. This develops because the same principles of current flow apply in the stratosphere as in the underlying troposphere.
That is to say, air always flows from a high-pressure area, in this case the upper-level high-pressure area above the equator, to a low-pressure area, here the upperlevel low-pressure area above the polar regions. However, the air current moving toward the pole is again deflected to the right because of the Earth’s rotation, which for the northern hemisphere results in the creation of a westerly wind. The polar night jet is thus located at an ­altitude above ten kilometres and blows from the west towards the east completely encircling the North Pole. The wind attains its highest velocity at a latitude of about 60 degrees. Here it forms a kind of barrier that isolates the upper-level polar low-pressure area from the air mas-ses coming from the equatorial region, thus preventing the high-altitude warm air masses from advancing further toward the pole.
During the course of the winter, the polar night jet gains strength because, with increased cooling in the ­stratosphere, more of the air masses in the low-pressure area continue to descend enabling more air to flow in, which boosts the wind strength. However, as soon as the first rays of the sun reach the polar region in spring, the air in the low-pressure area warms up. The differences in density and pressure equilibrate and the wind weakens again.
Comparing the stratospheric polar vortex in the Arctic with the one in the Antarctic, it is notable that the wind in the south maintains a more circular path and is significantly stronger than in the high north. On a normal winter day, the winds of the Antarctic polar night jet can achieve velocities of up to 80 metres per second. This is equal to 288 kilometres per hour. By contrast, in the northern hemisphere they blow with an average velocity of only 180 kilometres per hour. In addition, the stratospheric polar vortex over the Antarctic is significantly larger and the temperatures in its interior are lower than those in the north.
The fact that the two stratospheric polar vortices are so different in character is partly due to Rossby waves, which are also referred to in the special literature as planetary waves. These are large masses of air in the troposphere that meander around the globe with the westerlies. This kind of air mass possesses a certain vorticity (spin strength) due to the Coriolis force, which is related to its own rota­tion. The rotational speed of the air mass depends on the geographical latitude along which it is moving, because the Coriolis force becomes stronger with increasing distance from the equator. Air masses forming at higher latitudes basically rotate faster than those at lower latitudes.
If such a rotating mass of air encounters a mountain or high plateau during its meandering, such as the Rocky Mountains in the USA, or the Urals in Russia, or the high ice sheet of Greenland, it is deflected upwards by the obstacle. When this happens the air mass rises and also forces the overlying air masses to higher altitudes. With this ascent, the vorticity of the air mass changes and it is deflected towards the equator. Here the distance to the Earth’s axis is greater than on the original meander path of the air mass. The vorticity of the air mass no longer ­matches the latitude-dependent vorticity at its new location. As a result, the direction of movement of the air mass changes back toward the pole. It overshoots its original geographical latitude in the opposite direction, then turns back again due to the opposing effect. It thus establishes an oscillating pattern.
2.8 > The polar night jet is a band of wind in the stratosphere that feeds off air masses flowing at high altitudes from the equatorial region towards the high northern latitudes. The polar jet stream, on the other hand, meanders through the troposphere, one ­atmospheric level lower.
fig. 2.8 © maribus
A mass of air that was originally located at 50 degrees north latitude may fluctuate back and forth within the ­troposphere between 40 and 60 degrees, its path defining a waving line that snakes around the entire globe. This is the Rossby wave, named after the US American meteoro­logist Carl-Gustaf Arvid Rossby (1898–1957).
Because the Rossby wave also spreads upwards, under certain conditions its effect may extend into the stratosphere and disrupt the polar vortex to such an extent that it is weakened or even collapses completely. If this natural barrier is eliminated, warm air from the mid-latitudes can flow in and lead to a rapid warming of the stratospheric polar region. In the Arctic, scientists observe such a surge and the associated sudden rise of temperature in the stratosphere about every two years. The presence of mountains as well as the stark temperature differences between land and sea surfaces in the northern hemisphere are conducive to the formation of strong planetary waves.
So far, however, researchers have not been able to predict which waves can be destructive to the stratospheric polar vortex or when a surge can be expected. In the ­southern hemisphere, in contrast, there are no extremely high mountains except for the Andes. Moreover, large ­portions of the southern hemisphere are covered by seas, which impede the formation of planetary waves. Since observations began there has only been one case of an abrupt warming of the Antarctic winter stratosphere. That was in September 2002.
2.9 > The two faces of the polar jet stream: When the polar vortex rotates at full strength in the ­stratosphere, the winds in the troposphere blow parallel to the equator and effectively block warm air masses from ad­vancing into the Arctic. But if the polar vortex is weaker, the course of the jet stream begins to meander. As a result, cold polar air penetrates southward over North ­America and ­Siberia and moist, mild air over the North Atlantic ­migrates into the Arctic region.
fig. 2.9 © after Martin Künsting/Alfred-Wegener-Institut


