We addressed the dynamic impacts of atmospheric circulation in the previous article, where numerous elements such as thermal radiation, air pressure, and various forces intrinsically tend to affect the general pattern of circulation, resulting in the formation of secondary and third-order circulation. Furthermore, we have explored how the climatic relevance of atmospheric disturbances impacts different latitudinal regions; i.e., in temperate regions, the general circulation and pressure systems are superimposed with multiple wave cyclones and vortices; whereas in tropical regions, which were assumed to be uneventful and repetitive, it is not so; rather, a complex atmospheric disturbance often disrupts the general pattern that is not only limited in number but also in the area as well. There was also a brief discussion about the phenomena of cyclones and anticyclones. Furthermore, the nomenclature of tropical cyclones was also a part of our discussion, with a special emphasis on the North Indian Ocean.
As previously stated, the Earth’s atmosphere is a unique and complex system that is constantly interacting with a variety of forces. So, in temperate zones, the region is frequently depicted with highly variable and overcast weather, which is a direct result of travelling cyclones. The terms “extratropical cyclones,” “temperate cyclones,” or “depressions” are all used interchangeably to describe these moving cyclones in the mid-latitude zone. Because the mid-latitudes represent a point of convergence for contrasting air masses, it is there that the cyclones and anticyclones traverse with varying regularity with the prevailing westerly winds. As centres for converging and rising air, these moving cyclones produce cloudiness and precipitation, which further causes change in temperature and air pressure. Hence, this article will focus on the development of extratropical cyclones, their structure and movement in the regions, and its origin and related theories.
Extratropical Cyclones: Development, Structure, and Movement
In both hemispheres, extratropical cyclones develop in areas between 30o and 65o north and south latitudes. The polar and tropical air masses collide in these latitude zones, forming what are known as polar fronts. On these fronts, most of these cyclones arise as a result of a wavelike twist or disruption. Hence, there is a great degree of variation in the shape and size of middle-latitude cyclonic storms. No temperate cyclone is ever exactly like any other. Generally, the isobars are almost circular or elliptical. However, in certain depressions, the isobars take the shape of the letter V, which is often termed “V-shaped depressions.” At times, the cyclones become so broad and shallow that they are referred to as troughs of low pressure. There are occasions when these storms become greatly elongated and gradually lose some of the common characteristics of an ordinary temperate cyclone.
In general, V-shaped depressions are oval in shape, with one part being wider than the other. The long axis of this sort of depression runs southeast to northeast, with the larger part of the depression facing north. The northwest to southeast axis is positioned on the short axis. The long axis is frequently twice as long as the short axis. The size, severity, and other characteristics of these storms vary, as do their speed, wind strength, and also the amount and type of cloud cover. The diameter of temperate cyclones can range from 160 kilometres to 3,200 kilometres, but most cyclones have a diameter of 300 to 1500 kilometres. Extratropical cyclones in the United States of America have an average length of 1600 kilometres. The average length of extra tropical cyclones in the United States of America is about 1600 km. The vertical extent of an average-sized cyclone is estimated to be 10 to 12 km.
A cyclone’s air pressure is lowest in the centre and rises as it moves outward. From one storm to the next, the pressure at the centre varies. At its centre, a powerful cyclonic circulation can have pressures as low as 940 to 930 mb, while a mild cyclonic storm can have pressures around 1000 mb. The pressure differential between a low’s centre and its outside boundary might range from 10 to 20 millibars. This pressure difference could be as much as 35 mb in a very large and intense cyclone.
If we look onto to the general pattern of cyclonic distribution, it shows movement of temperate cyclones is from west to east with frequent trends towards southeast to north-east. In other words, the mid-latitude cyclones are subjected to the general westerly flow of the atmosphere in the temperate zone. Even though there is no definite path which most of the cyclones follow, it is at least true that there are certain tracks which are most commonly followed. The heavy concentration of storm tracks in the vicinity of the Aleutian and Icelandic lows is the most important feature of the world distribution of the paths followed by mid-latitude cyclones. However, each individual storm follows its independent path. These storm tracks, as they are called, vary with the changing season. Like the latitudinal shifting of the wind and pressure belts, there is a definite seasonal shifting of the paths of the cyclones. Because these opposing air masses have greater contrasts during the winter months, the frequency and intensity of winter cyclones forming in the middle latitude zone increase. This explains why the weather in the temperate zone is more changeable in the winter than in the summer when the entire atmospheric circulation slows down.
