monsoonal currents

monsoonal currents Monsoon systems constitute the largest and most energetic disturbances of the atmosphere. Not surprisingly, monsoons have assumed historical importance, bringing, for example, the coast of East Africa under the influence of Arabic culture through the reliability of seasonal wind systems.

Monsoons arise from the differential heating and cooling of the land and sea surface. At the most basic level they are akin to land and sea breezes. Three simultaneous criteria define a monsoonal climate:(1) prevailing wind direction shifts by at least 120° between winter and summer;(2) the average frequency of prevailing wind directions in winter and summer exceeds 40 per cent; and(3) the mean resultant wind speed is at least 31 m s⁻¹.

These conditions reach their largest expanse over Asia. At its height, the Asian monsoon dominates much of the global tropics. In summer, the Tibetan plateau and hence the overlying atmosphere is heated by solar radiation, leading to a reduction in surface pressure. The low surface pressure induces low-level flow of warm, moist air from the surrounding ocean to the land. Convergence and ascent lead to intense precipitation over India and the slopes of the Himalaya during the summer months. Release of latent heat in the rising air further heats the air column, thereby driving the system further. The summer monsoon begins with marked changes in temperature, air pressure, and an increase in wind flow from April to May. This coincides with the movement of warm surface waters from the central Arabian Sea and the Bay of Bengal. Simultaneously, a longitudinal surface pressure gradient extends from the south Indian Ocean anticyclone (high pressure) across the Equator into the south Asian continent. The typical zone of low pressure at the Equator disappears. Inflow into the heated Asian continent occurs as three main airstreams: over the Arabian Sea, the Bay of Bengal, and the South China Sea. The moist yet cool south-west airstreams undercut the dry and hot continental air, leading to vigorous and deep convection. There has been controversy over the importance of cross-equatorial transport of water vapour for the moisture budget of southern Asia. More recently, the role of the low-level East African Jet (over the Arabian Sea) in moisture import has been emphasized. The surface pressure trough extends from the desert regions of Iran and Pakistan across northern India. The principal belt of monsoon rainfall tends to migrate along a belt of roughly 500 mm of rainfall which moves from Indonesia to the foothills of the Himalaya.

Unlike the typical tropical Hadley cell, which is characterized by north–south overturning, much of the air flow in the summer monsoon is oriented east–west (Fig. 1a). The extent of this flow is massive, dominating much of the tropical atmosphere. As a result of the change towards a more negative radiation balance (long-wave loss exceeding short-wave solar radiation gain) and cold air advection in the westerlies, the seasonal cooling of the land surfaces of south Asia and the surrounding seas leads to the onset of the boreal winter phase of the monsoon. It is thought that the extensive cooling of the Asian landmass is more important than the more regional heating over Australia. As a result of the cooling, anticyclonic vortices develop over land, the Arabian Sea, and the Bay of Bengal by October, while the major monsoon convective zone migrates from the summertime position over north-eastern India to the maritime continent of Borneo and Indonesia. This occurs despite the much smaller area of land in the southern hemisphere compared with its Asian counterpart. When fully developed, the winter monsoon convection drives a gigantic planetary-scale overturning motion in the east–west and north–south direction (Fig. 1b). Between November and March, north-easterlies blowing out from the surface high pressure over southern Asia sweep much of the northern Indian Ocean, including the Arabian Sea, the Bay of Bengal, and the South China Sea. These airstreams then turn northwards to the north of the Equator and meet the southern hemisphere south-easterlies in a trough zone south of the Equator.

