Biogeochemical and ecological impacts of boundary currents in the Indian Ocean

• Published in: Progress in oceanography 2017-08, Vol.156, p.290-325
• Main Authors: Hood, Raleigh R., Beckley, Lynnath E., Wiggert, Jerry D.
• Format: Article
• Language: English
• Subjects: Boundary currents; Chlorophyll; Downwelling; Fish; Indian Ocean; Monsoon; Northeast Monsoon; Nutrients; Primary production; Southeast Monsoon; Southwest Monsoon; Transport; Upwelling; Zooplankton
• ISSN: 0079-6611; 1873-4472
• DOI: 10.1016/j.pocean.2017.04.011

•Indian Ocean (IO) boundary currents are complex compared to the Atlantic and Pacific.•The currents in the northern IO reverse seasonally and have large biological impacts.•The Leeuwin Current flows poleward and sheds seaward-propagating eddies.•The Agulhas Current is very large and generates eddies that propagate alongshore.•Leeuwin and Agulhas Current eddies also have large biological impacts. Monsoon forcing and the unique geomorphology of the Indian Ocean basin result in complex boundary currents, which are unique in many respects. In the northern Indian Ocean, several boundary current systems reverse seasonally. For example, upwelling coincident with northward-flowing currents along the coast of Oman during the Southwest Monsoon gives rise to high productivity which also alters nutrient stoichiometry and therefore, the species composition of the resulting phytoplankton blooms. During the Northeast Monsoon most of the northern Indian Ocean boundary currents reverse and favor downwelling. Higher trophic level species have evolved behavioral responses to these seasonally changing conditions. Examples from the western Arabian Sea include vertical feeding migrations of a copepod (Calanoides carinatus) and the reproductive cycle of a large pelagic fish (Scomberomorus commerson). The impacts of these seasonal current reversals and changes in upwelling and downwelling circulations are also manifested in West Indian coastal waters, where they influence dissolved oxygen concentrations and have been implicated in massive fish kills. The winds and boundary currents reverse seasonally in the Bay of Bengal, though the associated changes in upwelling and productivity are less pronounced. Nonetheless, their effects are observed on the East Indian shelf as, for example, seasonal changes in copepod abundance and zooplankton community structure. In contrast, south of Sri Lanka seasonal reversals in the boundary currents are associated with dramatic changes in the intensity of coastal upwelling, chlorophyll concentration, and catch per unit effort of fishes. Off the coast of Java, monsoon-driven changes in the currents and upwelling strongly impact chlorophyll concentrations, seasonal vertical migrations of zooplankton, and sardine catch in Bali Strait. In the southern hemisphere the Leeuwin is a downwelling-favorable current that flows southward along western Australia, though local wind forcing can lead to transient near shore current reversals and localized coastal up