Indian Ocean Basin Brief CDR Carl Allen Hager

Indian Ocean Basin Brief CDR Carl Allen Hager, USN 410 -293 -6561(w) CDR Carl Allen Hager hager@usna. edu

The Indian Ocean

Indian Ocean Outline • Basin Characteristics – Hydrology – Geology • Monsoons – Northeast – Southeast- • • • Surface Currents References Summary

Basin Characteristics Indian Ocean • Location: body of water between Africa, Antarctica, Asia, and Australia • Includes Andaman Sea, Arabian Sea, Bay of Bengal, Great Australian Bight, Gulf of Aden, Gulf of Oman, Mozambique Channel, Persian Gulf, Red Sea, Strait of Malacca, and other tributary water bodies • Nearly 10, 000 km wide at the tips of Africa and Australia and 10, 000 km from Antarctica to inner Bay of Bengal; its area is 73. 6 million sq km (20% of earth’s water surface); slightly less than 8 x the size of the US • Major sea routes connect the Middle East, Africa, and East Asia with Europe and the Americas; strategic importance far outweighs the economic value of its minerals or marine life

Basin Characteristics Indian Ocean • Surface currents controlled by the monsoon. • Deepwater circulation controlled by inflows from the Atlantic Ocean, the Red Sea, and Antarctic currents. • North of 20 o S, minimum sfc temp is 22 o. C (72 o. F), exceeding 28 o. C (82 o. F) to the east; southward of 40 o S, temps drop quickly. • Surface salinities range from 32 -37, with the highest concentration in Arabian Sea. • Pack ice and icebergs are found throughout the year south of 65 o S (ave. north limit~45 o).

Basin Characteristics • The African, Australian, Indian, and Antarctic crustal plates converge; juncture marked by inverted Y • Ridge systems: Central Indian Ridge, SW Indian Ridge, SE Indian Ridge and 90 o. E Ridge • Deepest point: Java Trench (7, 450 m) • 86% of main basin is pelagic sediments; remaining 14% is terrigenous sediments; glacial outwash dominates extreme southern latitudes Indian Ocean

Basin Characteristics Continental Drift Indian Ocean

Basin Characteristics Age of Ocean Crust Indian Ocean

Basin Characteristics Plate Motion Indian Ocean

Image source: Woods Hole Institute

Climate / Monsoons Indian Ocean • Climate is controlled by: – Northeast Monsoon (December to April) • Winter / Dry – Southwest Monsoon (June to October) • Summer / Wet • Tropical cyclones occur during May/June and Oct/Nov in northern Indian Ocean and Jan/Feb in southern Indian Ocean

Climate / Monsoons Indian Ocean • ‘Monsoon’ derived from an Arab word meaning ‘winds that change seasonally’ • …. or the Latin word “Monswine” – which means “its gonna rain till pigs float on the roof” • Due to differential heating of land water; water has a higher capacity for storing heat than does land (diurnal variability of differential heating results in ‘sea/land breezes’)

Climate / Monsoons Sea Breeze Indian Ocean Land Breeze

Climate / Monsoons Indian Ocean • North-East Monsoon (northern winter) – Air over southern Asia is cooler and denser than air over the ocean; continental air sinks. – Sinking air over Asia results in greater atmospheric pressure over land than ocean. – Resulting pressure gradient leads to a northerly (northeasterly) flow of air from Asia to south of the Equator; Prevailing winds drive a southward flowing Somali current. – Characterized by dry, cool air over India and surrounding areas.

January global atmosphere circulation Lutgens & Tarbuck, The Atmosphere p. 203

January Rainfall and Wind Vectors ITCZ

So m ali a Climate / Monsoons (Jan / NE) ITCZ Migration and Wind Direction off Somalia Indian Ocean

Climate / Monsoons Indian Ocean • South-West Monsoon (northern summer) – Air over southern Asia is warmer and less dense; continental air rises. – Rising air over Asia results in an atmospheric low pressure system over land. This is the “Wet” Season!! – Resulting pressure gradient leads to a southerly (southwesterly) flow of air from the ocean to the continent. – Prevailing winds drive a northward flowing Somali current.

July global atmosphere circulation Lutgens & Tarbuck, The Atmosphere p. 203

Climate / Monsoons So m ali a ITCZ Migration and Wind Direction off Somalia Indian Ocean

July Rainfall and Wind Vectors ITCZ

Surface Currents Indian Ocean

Surface Currents NEC ECC SEC Eastern Pacific Equatorial Surface Currents Indian Ocean

Winter (January) Indian Ocean circulation EUC Open University, Ocean Circulation p. 136

Summer (July) Indian Ocean circulation Open University, Ocean Circulation p. 136

Miscellaneous Indian Ocean • East African rift valley and the Red Sea represent the youngest stages of development of ocean basins (embryonic and young, respectively). The Himalaya Mountains exemplify the final stage of ocean evolution (relict scar). • Agulhas Current (warm, southward flowing current) is the second swiftest current in all the world’s oceans and is the deadliest. Rogue waves abound. • The Indian Ocean has its own El Nino-like phenomenon with a warm pool of water that moves eastward in a cycle of 3 -7 years.

Indian Ocean References • • Clark, C. O. , J. E. Cole, and P. J. Webster, 2000: Indian Ocean SST and Indian summer rainfall: predictive relationships and their decadal variability. Journal of Climate, 13(24), 2503 -2519. Holton, J. R. , 1992: An Introduction to Dynamic Meteorology. Academic Press San Diego, 511 pp. Lau, K. –M. and S. Yang, 1997: Climatology and interannual variability of the Southeast Asian summer monsoon. Advances in Atmospheric Sciences, 141— 162. Open University Course Team, 1989: Ocean Circulation. Butterworth Heinemann, Milton Keynes, England, 238 pp. Open University Course Team, 1997: The Ocean Basins: Their Structure and Evolution. Butterworth Heinemann, Milton Keynes, England, 185 pp. Sempere, J. –C. and E. M. Klein, 1995: New insights in crustal accretion from Indian Ocean spreading centers. EOS Transactions, 76(11), 113, 116. Tomczak, M. and J. S. Godfrey, 1994: Regional Oceanography: An Introduction. Pergamon, Oxford, 422 pp.

Indian Ocean Summary - Basin Characteristics - Hydrology - Geology - Monsoons - Northeast Southeast- - Surface Currents - References