Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean

Abstract

Abstract. The Indian Ocean presents two distinct climate regimes. The North Indian Ocean is dominated by the monsoons, whereas the seasonal variation is less pronounced in the south. The prevailing wind pattern produces upwelling along different parts of the coast in both hemispheres during different times of the year. Additionally, dynamical processes and eddies either cause or enhance upwelling. This paper reviews the phenomena of upwelling along the coast of the Indian Ocean extending from the tip of South Africa to the southern tip of the west coast of Australia. Observed features, underlying mechanisms, and the impact of upwelling on the ecosystem are presented. In the Agulhas Current region, cyclonic eddies associated with the Natal pulses drive slope upwelling and enhances chlorophyll concentration along the continental margin. The Durban break-away eddy spun-up by the Agulhas upwells cold nutrient-rich water. Besides, topographically induced upwelling occurs along the inshore edges of Agulhas Current. Wind-driven coastal upwelling occurs along the South coast of Africa and augments the dynamical upwelling in the Agulhas Current. Upwelling hotspots along Mozambique are present in the northern and southern sectors of the channel, and they are ascribed to dynamical effects of ocean circulation in addition to wind forcing. Interaction of mesoscale eddies with the western boundary, anticyclonic eddy pair interactions, and passage of cyclonic eddies cause upwelling. Upwelling along the southern coast of Madagascar is caused by Ekman wind-driven mechanism and by eddy generation and inhibited by Southwest Madagascar Coastal Current. The seasonal upwelling that occurs along the East African coast is primarily driven by the Northeast monsoon winds and enhanced by topographically induced shelf-breaking and shear instability between the East African Coastal Current and the island chains. Somali coast presents a strong case for the classical Ekman type of upwelling. This upwelling can be inhibited by the arrival of deeper thermocline signals generated in the offshore region by wind stress curl. The upwelling is nearly uniform along the coast of Arabia, it is caused by the alongshore component of the summer monsoon winds and modulated by the arrival of Rossby waves generated in the offshore region by cyclonic wind stress curl. Along the west coast of India, upwelling is driven by coastally trapped waves together with the alongshore component of the southwesterlies. Along the southern tip of India and Sri Lanka, the strong Ekman transport dives upwelling. Upwelling is feeble along the east coast of India and occurs during the summer, caused by alongshore winds. In addition, mesoscale eddies lead to upwelling but the arrival of river water plumes inhibits upwelling along this coast. Southeasterly winds drive upwelling along the coast of Sumatra and Java during summer. Kelvin wave propagation originating from the Equatorial Indian Ocean affects the magnitude and extent of Sumatra and Java upwelling. Both ENSO and IOD events cause large variability of upwelling here. Along the west coast of Australia, southerly winds can dominate over the Leeuwin Current, causing sporadic upwelling, which is prominent along the southwest, central, and Gascoyne coasts during summer. The open ocean upwelling in the southern tropical Indian Ocean and within the Sri Lanka Dome is driven primarily by the wind stress curl but also impacted by Rossby wave propagations. Upwelling is a key driver in enhancing biological productivity in all sectors of the coast, as indicated by enhanced sea surface chlorophyll concentrations. Additional knowledge at varying levels has been gained through in situ observations and model simulations. In the Mozambique channel, upwelling simulates new production, and circulation redistributes the production generated by upwelling and mesoscale eddies leading to observations of higher ecosystem impact along the edges of eddies. Similarly, along the southern Madagascar coast, biological connectivity is influenced by the transport of phytoplankton from upwelling zones. Along the coast of Kenya, both productivity rates and zooplankton biomass are higher during the upwelling season. Along the Somali coast, accumulation of upwelled nutrients in the northern part of the coast leads to spatial inhomogeneity in productivity. On the other hand, productivity is more uniform along the coasts of Yemen and Oman. Upwelling along the west coast of India has several biogeochemical implications, including oxygen depletion, denitrification, and high production of CH4 and dimethyl sulfide. Though feeble, wind-driven upwelling leads to significant enhancement of phytoplankton in the northwest Bay of Bengal during the summer monsoon. Along the Sumatra and Java coasts, upwelling affects the phytoplankton composition and assemblages. Dissimilarities in copepod assemblages occur during the upwelling periods along the west coast of Australia. Phytoplankton abundance characterizes inshore edges of the slope during upwelling season, and upwelling eddies are associated with abundance in Krill. The review identifies the northern coast of the Arabian Sea and eastern coasts of the Bay of Bengal as the least observed sectors. Further, sustained long-term observations with high temporal and spatial resolutions along with high-resolution modeling efforts are suggested for a proper description of upwelling, its variability, and its relationship to the ecosystem.

Comment on bg-2020-486

“…Murty and Varadachari (1968) found stronger upwelling at Visakhapatnam compared to Chennai during both spring and summer seasons. Similarly, upwelling at the northern part of the east coast of India (Figure 18) was also reported by several investigators (Murty, 1958;Murthy, 1981;Gopalakrishna and Sastry, 1984;Rao et al, 1986.…”

“…Therefore, they advocated that the open ocean cooling off Oman and the associated biological response is primarily driven by advection of cold nutrient-rich upwelled water from Oman coast and the wind-driven mixing entrainment at the bottom of the mixed layer, which deepens the thermocline offshore. This intense upwelling all along the coast of Yemen and Oman in the western Arabian Sea also drives one of the strongest primary productivity of this region at a rate of more than 2.5 gCm -2 day -1 (Marra et al, 1998;Morrison et al, 1998). Unlike, Somali coast, here chlorophyll concentration is found to be more uniform and extend up to 400 km offshore.…”

“…The first hydrographic data used to report on upwelling phenomena in the Mozambique Channel, as inferred from sloping isotherms and isohalines in the upper 500 m of the water column, were collected onboard RV Dr. Fridtjof Nansen, which surveyed the entire coast of Mozambique four times between August 1977and June 1978(IMR 1977aIMR 1978a, b, c). Saetre and de Paula e Silva (1979) concluded that, during the NE monsoon (Nov-April), wind-induced upwelling occurs in a narrow strip of the ocean along the northern Mozambique coast between 11 and 16°S. Although they did not observe any associated low temperatures or high nutrient concentrations in the surface waters, they observed cyclonic eddies off Angoche in September and November 1977 and further south off Inhambane and along a transect off ~27°S during the September 1977 and January-March 1978 surveys.…”

“…along the northeast coast (Krakstad et al, 2008). One year later, from 25 August to October 2009, the Nansen revisited the western sector of the south coast and continued sampling along the southwestern and north-western coasts, ending once more at Antsiranana (Diego Suárez) in the north, completing 10 transects and 182 hydrographic stations (Alvheim et al 2009). Once again, hydrographic sampling provided evidence of coastal upwelling on the southern coast (26°S), as well as at two locations on the west coast, near Cap Sainte André (16°S) and Nosy Be Island (13°S), with salinity maxima indicating upwelling of Subtropical Surface Water in the south and Equatorial Surface Water in the northern region (Pripp et al, 2014).…”

“…The first evidence of coastal upwelling along WBoB was observed between 1952-1965, during IIOE. The first published report, although insufficient to present evidences of coastal upwelling or downwelling for a season, along WBoB using hydrographic data, was by La Fond (1954Fond ( , 1957Fond ( , 1958Fond ( , 1959.…”