Creation of a monsoon
The emergence of a monsoon is shaped by a large number of influencing factors, their composition and thus the severity and strength of a monsoon phenomenon being site-specific. However, there are factors that are common to all regional monsoons or at least unspecific enough to explain the basis for the development of a monsoon phenomenon. The main goal is to find criteria which can be applied equally to all monsoon phenomena and which can be used for their classification - as for the definition of a monsoon in general. The criteria established on this basis are called monsoon criteria .
Summary
Seasonal, large wind direction changes ( monsoon angle criterion) arise initially because of the shift of the intra-tropical convergence zone (ITC - inter tropic conversion ), a low pressure channel that is created by the warming and the rise of the air near the equator . The comparatively low pressure of the ITC attracts air and creates winds, the trade winds . The intra-tropical convergence zone follows with a slight delay the migration of the zenith of the sun between the tropics caused by the inclination of the earth's axis . The ITC, in the case of a monsoon phenomenon influenced by a continental soil depth, which are also called monsoon low designated and by the strong heating of the over the continents located air masses caused. The reason for the stronger warming of the air over the continents is the different thermal properties of the land and sea surfaces . The warming, but also the cooling of the land surface occurs about two to three times as fast as that of the water surface.
The Indus plain and the Tibetan plateau , for example, form the core areas of ITC influence by a monsoon low ( Lit .: Weischet 2002). Due to this influence on the ITC, however, the Passats are also shifting. The winds in the northern hemisphere receive a western component due to the Coriolis force deflecting to the right in the direction of movement and the southwest monsoon (actually southwest monsoon wind) is created. In the southern hemisphere , the trade wind is deflected in the opposite direction to the left, i.e. also to the west, to a northeast monsoon (actually northeast monsoon wind) ( Lit .: Borchert 1993).
On its way from the ocean to the continent, the monsoon wind absorbs moisture over the surface of the water and rains it down to a large extent on the windward side of weather divisions such as the Himalayas . The summer monsoon in these regions is therefore characterized by very humid conditions, which can take on the character of a rainy season and usually do so when the monsoons are fully developed ( monsoon rains ).
In the respective winter, however, high pressure areas develop over the continents. The ITC then shifts again towards the equator or crosses it towards the other hemisphere. As a result, the northeast trade wind in the northern hemisphere and the southeast trade wind in the southern hemisphere are the dominant winds. These are also known as the winter monsoons and carry dry, continental air masses with them. They are therefore usually expressed in a pronounced dry season .
Basics
Thermal behavior of surfaces
Monsoon phenomena are mainly caused by the uneven heating of the surfaces of continents and oceans . Understanding how and why these surfaces heat up differently is therefore also a basic prerequisite for understanding a monsoon phenomenon in general.
All surfaces within a limited area, ie in particular without the effect of shadows , clouds and sun to be considered are from the sun equally irradiated , so also get the same amount of solar energy . This amount of radiation is also known as global radiation . It turns out, however, that different surfaces nevertheless have different temperatures, whereby these temperature differences mostly continue into the depth. With high global radiation, dark surfaces have higher temperatures than light surfaces with otherwise identical properties, which is due to the lower albedo of dark surfaces.
This different heating becomes even clearer with surfaces of different chemical composition. In contrast to water surfaces, sandy beaches , deserts and, as an extreme, metallic surfaces such as lead alloys show a very high surface temperature. These are all ordinary to very good heat conductors , but with a comparatively low specific heat capacity (for lead 129 J / (kg · K)). This means that these materials heat up relatively strongly due to low energies. However, this not only requires rapid heating, but also rapid cooling of such surfaces, as they cannot store a lot of energy and therefore react very quickly to changing ambient temperatures even with smaller heat flows. With 4187 J / (kg · K), water has a comparatively very high specific heat capacity and can therefore store or release a lot of energy even with a comparatively small change in temperature .
With a coefficient of thermal conductivity of 0.6 W / (m · K), water is only a poor conductor of heat , comparable to glass , for example . The energy absorbed on the surface of the water during the day is therefore not immediately "swallowed" by the depths of the water. You can understand this effect by trying to dive in a lake or unheated outdoor pool . You can quickly see that as long as the sun shines on the surface of the water, it is also the warmest and shortly below it, usually only a few decimeters, much colder layers of water appear.
But just as a lake or, analogously, an ocean heats up from above, it also cools down from above. In autumn , therefore, waters usually show a significantly higher temperature than their surroundings, and the more so the longer they have been able to slowly heat up undisturbed in summer from top to bottom, although this is by no means linear and the density anomaly of the water plays a major role here plays.
In summary, water can be viewed as a kind of natural air conditioning system. They always tend to compensate for temperature extremes and, for example, temperatures are much milder on the north German coast than in southern Germany . This applies both in the daytime course between day and night and in the annual course between summer and winter and is due to the influence of the maritime climate .
