The monsoon is primarily responsible for controlling ocean currents. The primary flow pattern consists of two huge gyres, one in the northern hemisphere traveling clockwise and one south of the equator traveling anticlockwise. The circulation is reversed north of 30°S during the winter monsoon (November-February), and winds are decreased during the winter and transitional periods between the monsoons.
The Subtropical Anticyclonic Gyre dominates water circulation in the Indian Ocean, with the Southeast Indian Ridge and the 90°E Ridge blocking its eastern extension. Three cells are separated south of Madagascar and off the coast of South Africa by Madagascar and the Southwest Indian Ridge. The minimum surface temperature north of 20° south latitude is 22 °C, rising to 28 °C to the east. Temperatures drop rapidly south of 40° south latitude.
The Bay of Bengal provides more than half of the Indian Ocean’s runoff water. This runoff primarily flows into the Arabian Sea during the summer, but it also travels south across the Equator, where it mixes with fresher seawater from the Indonesian Throughflow. In the southern tropical Indian Ocean, this mixed freshwater enters the South Equatorial Current.
The Arabian Sea has the highest sea surface salinity because evaporation surpasses precipitation there. It is the lowest in the Bay of Bengal due to river runoff and precipitation. The Indonesian Throughflow and rains cause decreased salinity near the west coast of Sumatra. Monsoonal variation causes saltier water to be transported eastward from the Arabian Sea to the Bay of Bengal from June to September, and westward by the East India Coastal Current to the Arabian Sea from January to April.
In 2010, an Indian Ocean waste patch measuring at least 5 million square kilometers was found. Except for trash that becomes forever lodged in the center of the gyre, this swirl of plastic rubbish regularly circles the ocean from Australia to Africa, down the Mozambique Channel, and back to Australia during a six-year period. Similar to the Pacific, icebergs drift as far north as 55° south latitude, though less so than in the Atlantic where they can reach up to 45°S.