Unit 14 Monsoons

Monsoons def

seasonally reversing winds

Somali current

During the North-East Monsoon the Somali Current flows to the south-west, but for the rest of the year it flows to the north-east. During the South-West Monsoon it becomes a major western boundary current (Fig. 11.2d). It surface velocity has been observed to reach 3.7 m/s (!)

Northern summer current

In the northern summer, by contrast, the flow in the North Equatorial Current reverses and combines with a weakened Equatorial Counter-Current to form the South-West Monsoon Current (Fig. 11.2 d, e, next slide). The South Equatorial Current is still present, although its flow is not as strong as during the North-East Monsoon

North-East Indian monsoon

In the northern winter, the air over southern Asia is cooler and denser than air over the ocean, and so the surface atmospheric pressure is greater over the continent than over the ocean. The resulting pressure gradient leads to a low-level northerly or north-easterly flow of air from the Asian landmass to south of the Equator. This flow of air is the North-East Monsoon and goes to left bc of coriolis force

Summer monsoon circulation

In the summer, the situation is reversed: air over the Eurasian land mass heats up, and becomes less dense There is a region of warm rising air and low pressure which winds blow towards

Winter monsoon circulation

In winter, the air above the Eurasian land mass is cooled and becomes denser, so that a large shallow high pressure area develops, from which winds blow out towards regions of lower pressure

Areas monsoons affect most

Indian Ocean and the western tropical Pacific

South West Indian Monsoon

The South-West Monsoon is the stronger of the two monsoons In May and Jun, during the South-West Monsoon, the winds cross the Arabian Sea and bring humid maritime air to India The moisture that provides the heavy monsoon rains is partly a direct result of evaporation from the warmed surface of the Arabian Sea, and partly the result of upward convection of warm moist air above the Arabian Sea

Current systems of the Indian Ocean

The change of atmospheric circulation due to monsoons results in the change of the pattern of surface currents in the Indian Ocean

How is it possible for the ocean to react so fast to the atmosphere?

The equatorial wave-guide provides a pathway for Rossby and Kelvin waves traveling westward and eastward directions respectively. As a result, the information about changing conditions on one coast of the ocean can propagate relatively fast to the other side of the ocean, which is not possible in mid and high latitudes

The South-West Monsoon is associated with ...

an intense, southerly low-level atmospheric jet over the western side of the Indian Ocean

Equatorial Undercurrent

has been observed in the Indian Ocean only for that part of year when the wind circulation resembles that in the Pacific and the Atlantic Ocean, that is, during the North-East Monsoon (winter time in the Northern Hemisphere)

Complexity of surface circulation in Indian ocean

is a result of the seasonally changing wind circulation pattern

Cause of Monsoons

main physical cause of monsoons is that the thermal capacity of continents is lower than that of the ocean

Monsoons and ITCZ

seasonal shift in the position of the ITCZ, from about 20oS in January to about 25oN, over Asia, in July

Continental Masses vs Ocean

the continental masses cool down and heat up faster than the oceans: in winter they are colder than oceans and in summer they are warmer

Somali current generation

the low-level atmospheric jet mentioned above is thought to play an important role in the generation of the intense coastal upwelling that occurs off Somalia during the South-West Monsoon. At this time of year, upwelling also occurs off Arabia. The Somali and Arabian regions of upwelling are the most vigorous in the Indian Ocean