Observations of the southern East Madagascar Current and undercurrent and countercurrent system

Abstract

[1] In April 2001 four hydrographic sections perpendicular to the southern East Madagascar Current were surveyed as part of the Agulhas Current Sources Experiment. Observations with a vessel mounted and a lowered ADCP produced information on the current field while temperature, salinity, oxygen and nutrient data obtained with a CTDRosette system, gave information on the water mass structure of the currents southeast of Madagascar. The peak velocity in the pole-ward East Madagascar Current through these four sections had a typical magnitude of $110 cm/s, while the width of this current was of the order of 120 km. The mean pole-ward volume transport rate of this current during the survey above the 5°C isotherm was estimated to be 37 ± 10 Sv. On all four sections an undercurrent was observed at intermediate depths below the East Madagascar Current. Its equator-ward transport rate amounted to 2.8 ± 1.4 Sv. Offshore of the East Madagascar Current the shallow South Indian Ocean Countercurrent was observed. This eastward frontal jet coincided with the barotropic and thermohaline front that separates the saline Subtropical Surface Water from the fresher Tropical Surface Water in the East Madagascar Current. The near-surface geostrophic flow of the East Madagascar Current, derived from satellite altimetry data from 1992 to 2005, suggests a strong variability of this transport due to eddy variability and interannual changes. The long-term pole-ward mean transport of the East Madagascar Current, roughly estimated from those altimetry data amounts to 32 Sv. The upper-ocean water mass of the East Madagascar Current was very saline in 2001, compared to WOCE surveys from 1995. Comparison of our undercurrent data with those of the WOCE surveys in 1995 confirms that the undercurrent is a recurrent feature. Its water mass properties are relatively saline, due to the presence of water originating from the Red Sea outflow at intermediate levels. The saline water was advected from the Mozambique Channel to the eastern slope of Madagascar.

The connection of the Indonesian Throughflow, South Indian Ocean Countercurrent and the Leeuwin Current

“…Observations in the west have suggested the Agulhas Retroflection and Return Current as the origin of the SICC (Nauw et al, 2008). In the east, observations (Smith et al, 1991;Woo et al, 2006;Woo and Pattiaratchi, 2008;Weller et al, 2011) and Lagrangian models (Song et al, 2004;Domingues et al, 2007;Valsala and Ikeda, 2007;van Sebille et al, 2014) have identified both tropical (including ITF) and subtropical waters as a source for the LC.…”

The East Madagascar Current: Volume Transport and Variability Based on Long-Term Observations

“…2a, gray shaded areas). Initially, two opposite options are considered: (i) assume that velocities decrease linearly to zero at the continental slope, obeying a no-slip boundary condition, and (ii) extrapolate horizontally the nearest measurement up to the continental slope, accomplishing a fullslip boundary condition (Beal and Bryden 1997;Nauw et al 2008;Beal 2009). For the sake of completeness, we apply both boundary conditions to our observations as well as a third method where empty regions are gridded with an (iii) optimal interpolation scheme (Carter and Robinson 1987;da Silveira et al 2004).…”

“…Regarding the vertical structure of the EMC system, at intermediate depths (around 1300 m), beneath the surface current and hugging the continental slope, an equatorward undercurrent was first reported by Nauw et al (2008). More recently, a detailed study of this East Madagascar Undercurrent (EMUC) estimated a mean equatorward volume transport to be 1.33 (61.41) Sv, with maxima up to 6 Sv (Ponsoni et al 2015a).…”

“…Through a quasi-synoptic survey, where velocities were sampled by lowered acoustic Doppler current profiler (L-ADCP), Nauw et al (2008) found a volume transport of 30 Sv at 258S. Schott et al (1988) analyzed 11 months of continuous observations, from three vertical lines of moorings longitudinally aligned off 238S, and found a mean (standard deviation) transport of 20.3 (66.6) Sv.…”

Variability and Coherence of the Agulhas Undercurrent in a High-Resolution Ocean General Circulation Model

“…Subsequent to this discovery, undercurrents have also been found in the Mozambique Channel and under the southern branch of the East Madagascar Current (Nauw et al 2008). The regions where these undercurrents have been found are both source regions for the Agulhas Current, leading to speculation on the possible connection between them.…”