First observational evidence of a North Madagascar Undercurrent

<i>In situ</i> observations reveal a southeastward-directed North Madagascar Undercurrent (NMUC) below and opposite to the equatorward-directed North Madagascar Current (NMC) off Cape Amber, at the northern tip of Madagascar. Results show an undercurrent hugging the continental slope with its core at 460 m depth and velocities over 0.7 m s-1. Its volume transport is estimated to be 3.1–3.8 Sv, depending on the velocity extrapolation methods used to fill in the data gaps near the slope (no-slip and full-slip, respectively). The thermohaline characteristics show a saltier and warmer NMUC, compared to the surrounding offshore waters, transporting mainly South Indian Central Water. Also, strong horizontal gradients of density are found in the NMUC domain. An inshore cell of coastal downwelling due to Ekman Transport toward the coast is identified, which can explain, at least in part, the strong baroclinic pressure gradients as well as the NMUC development and possible persistence

On the double sampling Method

An array of five moorings was deployed at 23°S off eastern Madagascar and maintained for about 2.5 years as part of the “INdian-ATlantic EXchange in present and past climate” (INATEX) experiment. The observations reveal a recurrent equatorward undercurrent (during 692 of 888 days), the East Madagascar Undercurrent (EMUC), flowing below the poleward surface East Madagascar Current (EMC). The average core of the undercurrent was found near the continental slope, at a depth of 1260 m and at an approximate distance of 29 km from the coast, with mean velocities of 6.4 (±4.8) cm s-1. Maximum speeds reach 20 cm s-1. The mean equatorward volume transport is estimated to be 1.33 (±1.41) Sv with maxima up to 6 Sv. The baroclinic/barotropic partitioning of the geostrophic flow shows a persistent equatorward baroclinic velocity in the undercurrent core, which is sometimes inhibited by a stronger poleward barotropic contribution. The wavelet spectrum analysis of the transport time series displays two dominant frequency bands: (i) nearly bi-monthly (46–79 days), previously observed in the surface EMC, and attributed to the forcing of barotropic waves generated in the Mascarene Basin; and (ii) nearly semi-annual (132–187 days), which seems to be related to the semi-annual cycle in the equatorial winds near the Indian Ocean eastern boundary. A historical dataset of temperature–salinity Argo profiles was used to investigate the spatial variability of the thermohaline properties at intermediate levels. Lastly, Argo-derived velocities suggest an undercurrent flowing upstream until approximately 17°S