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 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 MAD-Ridge 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 the 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 southnorth transect: i) one filament highly concentrated in chlorophyll-a demonstrating the capacity of the eddy to export shelf water offshore; ii) intrusions of a more southern-type of 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 Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.dipole, unmistakable fine-scale dynamics were also observed in the vicinity of the MAD-Ridge seamount, superimposed onto the mesoscale eddy flow.

The MADRidge project ― a major contribution to IIOE2-WIOURI focussing on three shallow seamounts and their pelagic ecosystems in the vicinity of the Madagascar Ridge

“…Modelling and remote sensing studies were brought into MADRidge to support the field campaigns. (Vianello et al,2020a;this issue;Vianello et al,2020b; this issue) and PhD student (Annasawmy et al, 2020a;this issue;Annasawmy et al, 2020b;this issue). A Malagasy student obtained a MSc degree through the project and is a co-author of a paper in this special edition (Noyon et al, 2020;this issue).…”

“…Demarcq et al (2020; this issue) similarly undertakes a regional view using satellite seasurface colour (SSC) data and a new chlorophyll-a enrichment index to highlight the role of seamounts on chlorophyll production. These are then followed by the more focused papers, the first of which describes the currents, hydrology and biogeochemical variables (oxygen, nutrients, chlorophyll-a) around the MAD-Ridge (Vianello et al, 2020b; this issue) and La Pérouse (Marsac et al, 2020a;this issue) seamounts. The analyses showed little evidence of eddy-topography interactions, with the main driver being the ambient mesoscale eddy field.…”

Inter‐Annual Variability of the Along‐Shore Lagrangian Transport Success in the Southern Benguela Current Upwelling System

“…An eddy detection and tracking algorithm, similar to the one used by Halo et al (2014) and Vianello et al (2020) over the South-West Indian Ocean, is here applied to detect and track mesoscale eddies in the SBUS within our R3KM simulation. The values of a typical eddy displacement (X 0 ), eddy radius (R 0 ), and vorticity scale (ξ 0 ) were defined as 20 km, 50 km, and 10 −5 s −1 , respectively.…”

The MADRidge project: Bio-physical coupling around three shallow seamounts in the South West Indian Ocean

“…The circulation and hydrography in the immediate vicinity of MAD-Ridge and La Pérouse are then investigated by Vianello et al, (2020b) and Marsac et al, (2020a), respectively. MAD-Ridge being in a very energetic environment, it was expected that the mesoscale dynamics there in the form of strong eddies and dipoles would be found to be the overriding driver of the physical processes, strongly influencing water masses, nutrient and chl-a distributions (Vianello et al, 2020b), and moreover in our 'snapshot' cruise survey, somewhat obscuring any in situ observations of current-topography interactions and processes. Nevertheless, currents at MAD-Ridge were strong, and definitely (albeit small) changes in vertical density and temperature structures were observed around the summit and upper 400 m of slope, but any biological repercussions were outweighed by the presence of a powerful eddy dipole at the time of sampling.…”