Monsoon Winds Over The Indian Ocean

As discussed in Section 2.1, the winds over the Indian Ocean change dramatically with the seasons. In the northern winter (Figure 2.3(b)), 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. After crossing the Equator, the flow is turned to the left by the Coriolis force and converges with the SouthEast Trades at about 10-20° S. As the year progresses, the Asian landmass heats up and the high pressure over southern Asia weakens. By May/June, a low has developed; suddenly the wind direction changes, and a southerly or south-westerly wind blows across the region until September. This is the South-West Monsoon, the stronger of the two monsoons.

During which season of the year. i.e. during which monsoon, does the w ind pattern over the Indian Ocean most resemble thai in the other two oceans ?

Figure 2.3(a) and (b) show that the wind pattern over the Indian Ocean most resembles that over the other two oceans in the southern summer/northern winter. At this time of year, the North-East Monsoon blows across the ocean, and the wind field resembles that associated with the North-East Trade Winds.

The different types of weather characteristic of the two monsoons are well known. In January and February, during the North-East Monsoon, the winds bring dry cool air to India from the Asian landmass. In May and June, 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, which leads to the formation of cyclonic vortices which draw in more moisture-laden air from adjacent regions (cf. Section 2.3.1).

The monsoonal nature of the winds may also be thought of as a manifestation of the seasonal shift in the position of the ITCZ, from about 20° S in January to about 25° N, over Asia, in July. The summer convection over Asia is driven not only by direct heating by the warm continental mass below, but also by the condensation of moisture from air rising over high land (particularly the southern Himalayas), to form the monsoon rains. This releases latent heat originally taken up from the ocean as latent heat of evaporation, warming the air. causing it to rise still further, and intensifying the convection.

A particularly interesting aspect of the South-West Monsoon is the appearance over the western side of the ocean of an intense, southerly low-level atmospheric jet (Figure 5.11). This resembles an oceanic western boundary current, although here it is the high tablelands of eastern Africa that form the western boundary.

Figure 5.11 Monthly positions of the core of the low-level (1-2 km) atmospheric jet over the Indian Ocean. Brown shading represents land higher than 1 km above sea-level.

Figure 5.11 Monthly positions of the core of the low-level (1-2 km) atmospheric jet over the Indian Ocean. Brown shading represents land higher than 1 km above sea-level.

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