Cyclones can travel up to 1000 kilometres each day on average. Individual storms move at speeds ranging from 500 to 2000 kilometres per day, with cyclones always moving towards higher latitudes. Most temperate cyclones that form in the north Pacific off the eastern coast of Asia proceed northward towards the Gulf of Alaska, where they join the Aleutian low. Winter storms in the Pacific take a more southerly route, reaching as far south as Southern California. When the Pacific western disturbances reach the windward slope of the Rockies, they dissipate. However, some of these storms regenerate on the eastern side of these high mountain chains. The most favourite areas for the rejuvenation of winter storms are Colorado and Alberta. Cyclones that form in Canada migrate southerly towards the Great Lakes region and then turn towards the northeast and move out into the Atlantic Ocean. The stormiest region in North America is the Great Lakes region. A few storms that originate in North America have matured to the point that they can travel over the Atlantic to Europe. There are several high-intensity storms that move across the British Isles and enter Russia. In the southern hemisphere, around the Antarctic ice pack in the southern hemisphere, the Atlantic frontal zone and associated storms occur all year.
Extratropical Cyclone: Theories
Weather scientists were driven to investigate and research the fickleness and variety of weather characteristics created by air disturbances travelling from west to east in the belt of predominant westerlies. Fitzroy was a pioneer in the topic, explaining in 1863 that most extratropical cyclones are caused by the collision of air masses with varying physical properties. His research found that warm and humid air currents originated in the subtropical regions, whereas cold and dry air currents originated in the polar and Arctic regions. It should be noted that the concept of air masses playing a role in the creation of temperate depressions was fresh at the time, and it served as a platform for future research into the causes of their formation and structure.
The Norwegian meteorologists V. Bjerknes and his son J. Bjerknes worked together towards the end of World War I. Bjerknes and colleagues studied the structure of a number of cyclones over Europe by collecting systematic synoptic observations. Their work resulted in significant improvements in our understanding of extratropical cyclones. Their explorations and research led to the development of the polar front theory of cyclones, sometimes known as the Bergen theory of cyclone formation or simply wave theory. The application of this theory to the examination of weather maps or synoptic charts by Tor Bergeron, a renowned Swedish weather scientist, increased its popularity. Even if this theory might explain cyclonic development adequately, more thorough theories for cyclonic development, such as the Baroclinic wave theory, have recently been proposed. As more data from satellite images of the middle and upper troposphere became available, certain changes to the polar front theory were essential for modifications. However, the Bjerknes model is still widely used in the interpretation of mid-latitude cyclones today.
Polar Front Theory
Bjerknes and other Norwegian meteorologists devised a theory that adequately explains the origin and development of a middle-latitude cyclone. This theory recognises that the polar front, which separates polar and tropical air masses, causes cyclonic disturbances to strengthen and proceed along the front, following a fairly predictable life cycle. Cyclones, according to Bjerknes, form along a front where polar and tropical air masses with contrasting physical properties (temperature and density) are moving parallel to it in opposite directions. It should be noted, however, that the middle latitudes constitute a convergence zone, and it is here that opposing air masses such as cold polar air and warm tropical or subtropical air generally collide. It’s worth noting that the polar front isn’t a permanent boundary. A polar front may appear as multiple lines on a synoptic chart. Another characteristic feature of the polar front is the fact that the changing seasons, as well as positioning of the jet streams aloft, bring about a marked north-south shift in its location. Any unstable wave that forms along these fronts can evolve into a full-fledged cyclone if the conditions are favourable.
Extratropical cyclones, according to wave theory, form at a wavelike twist or perturbation on the front. There are two sorts of waves that can result from this process: stable waves and unstable waves. The weather has no obvious effect on the stable waves as they build and disappear. In the case of unstable waves, they increase in amplitude and cycle. As a result, when a frontal surface is warped into a wave-shaped discontinuity, cyclogenesis (cyclone formation) happens.