Due to their sheer diversity, it is not possible to give generally valid characteristic values for land surfaces, but it is true that the specific heat capacity is much smaller than that of water surfaces and the thermal conductivity is also rather lower, which, however, depends much more on the type and coverage of the soil . The more vegetation there is, the smaller the difference to water surfaces, since living things consist to a large extent of water.
As a result of the different surface temperatures, there is also a difference in the temperatures of the air masses above. This is primarily caused by the temperature gradient and the natural convection it causes , but also by the relatively long-wave thermal radiation emanating from the surface . The decisive factor for the former is the surface's heat transfer coefficient, which is determined by various influences .
Weather dynamics
Pressure gradient force and circulation systems
If the thermal surface properties of continents and oceans presented in the section above are transferred to very large water and land surfaces, significant temperature differences can arise in the air masses heated or cooled over these surfaces, even on a global scale .
This is illustrated in the illustration on the right for the case of the summer monsoon. Since the air masses over the continent heat up much faster in summer, i.e. with high irradiance , than over the ocean, the air particles accelerated by the thermal energy are much better able to counteract gravity than the slower particles overhead the ocean. In this way, the pressure gradient is reduced over the continent and a strong thermal bottom is formed. The air pressure at the bottom of the continent is therefore lower than above the surface of the ocean, but it also drops less quickly. Viewed relative to the continent, this requires a thermal high above the ocean, an altitude high above the continent and, accordingly, a high altitude low above the ocean.
Since wind or air always flows from the location of the higher to the location of the lower pressure, i.e. from high to low, a cycle typical for this dynamic develops. The force that triggers this circulation is called the pressure gradient force (figure on the right) . It is caused by horizontal and vertical pressure differences, with the horizontal component of the resulting wind vector playing the main role. At the thermal low, or also at the bottom, the air begins to rise ( convection ) and creates the pressure difference, which now allows the air to flow horizontally from the bottom to the bottom ( convergence ). This ground wind is actually the wind that can be felt by humans and has been given a wide variety of names depending on the type and region . When the air rises from the ground low to high above the continent, the temperature of the air is lowered further and further, which at some point leads to falling below the dew point , i.e. to condensation and cloud formation ( Joule-Thomson effect ). The resulting precipitation and the cloud cover , which shields against sun rays , make the depth of the ground above the continent appear as a bad weather phenomenon. Since the air that has risen cannot accumulate at the high altitude and a pressure gradient force acts here as well in the direction of the low altitude, a wind now flows back to the ocean in the opposite direction to the ground wind and sinks there again, thus completing the cycle.
Hadley cell, the intertropical convergence zone and trade winds
The so-called Hadley cell represents an important cycle of this type . It is also caused by different heating, but not due to the different surface properties, but due to the latitude of the solar radiation , caused by the different angles of incidence of the sun's rays . This provides, together with the intertropical convergence zone (ITC - intertropical convergence ), a trough of extremely stable and strong low-pressure systems and part of the Hadley cell, the basis for the expression of a monsoon represents the surface wind does not move while in the Hadley cell. along the pressure gradient directly from the subtropical high pressure belt to the equatorial low pressure trough, but is deflected by the Coriolis force . There are therefore no north or south winds, but north-east or south-east winds, which are known as trade winds .
Other influencing factors
In addition to the pressure gradient force, one also has to take into account morphological factors (land-sea distribution, mountains as weather divisions), wind friction , centrifugal force and above all the Coriolis force in order to obtain a more realistic, but still quite idealized picture of the ultimate weather dynamics of a monsoon . Both the Coriolis and the centrifugal force are pseudo-forces that do not exist for the observer who is not in motion. With centrifugal force, only the rotational movement is decisive, which is why it has no special meaning for monsoons as such.
The Coriolis force, on the other hand, acts on all moving bodies that do not move parallel to the earth's axis and allows them to be distracted from the view of a co-rotating observer. Since this is the case for all observers in the rotation system of the earth, the origin of this deflection is subjectively ascribed to a force , precisely the Coriolis force. This increases in its horizontal component with increasing latitude in strength. It is therefore at its maximum at the poles and, with decreasing distance from the equator, shows an ever smaller expression until it is finally zero at the equator itself. Each wind on the northern hemisphere is by the Coriolis force in the direction of movement to the right, at any wind on the southern hemisphere deflected in the direction of movement to the left. This is crucial, since only in this way can a wind flow with east, west or even earth axis-parallel components, i.e. also the trade winds and jet streams , be explained in apparent contradiction to the pressure gradient force.