A wall of wind

Besides the stratospheric polar vortex and its associated polar night jet, the polar regions possess another atmospheric protective shield called the tropospheric polar vortex. This also has an accompanying strong wind that is variously referred to as the polar front jet stream, polar jet, or simply the jet stream. Depending on the prevailing ­weather conditions, this belt of wind lies about eight kilometres above sea level between the 40th and 60th degrees of latitude, i.e. above the zone of westerly winds. In the northern hemisphere the wind speeds here can range from 200 to 500 kilometres per hour and they are sustained throughout the year.
As long as the northern jet stream blows around the Arctic at full strength parallel to the equator, it prevents warm, moist air masses from the south from moving into the northern polar region, just as the polar night jet does in the stratosphere. At the same time, it blocks cold Arctic air from advancing into the mid-latitudes. The jet stream is constantly affected by the Rossby waves. These ultimately contribute to our weekly changing weather conditions, as they are sometimes stronger, sometimes weaker and change their position. The high- and low-pressure areas that define our weather form in the peaks or valleys of the waves.
2.10 > Exceptional atmospheric situation: At the end of February 2018 there was an unusual heat wave in the Arctic. The polar vortex split, causing the jet stream to lose strength, and allowing the penetration of warm air far into the Arctic. In the ­Labrador Sea and in the Siberian area of the Arctic Ocean the air temperatures rose to as high as 15 degrees Celsius above normal. At the same time, Central Europe was experiencing spells of extreme cold.
fig. 2.10 © after
In winter, the strength and durability of the jet stream in the troposphere depend in part on the stability of the polar vortex in the overlying stratosphere. Rossby waves, which can destroy the stratospheric polar vortex and ­cause an abrupt warming of the polar stratosphere, also change the jet stream in the troposphere. The wind in the troposphere weakens and assumes a meandering course over the northern hemisphere. The result is that over North America and northern Europe the tropospheric ­vortex expands to the south, and cold polar air can penetrate deeper into North America and Central Europe. Over East Greenland it retreats back to the far north, allowing damp, warm air to migrate into the Arctic region.
In February 2018, for example, the northern hemisphere experienced this kind of exceptional atmospheric situation. At that time, Rossby waves were able to split the polar vortex, which caused a rapid warming of the stratosphere above the Arctic region to temperatures as high as 50 degrees Celsius. This, in turn, caused a wea­kening of the polar front jet stream in the underlying troposphere, which had far-reaching impacts. While Central Europe suffered from extreme cold in February, with snowfall even in Rome, mild spring temperatures were predominant in the Arctic in spite of the polar night. In Siberia the temperatures at times reached values up to 35 degrees Celsius above the normal average for February. The weather ­station at Cape Morris Jesup, the northernmost point of Greenland, recorded ten winter days in a row in which the temperature did not drop below the ­freezing point. And off the west coast of Alaska, one-third of the sea ice that is normally present at this time of year melted within a period of eight days.

fig. 2.12: [M] mare, photo © Jacob Aue Sobol/Magnum Photos/Agentur Focus;

2.12 > A Piteraq blows through the Greenland settlement of Tiniteqilaaq. Gusts of this katabatic wind can reach speeds of up to 300 kilometres per hour.