On an unperturbed front, there are a number of conditions that can cause an unstable wave to form. Mountains, temperature differences between land and sea, ocean current differences, or adjacent disturbances can all impact air movement and create a wave. A cyclone system can be generated by the flow aloft on rare occasions. Surface cyclonic activity has been observed to be amplified by meandering air streams in the upper troposphere. Surface cyclones occur invariably ahead of an upper-air trough.
According to polar front theory, a cyclone-forming waveform along the front when cold polar air is deflected equatorward and warm tropical air is deflected poleward. The resulting wave is split into two sections. The eastern part of the wave where the warm air advancing towards the east ascends over a wedge of cold air mass is called the warm front. The cold front is the western part of the discontinuity, where cold polar air replaces warm air by under-running the warm and lighter tropical air mass. The frontal wave’s amplitude is increased by the curbing motion imparted by the Coriolis force when the conditions are favourable. In addition, the curvature has been enhanced. The depression has now progressed to a fully-fledged state, with two separate warm and cold air sectors.
Condensation, cold formation, and precipitation result from the ascent of warm air along the warm front as the wave develops. Because the cold front moves quicker than the warm front, the warm air sector shrinks. The process continues, and the slow-moving warm front is eventually overtaken by the cold front. The occlusion procedure now begins. Regardless of whether the occlusion is warm-front or cold-front, the warm-air sector is lifted aloft, and cold air behind the cold front now meets cold air ahead of the warm front. The cyclone is believed to have gained maturity at this vital stage. The cyclone’s supply of energy has been cut off due to the occlusion of the warm sector.
The centre of occlusion becomes filled with cold air from all sides. The vertical motion comes to a halt, and the surface pressure rises all around. It’s worth noting that combining warm and cold fronts results in a protracted backward-swinging front. An occluded front is a term for this situation. When the occlusion process begins, a storm is on the verge of dying. Thus, continued occlusion hastens the dissipation of the original depression. In the end, the cyclone is just a weak vortex of air that is very uniform.
Baroclinic Wave Theory
The frontal theory of cyclones has proven to be beneficial in forecasting cyclonic storm formation and movement. But this theory is not in conformity with a mathematical model for the origin and development of a wave cyclone on a sloping frontal surface. To put it another way, it has no mathematical basis for describing the initial phases of cyclone formation.
Baroclinic Wave Theory is based on the notion that extratropical cyclones can form even if there is no pre-existing front between the polar and tropical air masses. It also considers cyclones and anticyclones to be an important component of the overall atmospheric circulation. The most notable aspect of this theory is that it was developed using mathematical approaches and numerical weather forecasting analyses.
It is necessary to define the term “Baroclinicity” before analysing the key elements of the Baroclinic wave theory of cyclones. The state of stratification in the atmosphere when surfaces of constant pressure intersect surfaces of constant density is known as Baroclinicity (which depends mostly on temperature). Barotrophy, on the other hand, is a type of stratification in the atmosphere characterised by parallel surfaces of constant pressure and density. Those conditions represent atmospheric factors that influence stability or instability. Another word for this type of overall stability of the atmosphere in terms of cyclone formation is ‘hydrodynamic stability.’
According to the Baroclinic theory, baroclinic instability causes cyclones and anticyclones to form in the temperate zone. The kinetic energy of the eddies is transformed from the potential energy of the zonal flow. A frontal zone in the atmosphere represents Baroclinicity. The temperature of the troposphere decreases continuously from the equator to the poles. The meridional temperature gradient continues to rise, causing the zonal flow to become unstable. The flow is broken down into a number of cyclonic and anticyclonic circulations at a given point. It should be noted that the wind and temperature gradients at altitudes differ from those at ground level. We also know that the higher airflow is ridged and troughed due to the meandering jet streams. Warm air is drawn towards the pole by a ridge of high pressure. Low-pressure troughs, on the other hand, allow cold air to migrate closer to the equator. As a result, the upper westerly wave flow serves as a crucial mechanism for the development of cyclonic storms, which then re-distribute energy. The cyclones and anticyclones affect the heat transfer that occurs across the temperature zone. The presence of waves and eddies in the global flow pattern is necessary for maintaining latitudinal thermal balance. The pressure patterns at the surface are mostly determined by the wavy pattern aloft.
