Monsoon is traditionally defined as seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea.
However, because of the winds and weather usually associated with the monsoons in India and Southeast Asia, the word monsoon is often used to mean the prevailing wind and associated weather of these regions.
The Monsoon Process
Continental land masses warm up considerably in summer and cool down in winter. The biggest temperature variations are found in the land masses of North America and Asia.
Monsoons are caused by the larger amplitude of the seasonal cycle of land temperature compared to that of nearby oceans. This differential warming happens because heat in the ocean is mixed vertically through a "mixed layer" that may be fifty metres deep, through the action of wind and buoyancy-generated turbulence, whereas the land surface conducts heat slowly, with the seasonal signal penetrating perhaps a metre or so. Additionally, the specific heat capacity of liquid water is significantly higher than that of most materials that make up land. Together, these factors mean that the heat capacity of the layer participating in the seasonal cycle is much larger over the oceans than over land, with the consequence that the air over the land warms faster and reaches a higher temperature than the air over the ocean.
The hot air over the land tends to rise, creating an area of low pressure. This creates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it. Rainfall is caused by the moist ocean air being lifted upwards by mountains (Orographic Lift), surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation and precipitation as the air becomes saturated.
Monsoon climates have clearly marked seasons, each with a specific type of weather and wind from a certain direction. Monsoon climates are found in many parts of the world: West Africa, Ethiopia, northwest Australia, northwest corner of South Africa and the biggest areas of east and south Asia (including East Indies and Philippines).
Over Asia the mountain ranges run mainly east to west and this, together with the fact that the land mass of Asia is the largest in the world, limits the transposition of warm and cold air masses and makes seasonal contrasts much greater.
In winter: In January, in the northern hemisphere, low temperatures over Asia in winter give rise to an intense anticyclone (called Syberian high) which extends its influence over most of Europe and Asia. Because of the intense cold in the heart of Siberia in winter (average of minus 40 degress Celsius) the pressure in the anticyclone reaches very high values (sometimes even 1070 hPa). Air flows out from the high pressure region and gives rise to the winter monsoon, with north westerly winds in northern China. Further south the winds become northerly and finally north easterly as they take up the trade wind flow to become the northeast monsoon of Southeast Asia and Indonesia.
India is cut off from the Siberian anticyclone by the barrier of the Himalayas, but it develops its own high pressure system, centered in northwest India (in the Thar desert) and northern Pakistan and, as a result, there is an eastward flow of air out along the Ganges valley area which eventually joins the northeast monsoon over the Bay of Bengal.
Over China, Japan and eastern Asia, the air is generally very cold and warms up as it moves towards the equator. The flow from India is not as cold. Over the land the air remains dry, over the sea the monsoon becomes moist. The weather created by the monsoon depends on whether a land or sea track is followed and how much instability has developed.
In summer: The pressure distribution is completely changed because the high land temperatures lead to low pressure systems over the areas of southern Asia which are subject to the most heating and high pressure over the oceans. The flow is then from the sea to the land and the air in many cases actually comes from the southern hemisphere as the southeast trade winds which are then turned by the coriolis force on crossing the equator to form the southwest monsoon. The southwest monsoon has its direction changed again as it reaches the land masses of Asia (for example being diverted to flow from the southeast up the Ganges valley towards the low pressure area over northwest India).
Whilst the descriptions above have focused on the Asia monsoons, other parts of the world are subject to similar seasonal changes. The climatology of West Africa for example includes reference to the southwest monsoon and that of Australia, the northwest monsoon.
Monsoons are directly connected to the effect of the Intertropical Convergence Zone. Variation in the location of the ITCZ drastically affects rainfall in many equatorial nations, resulting in the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes. Longer term changes in the ITCZ can result in severe droughts or flooding in nearby areas.
La Niña is defined as abnormally high sea temperatures in the western Pacific ocean. This event, the opposite of El Niño, often follows an El Niño event. With La Niña, the waters of the western Pacific become a lot warmer than usual and the upwelling of cold waters along the west coast of South America becomes more pronounced than usual. This affects climatic conditions around the globe and can result in significantly increased monsoon rains over India.
Monsoon is accompanied by:
Accidents and Incidents
The following events occurred in Monsoon conditions:
- B744, Bangkok Thailand, 1999: On 23 September 1999, a Boeing 747-400 being operated by Qantas on a scheduled passenger service from Sydney Australia to Bangkok overran Runway 21L during an attempted night landing onto a wet runway.