Reviews and syntheses: Physical and biogeochemical processes associated with upwelling in the Indian Ocean
Authors: Vinayachandran Puthenveettil Narayana Menon , Masumoto Yukio , Roberts Michael J. , Huggett Jenny A. , Halo Issufo , Chatterjee Abhisek , Amol Prakash , Gupta Garuda V. M. , Singh Arvind , Mukherjee Arnab , Prakash Satya , Beckley Lynnath E. , Raes Eric Jorden , Hood Raleigh 
Published: November 2021

Abstract. The Indian Ocean presents two distinct climate regimes. The north
Indian Ocean is dominated by the monsoons, whereas the seasonal reversal 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 Natal pulses
drive slope upwelling and enhance chlorophyll concentrations along the
continental margin. The Durban break-away eddy spun up by the Agulhas
upwells cold nutrient-rich water. Additionally, topographically induced
upwelling occurs along the inshore edges of the 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 the
Mozambique coast are present in the northern and southern sectors of the
channel and are ascribed to dynamical effects of ocean circulation in
addition to wind forcing. Interaction of mesoscale eddies with the western
boundary, dipole eddy pair interactions, and passage of cyclonic eddies
cause upwelling. Upwelling along the southern coast of Madagascar is caused
by the Ekman wind-driven mechanism and by eddy generation and is inhibited by
the Southwest Madagascar Coastal Current. Seasonal upwelling 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. The Somali
coast presents a strong case for the classical Ekman type of upwelling; such
upwelling can be inhibited by the arrival of deeper thermocline signals
generated in the offshore region by wind stress curl. Upwelling is nearly
uniform along the coast of Arabia, 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 monsoon winds. Along the southern tip of India
and Sri Lanka, the strong Ekman transport drives upwelling. Upwelling along
the east coast of India is weak and occurs during 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, with Kelvin wave propagation originating from the equatorial
Indian Ocean affecting the magnitude and extent of the upwelling. Both El Niño–Southern Oscillation (ENSO)
and Indian Ocean
Dipole (IOD) events cause large variability in upwelling here. Along the west
coast of Australia, which is characterized by the anomalous Leeuwin Current,
southerly winds can cause sporadic upwelling, which is prominent along the
southwest, central, and Gascoyne coasts during summer. Open-ocean upwelling
in the southern tropical Indian Ocean and within the Sri Lanka Dome is
driven primarily by the wind stress curl but is also impacted by Rossby
wave propagations. Upwelling is a key driver 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 impacts 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
heterogeneity in productivity. In contrast, 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.
Although weak, 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 krill abundance. The review identifies the northern coast of the Arabian Sea and eastern
coasts of the Bay of Bengal as the least observed sectors. Additionally,
sustained long-term observations with high temporal and spatial resolutions
along with high-resolution modelling efforts are recommended for a deeper
understanding of upwelling, its variability, and its impact on the
ecosystem.