First observational evidence of a North Madagascar Undercurrent

- We report an undercurrent flowing below and opposite to the North Madagascar Current.
- The North Madagascar Undercurrent (NMUC) transport is estimated to be 3.1-3.8 Sv.
- The NMUC core is found hugging the slope, at 460 m depth and velocity over 0.7 m s−1.
- Thermohaline properties reveal the NMUC mainly carrying South Indian Central Water.
- Dowelling-favorable winds are identified, stronger in winter and weaker in summer.

In situ 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.