section_path: Oceanography
section_name: Oceanography
paragraph_text: Oceanography
  
  Forty percent of the sediment of the Indian Ocean is found in the Indus and Ganges fans. The oceanic basins adjacent to the continental slopes mostly contain terrigenous sediments. The ocean south of the polar front (roughly 50° south latitude) is high in biologic productivity and dominated by non-stratified sediment composed mostly of siliceous oozes. Near the three major mid-ocean ridges the ocean floor is relatively young and therefore bare of sediment, except for the Southwest Indian Ridge due to its ultra-slow spreading rate.
  
  The ocean's currents are mainly controlled by the monsoon. Two large gyres, one in the northern hemisphere flowing clockwise and one south of the equator moving anticlockwise (including the Agulhas Current and Agulhas Return Current), constitute the dominant flow pattern. During the winter monsoon (November–February), however, circulation is reversed north of 30°S and winds are weakened during winter and the transitional periods between the monsoons.
  
  The Indian Ocean contains the largest submarine fans of the world, the Bengal Fan and Indus Fan, and the largest areas of slope terraces and rift valleys.
  
  The inflow of deep water into the Indian Ocean is 11 Sv, most of which comes from the Circumpolar Deep Water (CDW). The CDW enters the Indian Ocean through the Crozet and Madagascar basins and crosses the Southwest Indian Ridge at 30°S. In the Mascarene Basin the CDW becomes a deep western boundary current before it is met by a re-circulated branch of itself, the North Indian Deep Water. This mixed water partly flows north into the Somali Basin whilst most of it flows clockwise in the Mascarene Basin where an oscillating flow is produced by Rossby waves.
  
  Water circulation in the Indian Ocean is dominated by the Subtropical Anticyclonic Gyre, the eastern extension of which is blocked by the Southeast Indian Ridge and the 90°E Ridge. Madagascar and the Southwest Indian Ridge separate three cells south of Madagascar and off South Africa. North Atlantic Deep Water reaches into the Indian Ocean south of Africa at a depth of 2,000–3,000 m (6,600–9,800 ft) and flows north along the eastern continental slope of Africa. Deeper than NADW, Antarctic Bottom Water flows from Enderby Basin to Agulhas Basin across deep channels (< 4,000 m (13,000 ft)) in the Southwest Indian Ridge, from where it continues into the Mozambique Channel and Prince Edward Fracture Zone.
  
  North of 20° south latitude the minimum surface temperature is 22 °C (72 °F), exceeding 28 °C (82 °F) to the east. Southward of 40° south latitude, temperatures drop quickly.
  
  The Bay of Bengal contributes more than half (2,950 km or 710 cu mi) of the runoff water to the Indian Ocean. Mainly in summer, this runoff flows into the Arabian Sea but also south across the Equator where it mixes with fresher seawater from the Indonesian Throughflow. This mixed freshwater joins the South Equatorial Current in the southern tropical Indian Ocean. Sea surface salinity is highest (more than 36 PSU) in the Arabian Sea because evaporation exceeds precipitation there. In the Southeast Arabian Sea salinity drops to less than 34 PSU. It is the lowest (c. 33 PSU) in the Bay of Bengal because of river runoff and precipitation. The Indonesian Throughflow and precipitation results in lower salinity (34 PSU) along the Sumatran west coast. Monsoonal variation results in eastward transportation of saltier water from the Arabian Sea to the Bay of Bengal from June to September and in westerly transport by the East India Coastal Current to the Arabian Sea from January to April.
  
  An Indian Ocean garbage patch was discovered in 2010 covering at least 5 million square kilometres (1.9 million square miles). Riding the southern Indian Ocean Gyre, this vortex of plastic garbage constantly circulates the ocean from Australia to Africa, down the Mozambique Channel, and back to Australia in a period of six years, except for debris that gets indefinitely stuck in the centre of the gyre. The garbage patch in the Indian Ocean will, according to a 2012 study, decrease in size after several decades to vanish completely over centuries. Over several millennia, however, the global system of garbage patches will accumulate in the North Pacific.
  
  There are two amphidromes of opposite rotation in the Indian Ocean, probably caused by Rossby wave propagation.
  
  Icebergs drift as far north as 55° south latitude, similar to the Pacific but less than in the Atlantic where icebergs reach up to 45°S. The volume of iceberg loss in the Indian Ocean between 2004 and 2012 was 24 Gt.
  
  Since the 1960s, anthropogenic warming of the global ocean combined with contributions of freshwater from retreating land ice causes a global rise in sea level. Sea level also increases in the Indian Ocean, except in the south tropical Indian Ocean where it decreases, a pattern most likely caused by rising levels of greenhouse gases.