Home of the blizzards

Antarctica is not only the coldest continent in the world, it also tops the list of windiest regions. At the French research station Dumont d’Urville, for example, scientists recorded a peak wind speed of 327 kilometres per hour in July 1972. This is more than double the strength of hurricane winds. A hurricane is defined as winds with average speeds of 120 kilometres per hour or more.
The documentation of such high wind speeds in the coastal region of Antarctica is not a coincidence. Apart from the global wind systems, the icy continent produces its own local wind system, which forces researchers to remain confined inside their stations, especially in winter, and which is largely responsible for the formation of sea ice in the Southern Ocean.
Winds normally develop when air masses flow from a high-pressure area into a low-pressure area to compensate for the difference in pressure. However, an air mass can also begin to move due to its own weight – for example, when it becomes colder and heavier than the surrounding air masses and sinks as a result. The near-surface air layer above the Antarctic Ice Sheet is particularly dense and heavy due to its altitude, low solar radiation input and high radiational cooling from the ice. The cooled air masses form a heavy 300-metre-thick layer above the ­central ice sheet. Because the ice sheet does not have a flat surface, but falls away at the edges, this extremely cold and heavy air from Central Antarctica at some point begins to slide down the slopes toward the coasts. It gains velo­city exclusively through its own weight and the steepness of the slope.
2.11 > The French ­research station Dumont d’Urville on Pétrel Island in Adélie Land is one of the windiest places in ­Antarctica. Particularly in winter, air masses that have cooled over the East Antarctic Ice Sheet gush down onto the sea as descending (katabatic) winds.
fig. 2.11 © Yann Arthus-Bertrand/Getty Images

Polar scientists refer to “whiteout” as the condition where fog, clouds or a snowstorm restrict visibility to such an extent that neither contours nor the horizon can be recognized in any direction.

However, the cold rushing air really begins to gain speed when its path toward the coast is partially blocked by mountains. When this happens the total air mass has to squeeze through narrow valleys, which accelerates the air current enormously. In extreme cases, when these winds, called katabatic winds, reach the coast they can attain storm or even hurricane strengths. This technical term derives from the Greek prefix kata, which means “des­cending” or “downwards”.
Polar researchers report that katabatic winds can arise unexpectedly from nowhere. At one moment the working conditions on a glacier or ice shelf can be totally windless and five minutes later, with no warning, a hurricane can sweep across the ice and lead to a condition known as whiteout.
Just as abruptly, however, the wind can again subside – depending on the area of the ice surface above which the cold air formed. In extreme cases, however, this kind of wind can persist for several days with sustained high velocities.
2.13 > (1) Katabatic winds form when near-surface air above an ice sheet cools and thus becomes denser and heavier. (2) The air mass then slides down the slope due to its own weight, (3) gains speed along its way through narrow valleys, and (4) pushes loose ice floes off the coast into the sea, where the air mass is deflected and slowed by coastal winds.
fig. 2.13 © maribus
These exceptional winds occur most notably in the coastal region of Adélie Land, the windiest part of ­Antarctica and location of the French research station ­Dumont d’Urville. This is due to the topography of the region. Here the cold air from a large area of Eastern Antarctica flows down the ice sheet, and newly formed cold air masses can flow down at any time, especially ­during the winter. In addition, mountains channel the air currents through narrow valleys, which amplifies their strength each time.
Katabatic winds, incidentally, also occur in regions outside Antarctica – for example, at the margins of the vast Greenland Ice Sheet, where the near-surface air layer ­above the high plateaus is cooled to a temperature between minus 20 and minus 40 degrees Celsius in winter. Greenland’s strongest winds occur on the southeast coast, in the region near the town of Tasiilaq. Storm winds can gust at speeds up to 300 kilometres per hour, and because of the great danger they are called “piteraq” by the ­natives, which in their local language means „that which attacks you“.
On 27 April 2013 a wind like this not only blew snow from large areas of the ice sheet. Blasting through the 85-kilometre-long Sermilik Fjord, the piteraq also pushed all of the sea ice and glacier ice floating in the fjord out into the sea, so that the fjord was virtually ice-free after the storm. Textende