What is the monsoon and what are its effects?

What is a monsoon?

Contrary to popular belief, the monsoon (Arabic for "season") does not primarily describe the heavy rainfall in the (sub)tropics, but a seasonal wind system. These seasons are not primarily distinguished by temperature fluctuations, as is the case here, but primarily by the amount of precipitation. Dry and rainy seasons alternate, and depending on the season and direction of the wind systems, either enormous amounts of precipitation can fall, or it remains mostly dry for months. The monsoon is a global and complex system that has a decisive and direct impact on about 60% of the world's population in large parts of the tropics and subtropics. About one-third of global precipitation falls in association with the monsoon, although monsoon regions account for barely 20% of the Earth's surface. The best known monsoon region is on the Indian subcontinent, but apart from Europe, all continents have such systems (more on this below). Monsoon regions are spatially defined based on the amount of precipitation that falls and its temporal component. According to the definition, at least 70% of the annual precipitation totals of a region must fall during the monsoon season (differentiation from equatorial regions, which record large precipitation totals all year round), and in addition, the differences between winter and winter periods must not exceed a certain amount.In addition, the differences between winter and summer precipitation must exceed a certain threshold (differentiation from arid/mediterranean climate regions, in which precipitation falls mainly in the winter months).

How does the monsoon develop?

The monsoon basically results as a balancing reaction of the atmosphere to different temperatures between different latitudes and surfaces. Also an important factor is the wandering zenith position of the sun during the course of a year. Where the sun is at its zenith (vertically above the ground), the land and ocean masses are warmed the most. The near-ground air masses above rise due to thermal expansion (lower density). This vertical air transport creates an area of low air pressure near the ground, a heat low. The formation of such heat lows occurs with an average delay of about 3 to 4 weeks to the zenith of the sun and extends as a low pressure trough around the globe. This low-pressure trough is also called the intra-tropical convergence zone (ITC) and thus moves behind the zenith of the sun during the course of the year. Depending on the ratio of land and water masses, the ITC is regionally deflected more to the north or south. Since land masses have a significantly lower heat storage capacity compared to water masses, the land warms more rapidly than the sea, and the ITC meanders more strongly.

The low pressure in the area of the ITC must be balanced by winds blowing to it (trade winds). These meridional winds (north-south) are deflected by the Coriolis force to the right in the northern hemisphere and to the left in the southern hemisphere. In addition, heat lows over land areas (also known as monsoon lows ) also influence their direction. Depending on the region, these winds thus flow over long stretches of warm ocean surfaces, picking up large masses of water in the process. When these moisture-enriched air masses reach the land surfaces, this leads to monsoon precipitation. Particularly on the windward side of mountains, the air masses forced to rise cause heavy uplift precipitation.

Typical regions and seasons

As mentioned at the beginning, the monsoon is a global system that occurs in large parts of the tropics and subtropics. In the following, we briefly discuss the most important systems:

Indian monsoon:

The summer monsoon typically begins in early June at the southern tip of India and Bangladesh and moves further north in the following weeks and months. The southwest trade wind brings cooling especially in the region of Rajasthan and Pakistan at the beginning of July, before that temperatures of sometimes over 50 degrees can be reached there. Most precipitation falls on average in Mawsynram with almost 12'000 mm per year, the absolute record within one calendar year was in Cherrapunji with unbelievable 26'461 mm rain (26 meters!). In autumn, the pressure distributions are reversed due to the position of the sun, the wind now comes from the northeast. This winter monsoon is characterized by cool and above all dry continental air masses, it is at the same time the start of the dry season with only little precipitation.

West African monsoon:

After the Indian Monsoon, the West African Monsoon is the second largest monsoon system in the world. The summer monsoon usually lasts from May to October and is characterized mainly by the large meridional differences. While the northern edge of the monsoon receives only about 100 mm of precipitation, the southern edge receives about 1500 mm. The monsoon extends from Cape Verde to the Ethiopian highlands of Abyssinia, and the temporal variability and intensity of precipitation vary greatly from year to year. In extreme cases, this monsoon can bring very little rainfall for years, causing great hardship in the affected countries. From February to November, the wind system reverses and the winter monsoon brings great drought.

Northern Australian monsoon:

In the austral summer, a heat low over northern Australia and a cold high over the Asian continent create a southward flow. The northeast winds prevailing just north of the equator are deflected to the left as they cross the equator, resulting in a wind from the northwest. The air soaks up a lot of moisture over the sea, which is the warmest in the year on average. January is the month with the greatest rainfall totals in northern Australia. During the southern winter the whole system turns around, a dry southeast trade wind blows mainly.

North American monsoon:

This is located between the Gulf of Mexico and the southern states of the United States. The associated heat low is located over the Colorado Valley and brings rain to the northern regions of Mexico beginning in June. The largest rainfall totals are usually measured in the Sierra Madre, where up to 350 mm accumulate. This monsoon provides about 70% of the annual precipitation in this region. Tropical hurricanes can amplify the whole effect significantly.

Influence of climate change

Over the last 30 years, an expansion of the monsoon areas has been observed, as well as an increase in the amount of rainfall. This trend is expected to continue in the 21st century, with affected areas increasing by 5-16% depending on the climate scenario, with 80-90% of this over the ocean. Precipitation totals will increase by 3.5-6%, but here there are large differences between the northern and southern hemispheres. While south of the equator precipitation is likely to be somewhat lower, in the north it will increase significantly, and the Indian monsoon could increase by up to 15%. These differences are due to the uneven distribution of land and water in the two hemispheres. In general, these changes occur primarily because of rising temperatures. On the one hand, this increases evaporation rates, and on the other hand, a warmer air parcel can absorb more moisture (about 7% per degree). In West Africa, melting Greenland ice could also have a negative impact on monsoon precipitation, resulting in devastating droughts.

The major changes in the landscape due to the monsoon are of great importance to the local population. While the accompanying rainfall is immensely important for irrigating agricultural land and thus feeding people, an imbalance of it has devastating consequences. The variability of the monsoon and the resulting droughts or floods are an integral part of life and crops in the affected areas, but with the increasing dynamics due to climate change, it also provides ever new and greater challenges.