Observation of a mesoscale eddy dipole on the northern Madagascar Ridge: Consequences for the circulation and hydrography in the vicinity of a seamount

Abstract

Based on satellite and in situ data, the dynamic characteristics and vertical structure of a surface intensified mesoscale eddy dipole recently expelled from the South East Madagascar Current (SEMC) is described for the first time. The dipole was surveyed 250 nautical miles south of Madagascar between 14 and 23 November 2016, during west-east and south-north transects carried out over the northern Madagascar Ridge. The dipole consisted of two counter-rotating vortices of similar size (100 km) and intensity (0.7 f), and an intense southwestward jet (150 cm s-1) in the frontal region between the two eddies. The cyclonic eddy was lying on the western side of the anticyclonic eddy. With azimuthal velocities reaching 100 cm s-1 at the surface and decreasing slowly with depth (40 cm s-1 at -600 m), this dipole was defined as a highly non-linear (Ro ~ 0.7) isolated eddy-type structure (cβ ~ 11 cm s–1 and U/cβ ~ 0.7) capable of trapping and advecting water masses over large distances. The enhanced concentration of chlorophyll-a found in the cyclone relative to the anticyclone could be tracked back to the spin-up phase of the two eddies and attributed to eddy-pumping. The eddy cores were located above the pycnocline (1026.4 kg m-3), within the upper 600 m, and consisted of varieties of Subtropical Underwater (STUW) found within the SEMC. The STUW found in the anticyclone was more saline and oxygenated than in the cyclone, highlighting mixing with the inshore shelf waters from the southeastern coastal upwelling cell off Madagascar. Observations suggest that the dipole interacted strongly with the chaotic bathymetry of the region, characterized by a group of five seamounts lying between -240 m and -1200 m. The bathymetry blocked its westward advection, trapping it in the vicinity of one shallow seamount for more than 4 weeks, so enhancing the role of the eddy-induced velocities in stirring the surrounding water masses. Squeezed between the southern Madagascan shelf and the northern flank of the anticyclone, two filament-like dynamic features with very different water-mass properties could be observed on the south-north transect: i) one filament highly concentrated in chlorophyll-a demonstrating the capacity of the eddy to export shelf water offshore; ii) intrusions of southern-type STUW generally found south of the South Indian Counter Current (SICC) recirculating on the external flanks of the anticyclone. Although the observed circulation and hydrography were largely constrained by the presence of the mesoscale eddy dipole, unmistakable fine-scale dynamics were also observed in the vicinity of the MAD-Ridge seamount, superimposed onto the mesoscale eddy flow.