Where and how the East Madagascar Current retroflection originates?

Type Article
Date 2021-11
Language English
Author(s) Ramanantsoa Juliano D.ORCID1, 2, 3, 4, Penven PierrickORCID5, Raj R. P.ORCID6, Renault L.ORCID7, 8, Ponsoni L.9, Ostrowski M.10, Dilmahamod A. F.ORCID11, 12, Rouault M.ORCID1, 3
Affiliation(s) 1 : Department of Oceanography University of Cape Town (UCT) ,South Africa
2 : Norwegian Research Center (NORCE) Bergen, Norway
3 : Nansen Tutu for Marine Environmental Research Ma‐Re Institute University of Cape Town (UCT) ,South Africa
4 : Institut Halieutique et des Sciences Marines (IH.SM) Toliara, Madagascar
5 : Univ. Brest CNRS IRD Ifremer Laboratoire d’Océanographie Physique et Spatiale (LOPS) IUEM Brest ,France
6 : Nansen Environmental and Remote Sensing Center (NERSC) Bjerknes Center for Climate Research (BCCR) Bergen ,Norway
7 : Department of Atmospheric and Oceanic Sciences University of California Los Angeles, Los Angeles, USA
8 : Laboratoire d'Étude en Geophysique et Océanographie Spatiale, IRD, Toulouse, France
9 : Georges Lemaître Centre for Earth and Climate Research (TECLIM) Earth and Life Institute Université catholique de Louvain Louvain‐la‐Neuve, Belgium
10 : Institute of Marine Research (IMR) Bergen, Norway
11 : GEOMAR Helmholtz Centre for Ocean Research Kiel Kiel ,Germany
12 : Department of Oceanography Dalhousie University HalifaxNova Scotia B3H 4R2, Canada
Source Journal Of Geophysical Research-oceans (2169-9275) (American Geophysical Union (AGU)), 2021-11 , Vol. 126 , N. 11 , P. e2020JC016203 (22p.)
DOI 10.1029/2020JC016203
WOS© Times Cited 1
Keyword(s) EMC, retroflection, eddies, SICC, bloom phytoplankton, Indian Ocean

The East Madagascar Current (EMC) is one of the western boundary currents of the South Indian Ocean. As such, it plays an important role in the climate system by transporting water and heat towards the pole and recirculating to the large-scale Indian Ocean through retroflection modes of its southern extension. Five cruise datasets and remote sensing data from different sensors are used to identify three states of the southern extension of the East Madagascar Current (EMC): early retroflection, canonical retroflection and no retroflection. Retroflections occur 47% of the time. EMC strength regulates the retroflection state, although impinged mesoscale eddies also contribute to retroflection formation. Early retroflection is linked with EMC volume transport. Anticyclonic eddies drifting from the central Indian Ocean to the coast favour early retroflection formation, anticyclonic eddies near the southern tip of Madagascar promote the generation of canonical retroflection, and no retroflection appears to be associated with a lower eddy kinetic energy (EKE). Knowledge of the EMC retroflection state could help predict (1) coastal upwelling south of Madagascar, (2) the southeastern Madagascar phytoplankton bloom, and (3) the formation of the South Indian Ocean Counter Current (SICC).

Plain Language Summary

Using in situ and satellite observations, we show that the East Madagascar Current (EMC), a strong current flowing along the East Coast of Madagascar, often detaches from the coast before the southern tip of the island and goes directly into the Indian Ocean, the so-called EMC retroflection. The EMC retroflection is characterized by three well-defined forms: early retroflection, canonical retroflection, and no retroflection. The EMC Early Retroflection is an unusual abrupt return current straight to the Indian Ocean without reaching the detachment point, while the EMC Canonical Retroflection returns the mass flow in the vicinity of the southern tip of the island. No retroflection is characterized by the straight propagation of the flow towards the Agulhas Current. These three forms of retroflection are due to the strength of the EMC and the contribution of mesoscale eddies arriving from the Indian Ocean. Retroflections have implications for coastal upwelling strength, Southeast Madagascar phytoplankton bloom occurrences, and South Indian Ocean Counter Current (SICC) formation.

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Ramanantsoa Juliano D., Penven Pierrick, Raj R. P., Renault L., Ponsoni L., Ostrowski M., Dilmahamod A. F., Rouault M. (2021). Where and how the East Madagascar Current retroflection originates? Journal Of Geophysical Research-oceans, 126(11), e2020JC016203 (22p.). Publisher's official version : https://doi.org/10.1029/2020JC016203 , Open Access version : https://archimer.ifremer.fr/doc/00732/84401/