In the upper atmosphere during this season, westerlies dominate in the upper troposphere over most of the monsoon region. The Subtropical Westerly Jet is found to the south of the central Asian mountain massifs, in contrast to the boreal summer when the Tropical Easterly Jet occupies the upper troposphere to the south of the Himalayas. The fundamental difference between the winter and summer monsoon is that convection is based over land in July but over land and sea in January. There is, however, an important difference in the moisture supply between the two systems The daily cycle of winter monsoon convection over Indonesia is influenced by land–sea breeze effects, whereas the diurnal variation of the continental monsoon has continental features, for example, dry convection over Tibet. The divergent component of the equatorial winter monsoon is also much stronger. Overall, the boreal summer monsoon is much more vigorous than its winter counterpart, except over East Asia. This, together with the hemispheric asymmetry in land–sea distribution in the monsoon region, points to the role of hemispheric asymmetry in surface heating as the main driving mechanism of the monsoon.

Much can be learned by considering some of the basic energetics of the monsoon. Descent over Asia in January leads to a gain in geopotential and sensible energy which is huge compared with the small radiative heat and oceanic heat import. Latent heat from precipitation is non-existent, this form of energy being exported in the form of evaporative fluxes largely off the ocean. The gain in radiative heat input at the top of the atmosphere in July between 0° and 30° S is small. Added to this, heat is imported into the southern tropical Indian Ocean by the southward-directed currents. Both sensible and latent heat losses from the southern tropical Indian Ocean are far higher than the radiative heat gain. The latitudinal temperature gradient in July provides an important energy source in the form of available potential energy, providing the monsoon in this season with higher kinetic energy.

Much research effort in the past two decades has been invested in numerical modelling of the Asian monsoon. Since these models allow for changes in one variable at a time, the role of mountains in maintaining the south Asian low-pressure system, or the importance of sea-surface tem-perature anomalies in the Arabian Sea, have, for example, been investigated.

A remaining problem in monsoon studies concerns intraseasonal oscillations. In the course of the south-west monsoon, there are periods when the monsoon trough shifts northwards to the foot of the Himalayas, and rains decrease over much of India except along the slopes of the Himalayas. This is called the ‘break in the monsoon’. These breaks are most frequent in July and August and last for 2 to 3 weeks. Recent studies emphasize the propagation of low-frequency modes and middle-latitude interactions. There has similarly been a great deal of research aimed at predicting monsoon onset as well as understanding the interannual variability of the monsoon. A concerted effort to address such problems saw the initiation of the GARP (Global Atmospheric Research Programme) Monsoon Experiment (MONEX) conducted from 1978 to 1979. Initially staged in Malaysia during the winter of 1978–9 for the study of cold surges, rain-producing waves, and vortices, the entire monsoon trough and the planetary-scale structure of the monsoon were eventually examined. It is thought that the mode of migration of the region of heating from northern Burma in the month of May to the foothills of the Himalayas in June is a critical factor in determining the onset date.

It has long been known that the Asian monsoon is related to El Niño–Southern Oscillation (ENSO) events, which feature the interannual oscillation of air pressure and sea surface temperatures across the global tropics, particularly the Pacific. Indeed, in the 1920s that Sir Gilbert Walker uncovered this see-saw of pressure in an attempt to predict the Asian monsoon. Generally, higher sea-level pressure at Darwin is accompanied by lower monsoon rainfall in India. Research into this relationship continues.

The criteria outlined above are by no means confined to the atmosphere overlying Asia and Indonesia. Africa also has an important monsoon circulation. During the boreal winter, for example, the equatorial surface pressure trough stays closest to the Equator over Africa, while the Harmattan winds, a stream of dry desert air (trade winds) sweep far southwards, confining rains to a belt extending from the Gulf of Guinea eastward into the interior of the continent. During the boreal summer, the surface pressure trough shifts northwards, so that a cross-equatorial moist and cool monsoon airstream penetrates into the continent.

Work by Neil Ward has shown that the monsoon systems of the globe, including Asia and Africa, may be linked in an integral system, such that in some years a tropic-wide mode is triggered, as rainfall throughout the global monsoon system is above. These modes are thought to be oscillations internal to the atmosphere that are excited by sea-surface temperature conditions in the Pacific.

R. Washington

Bibliography

Grotjahn, R. (1993) Global atmospheric circulations: observations and theories. Oxford University Press.