According to the Baroclinic theory, north-south wind velocity perturbations are accompanied by vertical velocities in ascending warm and moist air currents and descending cold and dense air currents. Rising air currents cause inflowing air currents from adjoining places. Because of the action of Coriolis force, converging air currents generate a cyclonic spin. The rotational velocity momentum of the air increases as it approaches the axis of rotation. There is a counter-clockwise circulation around the centre of the northern hemisphere. The rise of air becomes more violent as the cyclone intensifies. As a result, condensation and the release of latent heat increases. The cyclone’s ongoing intensifications result in a proportionate drop in pressure in the cyclone.
To summarise, mathematical techniques were used to build this theory. The north-south temperature gradient, according to this theory, makes the upper airflow in the middle latitudes unstable. The airflow becomes wavy, and under certain conditions, it splits into cyclones and anticyclones. The largest heat exchange in the mid-latitude region is enabled by these atmospheric disturbances. According to this theory, cyclones and anticyclones are non-frontal in origin and can be considered part of the general circulatory pattern.
Extratropical Cyclone: Life Cycle
The lifecycle of this travelling wave cyclone has four stages of development: the Initial Stage, the Incipient Stage, the Mature Stage, and the Occlusion Stage.
The polar and tropical air currents on opposite sides of the polar front blow parallel to the isobars and the front in the initial stages. The flow of air in the cold air mass to the north of the polar front is from east to west. As a result, the wave disturbance is created, and the front is quasi-stationary and perfectly balanced. Underneath the warm air is a wedge of the cold air mass. The wind shift is completely absent, and the weather is pleasant. However, there is an abrupt change in wind direction over the slanting surface of discontinuity where the opposing air currents meet. This is known as wind shear.
On the front, a wave has formed in the second stage. Cold air is pushed to the southward, while warm air is pushed to the northward. Each air mass has begun to encroach on the domain of the other. As a result of this, the pressure field is readjusted, and isobars take on a nearly circular shape. Now, at the apex of the wave, a cyclonic circulation forms around a low centre. The whole cyclonic vortex is carried along with the winds prevailing in the warm air region at approximately the speed of the geostrophic component of the wind. It’s worth noting that the new depression forming at the crest of the wave is known as a nascent cyclone, and the process of this development is called cyclogenesis.
The cyclone’s intensity rises in the third stage. The wave’s curvature and amplitude both undergo a marked increase. The air in the warm sector begins to travel from the southwest to the southeast, where it meets the cold air. The cyclone has now reached its maximum strength and the warm and cold zones are clearly delineated. The warm air goes faster than the cold air at this temperature. The direction of movement is perpendicular to the warm front. In fact, the warm air is moving into an area that was previously inhabited by cold air. At the rear of the cyclone, cold polar air is under running the air of the warm sector, thus a cold front is generated there. Each of these fronts is convex in the direction of its movement.
There is ascending air along the entire surface of discontinuity throughout the cyclone. Cloudiness and precipitation will occur along warm and cold fronts if the ascending air mass is moist. The warm front precipitation is more extensive and consistent, whereas the cold front precipitation is confined to a narrow zone. Because the cold front advances quicker than the warm front, the warm sector becomes increasingly small. This is when occlusion begins. This particular occurrence signifies the cyclone’s maturity. This is, without a doubt, the most intense period.
The advancing cold front now overtakes the warm front in the final stage, resulting in the formation of an occluded front. Occlusion begins at the wave’s apex, where the warm front is closest to the cold front. Gradually, the occlusion process reaches the more open portion of the two fronts. As a result, the warm sector is gradually pinched off, and the two cold air masses eventually merge across the front. This removes the occluded front, and now, the cyclone dies out. A single frontal cyclone has a lifespan of around five to seven days.
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- D. S. – Climatology
- Singh Savinder, Climatology