Australia: The Land Where Time Began

A biography of the Australian continent 

Indian Ocean

At the northern end of the Indian Ocean there is land across the entire width of the ocean just north of the tropics. As the Indian Plate moved to the north it left in its wake many ridges on the oceanic crust that make the ocean floor topography very rough. Of the spreading ridges of the Earth the Central Indian Ridge and the southwest Indian Ridge are 2 of the slowest. According to the authors1 to understand the spatial distribution of deep mixing in the global ocean there must be an understanding of ocean floor roughness from abyssal hills and fracture zones being highest at slow spreading sites. The Sunda Trench where the Indian plate is being subducted beneath Indonesia is the only trench in the Indian Ocean. The Indian Ocean connects to the Pacific Ocean thorough the gaps between the Indonesian Archipelago in the eastern part of the ocean. The Andaman Sea, Red Sea and Persian Gulf are marginal seas of the Indian Ocean. To the east of India the open ocean is called the Bay of Bengal and west of India it is the Arabian Sea.

The monsoon weather system results from differential heating of land and ocean in the tropics. The best known and most dramatic monsoon  is in the northern Indian Ocean, though they are also known from many other places. Cool, dry winds of the Northeast Monsoon blow over the Indian subcontinent over the ocean from the continental land masses in the northeast between October and May. The system changes to the Southwest Monsoon from June to September bringing warm, wet conditions from the western tropical ocean to the Indian subcontinent. The climate in the western tropical and South Pacific is dominated by these monsoons.

From the Himalayas most of the rivers draining southwards, including such rivers as the Ganges River, Brahmaputra River, and the Irrawaddy River, empty into the Bay of Bengal to the east of India, with the result that the surface water is rather fresh. The Bengal Fan that slopes gently downwards for thousands of kilometres has formed from the huge quantities of sediment brought to the ocean from the erosion of the Himalayas. The waters of the Arabian Sea, Red Sea and Persian Gulf are very saline because of the high evaporation that results from the dry climate. The salinity of the water of the Red Sea causes it to sink to mid-levels in the Indian Ocean, as occurs where the water from the Mediterranean Sea enters the Atlantic Ocean, affecting the properties over a large part of the Arabian Sea and Western Indian Ocean.

The Indian Ocean, the smallest of the 3 major oceans, extends to 25o N at its northern extremity. To the south it is bounded by the ACC (Antarctic Circumpolar Current) in the Southern Ocean, connecting it to the Pacific Ocean and the Atlantic Ocean within the ACC as well as to the north of it. In the deep parts of the Indian Ocean it is much more geographically complex than either the Atlantic or Pacific Oceans. The deep circulation connected to the Southern Ocean is divided into many complicated paths by the many deep ridges.

The subtropical gyre of the south Indian Ocean and the circulation of the tropics and Northern Hemisphere that is forced by the monsoon are the main flow regimes in the upper layers of the ocean. The South Equatorial Current (SEC), a nearly zonal current, oceanographically separates these at about 10-12o S. The anticyclonic subtropical gyre, similar to those in the other 4 ocean basins, differs in that the Agulhas Current, its western boundary current, overshoots the African coast, resulting in a different type of separation from the western boundary, and the Leeuwin Current, its eastern boundary current, according to the authors1 flows the "wrong way" towards the south. The reversing Southwest and Northwest Monsoons  force the strongly seasonal circulations in the tropics and northern Indian Ocean. There is a strong contrast between oceanographic regimes of the Bay of Bengal and the Arabian Sea, the Bay of Bengal being dominated by runoff from all the major Indian rivers that make it fresher than normal seawater, and the Arabian Sea that is more saline than normal as a result of the high evaporation rates. At more than 29o C the surface waters of the Indian Ocean are the warmest of the global open ocean.

In the Indian Ocean a connection to the Southern Ocean is included in the intermediate flow and deep flow regimes, similar to the those in the South Pacific Ocean, what differences exist are largely the result of accidents of topography. For the Indian Ocean the authors1 suggest the main difference is a source of intermediate (deep) water in the Red Sea in the northwest Indian Ocean. As with the water overflowing from the Mediterranean Sea to the Atlantic, the water flowing from the Red Sea to the Indian Ocean is highly saline, hence making the intermediate and deep waters highly saline, though both have limited impact on rates of deep ventilation rates "as both have low transport"1.

As with the Pacific Ocean, the Indian Ocean role in the global overturning circulation is as an upwelling region. Also participating in a complicated upwelling pattern that the authors1 suggest probably includes Indian Ocean deep water return to the Southern Ocean as well as upwelling to near the surface, North Atlantic and Antarctic waters entering from the south. In the Pacific Ocean upper ocean waters participate in the global circulation also traversing the Indian Ocean (Indonesian Throughflow ITF), entering the Agulhas Current, eventually entering the Atlantic Ocean.

Wind and Buoyancy forcing

The authors1 say the wind forcing of the Indian Ocean is one of the most unique features of the south Indian Ocean. In the Indian Ocean the mean wind pattern is similar to that of the Atlantic and Pacific, all having westerly winds at high latitude (Southern Ocean), and at low latitudes, Trade Winds. The seasonally reversing monsoons dominate the northern Indian Ocean, the ocean circulation changing direction seasonally.

