The East Madagascar Current: Volume Transport and Variability Based on Long-Term Observations

Abstract

This study provides a long-term description of the poleward East Madagascar Current (EMC) in terms of its observed velocities, estimated volume transport, and variability based on both ~2.5 yr of continuous in situ measurements and ~21 yr of satellite altimeter data. An array of five moorings was deployed at 23 degrees S off eastern Madagascar as part of the Indian-Atlantic Exchange in present and past climate (INATEX) observational program. On average, the EMC has a horizontal scale of about 60-100 km and is found from the surface to about 1000-m depth. Its time-averaged core is positioned at the surface, at approximately 20 km from the coast, with velocity of 79 (+/- 21) cm s(-1). The EMC mean volume transport is estimated to be 18.3 (+/- 8.4) Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)). During the strongest events, maximum velocities and transport reach up to 170 cm s(-1) and 50 Sv, respectively. A good agreement is found between the in situ transport estimated over the first 8 m of water column [0.32 (+/- 0.13) Sv] with the altimetry-derived volume transport [0.28 (+/- 0.09) Sv]. Results from wavelet analysis display a dominant nearly bimonthly (45-85 days) frequency band of transport variability, which explains about 41% of the transport variance. Altimeter data suggest that this band of variability is induced by the arrival of westward-propagating sea level anomalies, which in turn are likely represented by mesoscale cyclonic and anticyclonic eddies. Annual averages of the altimeter-derived surface transport suggest that interannual variabilities also play a role in the EMC system.