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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 (c β ~ 11 cm s–1 and U / c β ~ 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. [ABSTRACT FROM AUTHOR]

This work is part of the MADRidge Project special issue which aims to describe pelagic ecosystems in the vicinity of three prominent shallow seamounts in the South West Indian Ocean: one here named MAD-Ridge (240 m below the surface) plus Walters Shoal (18 m) on the Madagascar Ridge, and La Pérouse (60 m) on the abyssal plain east of Madagascar. The three span latitudes 20°S and 33°S, some 1500 km. The study provides the background oceanography for the once-off, multidisciplinary snapshot cruise studies around the seamounts. As life on seamounts is determined by factors such as summit depth, proximity to the light layers of the ocean, and the ambient circulation, a first description of regional spatial-field climatologies (16–22 years) and monthly along-ridge gradients of surface wind (driving force), water column properties of sea surface temperature, mixed layer depth, chlorophyll- a and eddy kinetic energy, plus ocean currents is provided. Being relevant to many applications in the study domain, these properties in particular reveal contrasting environments along the Madagascar Ridge and between the three seamounts that should drive biological differences. Relative to the other two seamounts, MAD-Ridge is in the more extreme situation, being at the end of the East Madagascar Current, where it experiences sturdy, albeit variable, currents and the frequent passing of mesoscale eddies. [ABSTRACT FROM AUTHOR]

An investigation of the vertical and horizontal distributions of micronekton, as influenced by mesoscale eddies, the Madagascar shelf and shallow seamounts, was undertaken using acoustic data collected during two research cruises at an unnamed pinnacle (summit depth ~240 m) thereafter named "MAD-Ridge", and at La Pérouse seamount (~60 m) in the south-western Indian Ocean. MAD-Ridge is located to the south of Madagascar, in an "eddy corridor", known both for its high mesoscale activity and high primary productivity. In contrast, La Pérouse is located on the outskirts of the Indian South Subtropical Gyre (ISSG) province, characterised by low mesoscale activity and low primary productivity. During the MAD-Ridge cruise, a dipole was located in the vicinity of the seamount, with the anticyclone being almost stationary on the pinnacle. Total micronekton acoustic densities were greater at MAD-Ridge than at La Pérouse. Micronekton acoustic densities of the total water column were lower within the anticyclone than within the cyclone during MAD-Ridge. Micronekton followed the usual diel vertical migration (DVM) pattern, except within the cyclone during MAD-Ridge where greater acoustic densities were recorded in the daytime surface layer. The backscatter intensities were stronger at the 38 kHz than at the 70 and 120 kHz frequencies in the daytime surface layer at MAD-Ridge cyclonic stations. These backscatter intensities likely correspond to gas-filled swimbladders of epi- and mesopelagic fish actively swimming and feeding within the cyclone or gelatinous organisms with gas inclusions. Our findings evidenced that the distributions of micronekton and DVM patterns are complex and are influenced significantly by physical processes within mesoscale eddies. The mesoscale eddies' effects were dominant over any potential seamount effects at the highly dynamic environment prevailing at MAD-Ridge during the cruise. No significant increase in total micronekton acoustic densities was observed over either seamount, but dense aggregations of biological scatterers were observed on their summits during both day and night. [ABSTRACT FROM AUTHOR]