Mean wind forcing

The Ekman Downwelling in the Southern Hemisphere over the broad region of latitude from 50o S - 10o S results from the Southern Hemisphere mean winds, producing Sverdrup forcing for a standard anticyclonic subtropical gyre. The gyre forcing differs from that in the South Atlantic and the South Pacific as at about 35o S the southern cape of Africa is well within the major subtropical gyre forcing. As the authors1 say "The subtropical gyre 'runs out' of western boundary before it 'runs out' of wind forcing for the gyre". Consequent to this the Agulhas Current overshoots the tip of Africa, a situation different from what occurs with the western boundary currents in the subtropical gyres of the other 4 oceans. The subtropical circulation then continues to the west as far as the coast of South America, as a result of the continued wind forcing, where the Brazil Current, a western boundary current that flows to the south. After separating from the African coast the Agulhas Current turns back east, resulting in the actual current being much more complex, shedding large eddies at the retroflection that propagate to the west into the South Atlantic, instead of continuing as a smooth flow to the west to the coast of South America. The result of this wind forcing is that the subtropical gyres in the Indian Ocean and South Atlantic Ocean are connected.

There is some connection between the eastern part of the Indian Ocean subtropical gyre region with the subtropical circulation of the South Pacific, the subtropical circulation becoming more a part of the circulation of the South Pacific Ocean to the east of Tasmania, with part of the East Australian Current (EAC) leaking into the circulation of the Indian Ocean.

Between the equator and 15-20o S there is a region of net upwelling that results from the mean winds in the tropical and northern Indian Ocean, associated with a cyclonic gyre that is comprised of the westward SEC on the southern side, the eastward South Equatorial Countercurrent (SECC) on the northern side, and (The East African Coastal Current; EACC), a northward western boundary current.

In the Arabian Sea the mean winds are dominated by the Southwest Monsoon that produced net downwelling and Sverdrup transport (the ocean current pattern produced by the wind-induced Ekman transport) forcing for a mean anticyclonic circulation, though the Northwest  Monsoon is very different.

Monsoonal wind forcing

Monsoonal wind forcing occurs in the tropical Indian Ocean as well as in the northern parts of the ocean. The circulation of the ocean responds to the almost complete reversal of the winds from that of winter to wind direction in summer. Large-scale wind changes that occur seasonally are the monsoons, the winds arise as a result of sign changes of the large-scale temperatures difference between the ocean and the land mass. In winter the land mass is cold and the ocean waters are warm and in summer the land mass is warm, while the ocean surface waters remains at about the same temperature, though with small variations between summer and winter. The result is that during summer in the tropics the wind blows towards the warm land and towards the ocean in winter. A thorough explanation of the these winds is said by the authors1 to be much more complex.

The naming of the monsoons relates to the prevailing wind direction. The Southwest Monsoon in the northern Indian Ocean blows from the southwest to the northeast in summer, into India from the Arabian Sea and western Indian Ocean. In the Southern Hemisphere the southeast trade winds blow throughout the year, the Southwest Monsoon winds are a continuation of these across the equator. The Somali (Findlater) Jet is a narrow jet, it is the name for the concentrated Southeast Monsoon that are apparent as the wet season in July. In India, as well as most of Southeast Asia. The Northeast Monsoon blows from the continental land mass in the northeast to the ocean in the southwest in winter, November to March. In this dry season conditions are relatively cool.

The annual mean wind pattern looks like a weaker version of the Southwest Monsoon as the winds of this Monsoon are stronger than those of the Northeast Monsoon. The switchover between the Southwest Monsoon and the Northeast Monsoon occurs over periods of 4-6 weeks in April-June and October-November. The equatorial winds blow eastwards across the full width of the Indian Ocean.

Buoyancy Forcing

There are no high northern latitudes where substantial loss of heat could occur, though the Red Sea and the Persian Gulf are subject to cooling and evaporation, with the result that they form dense saline waters. The outflow from the Red Sea penetrates deep into the water column of the Indian Ocean as a result of its salinity, but as there is only a small volume transport of overturn the saline overflow water contributes little more than a salty "dye" to the deep waters of the northern Indian Ocean.

There is a region of net heating in the tropical Indian Ocean in the Somali Current along the coast of Africa that is associated with upwelling and large permanent eddies. In the east rising air above the very warm pool of surface water in the Indian Ocean that results in a net precipitation. These feature are reversed in the east-west direction when compared to regions of highest heating and precipitation in the Pacific Ocean and Atlantic Ocean, the warmest region of the Indian Ocean is in the eastern tropics, and there is no equatorial cold tongue. The result is that the winds are strongly seasonal in the tropical Indian Ocean, while in the Atlantic Ocean and the Pacific Ocean there are prevailing easterly trade winds.

The surface forcing of the Indian Ocean subtropical waters differs from the situation in the subtropical regions of the Atlantic Ocean and Pacific Ocean as the Leeuwin Current that flows southwards dominates the eastern boundary regime instead of the equatorwards eastern boundary current. There is thus heat loss in the Agulhas Current and the Leeuwin Current regions, with the heat loss region being in the Agulhas Current is higher than any other part of the Indian Ocean. Along the Agulhas Return Current the high heat loss extends far to the east. Net evaporation is also high in the subtropics, though the contribution to total buoyancy flux is small.

Monsoonal and tropical ocean circulation



Sources & Further reading

  1. Tally, Lynne, Pickard, George L., , William J.,  Emery, D., & Swift, James H., 2011, Descriptive Physical Oceanography, an Introduction, Elsevier Ltd.


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Author: M. H. Monroe
Last Updated 25/04/2012



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