In the land-sea distribution, there is a close connection between the strength of the monsoons and the north-south distribution of land masses and oceans. The strongest monsoons occur with a pronounced distribution of this type, as this shows the effect of the sun's moving zenith position in relation to the different thermal surface properties.
Bringing all the factors together
Up to this point it is not generally a really monsoon-specific development, since this does not require the creation of a stable pressure system, but, as already stated in the classification, a relatively stable wind phenomenon that changes in its main wind direction every six months must change a certain number of degrees. The main driving force here, as has also already been stated, is solar radiation and the resulting Hadley cell with its trade winds.
Idealized thought experiment
To illustrate the emergence of a monsoon, must first be a theoretical ideal case, the atmospheric circulation as a thought experiment to be based to thereto later show the difference to the real situation of a monsoon.
In this thought experiment there are no different surfaces on earth. With the same energy input, it would heat up to the same extent everywhere. The solar energy required for warming is not constant when the earth revolves around the sun (see article Seasons ) and the zenith of the sun therefore fluctuates between the tropics over the course of the year . Consequently, in this theoretical case too, weather fluctuates over the course of the year. This is expressed, for example, in the calms migrating along the lines of longitude . These are the windless zones in the area of the trade winds that were once feared in sailing shipping, i.e. around the center of the ITC. In the course of the year, the ITC would deviate equally north or south from the equator on all longitudes, but only to a very limited extent, since the necessary maximum radiation power would be very close to the equator. However, it must also be taken into account that the ITC only develops with a certain degree of inertia and only follows the zenith of the sun at intervals of about one month.
The monsoon
If one observes the real course of the ITC on a global level (see the web links), it becomes apparent that it moves in the direction of major continents. The ITC is therefore by no means evenly distributed over the longitudes or it is not located on a single geographical latitude, but sometimes fluctuates quite strongly by up to 30 degrees of latitude to the north and south. It should also be noted that the degree of this distortion by the respective continent depends on whether it is summer or winter on this continent. Provided the solar radiation on the respective continent is large enough at the time of viewing, a stable and strong ground depth is formed here.
If the ITC were to remain close to the equator, two large convergence zones would have to exist between the tropics. However, only a large convergence zone can develop, since only the relative, but not the absolute, warming of air layers plays a role for the convergence. As soon as the air layers over the continent are heated so much that the air pressure here drops below the air pressure over the ocean, the ITC will automatically move in the direction of the continent and thus incorporate the thermal depth above the continent into the low pressure trough of the ITC. The greater this pressure difference, the faster and more extensive the deflection of the ITC to the north or south. However, together with the ITC, its wind system, the Passat circulation , is also shifting . Strong ground winds flow from the subequatorial high pressure belts in the direction of the ITC from both the north and the south and are thereby deflected by the Coriolis force. This creates the Northeast Passat north of the ITC and the Southeast Passat south of the ITC.
In the following, one must always differentiate between the respective hemisphere due to the position of the continent relative to the ITC . In the summer of the northern hemisphere, i.e. when the respective land mass warms up, this is part of or below the ITC. As a result, the south-east trade wind crosses the equator to the north and is converted by the Coriolis force into a wind with an east component, the south-west monsoon, which blows from the ocean to the continent. In winter, however, due to the reversal of the circulation system, the land mass lies north of the ITC and a north-east trade wind occurs from the continent to the ocean, which is identical to the winter monsoon. In the southern hemisphere, the relative position is exactly the opposite, therefore a northwest monsoon occurs in the southern summer (southern summer) and a southwest monsoon in the southern winter (southern winter). The latter is identical to the SO Passat. However, it also shows that the summer monsoon of one hemisphere can feed the winter monsoon of the other hemisphere and therefore, for example, the Indian and North Australian monsoons are directly linked to each other (Indian-North Australian monsoon system).
It should also be noted here that the relative location, size and orography of the continent in relation to the ocean have different regional characteristics, which ultimately shape the decisive interplay of air pressure regimes. This results in a non-generalizable east-west component of the monsoon winds and also a different size monsoon angle. For the ultimately locally perceptible strength and form of a monsoon phenomenon, regional factors are therefore usually of far greater importance than the dynamics of a monsoon in its entirety. It is therefore better to speak of a summer or winter monsoon, for example, than of a southwest or northeast monsoon, as these terms are more generally applicable.
The transition times between the two dominant monsoon winds are referred to as the pre- monsoon season and the post-monsoon season relative to the summer monsoon . Depending on the regional conditions, it is also possible to define these as separate seasons. These names result from the fact that the summer monsoon is often called the monsoon for short , due to the associated monsoon rain of the solstitial rain type .
Dynamic monsoon theory
The dynamic monsoon theory states that the strong interannual fluctuations in precipitation are not caused by an annual process alone. They are said to have their cause in different lows.