Ocean currents and gradients of surface layer properties in the vicinity of the Madagascar Ridge (including seamounts) in the South West Indian Ocean

Abstract

As a 1 300 km longitudinally submerged extension of Madagascar, the Madagascar Ridge is a significant feature in the Southwest Indian Ocean. The ridge rises from abyssal depths to around 2000 m with several shallow (750-20m) seamounts occupying the northern and southern regions. Certainly the Walters Shoal is becoming recognised as a biological hotspot. The orientation of the ridge means that it traverses the background westward re-circulation of the Southwest Indian Ocean sub-gyre that apparently contributes to the Agulhas Current on the east coast of southern Africa. Because of its remoteness, little is known of the ridge's role in the local and regional oceanography and biology-although tuna fisheries operate in the north and around the Walters Shoal farther south. This remoteness has led to few in situ measurements having been made in the vicinity of the Madagascar Ridge. In this context, our study provides the first detailed description of the wind field and ocean properties along the Madagascar Ridge. In particular the study aims to provide a backdrop for several new investigations that focus on an unnamed seamount (260 m) on the northern sector of the ridge and the Walters Shoal (20 m) in the south. Spatial fields and along-ridge gradients of surface wind, SST, MLD, EKE, Heat Flux and Chl-a are produced to help biologists understand biological ramifications. These reveal contrasting environments between the northern and southern regions of the ridge. To gain an understanding of the circulation dynamics along the 1300 km long ridge and connectivity in the region-a novel approach of 'virtual moorings' is used. These comprised selected positions along the ridge with the compilation of 4year long time series of satellite-derived geostrophic currents. It is shown that currents (and EKE) decrease from north to south with greater variability. The northern ridge is highly dynamic due to the presence of the East Madagascar Current and its retroflection into the East Madagascar Return Current. By comparison, Walters Shoal in the south is in a quiescent zone.

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

“…Models suggest that some 50% of the total S-EMC transport heads toward the Agulhas Current and that about 40% flows back into the central Indian Ocean, some via the South Indian Ocean Counter Current. Owing to the southward flow of the warm S-EMC, sea surface temperatures over the northern ridge are high, ranging seasonally between 23 and 26°C (climatological values; Vianello et al, 2020a).…”

“…With Walters Shoal being in the Gyre, surface currents tend to be weaker. Sea surface temperature climatologies indicate a seasonal range between 23 and 18°C (Vianello et al, 2020a).…”

“…The oceanography papers start by providing broad perspectives centred around the Madagascar Ridge of ocean circulation, eddy kinetic energy (EKE), and meridional gradients of sea surface temperature (SST), mixed layer depth (MLD), Total Heat Flux and chlorophyll (chl-a), all based on 20-year climatologies of satellite remote sensing data (Vianello et al, 2020a). For the ocean circulation in this seldom visited region, a novel approach is used.…”

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

“…It is influenced by cold filaments, highly concentrated in chl-a, detaching from the adjacent southern Madagascar coastal upwelling cells (Lutjeharms and Machu, 2000;Quartly, 2006;Quartly et al, 2006;Ramanantsoa et al, 2018;Demarcq et al, 2020). It is also located in the very energetic retroflection region of the South East Madagascar Current (SEMC) (Pous et al, 2014;Vianello et al, 2020), which flows south along the east coast of Madagascar, transporting around 35 Sv 1 of warm, saline water from the subtropical South Indian Ocean (Siedler et al, 2009). It originates from the bifurcation, at 20°S, of the Indian Ocean South Equatorial Current (SEC) (DiMarco et al, 2002) and forms the northern part of the western boundary current of the South Indian Ocean subtropical gyre.…”

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).…”