Deepwater circulation is controlled primarily by inflows from the Atlantic Ocean, the Red Sea, and Antarctic currents. Southward of 40 deg south latitude, temperatures drop quickly. Surface water salinity ranges from 32 to 37 parts per 1,000, the highest occurring in the Arabian Sea and in a belt between southern Africa and southwestern Australia. Pack ice and icebergs are found throughout the year south of about 65 deg south latitude. The average northern limit of icebergs is 45 deg south latitude.
Fishing is confined to subsistence levels.
The ocean's most important function has been that of trade transport. Europeans, following the ancient seafarers, had crossed its waters to reach the East and returned with silks, rugs, tea, and spices. The Indian Ocean is also noted for its role in the shipment of petroleum from Southeast Asia to the West. Petroleum is the most significant mineral of the area, extracted primarily on the Persian Gulf. The earliest known civilizations, in the valleys of the Nile, Euphrates, Tigris, and Indus rivers and in Southeast Asia, have developed near the Indian Ocean.