Where and How the East Madagascar Current Retroflection Originates?. Issue 11 (14th November 2021)

• Record Type: Journal Article
• Authors: Ramanantsoa, Juliano D.; Penven, P.; Raj, R. P.; Renault, L.; Ponsoni, L.; Ostrowski, M.; Dilmahamod, A. F.; Rouault, M.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-14
• Subjects: EMC -- retroflection -- eddies -- SICC -- bloom phytoplankton -- Indian Ocean; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016203
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: 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 toward the pole and recirculating to the large‐scale Indian Ocean through retroflection modes of its southern extension. Five cruise data sets and remote sensing data from different sensors are used to identify three states of the southern extension of the 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 favor 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 (a) coastal upwelling south of Madagascar, (b) the southeastern Madagascar phytoplankton bloom, and (c) 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 toward 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. Key Points: The East Madagascar Current (EMC) retroflection is assessed. Evidence of EMC early retroflection is demonstrated for the first time Retroflection regimes are associated with EMC strength and mesoscale variability Knowledge of the EMC retroflection state helps understand regional ecosystem variability