Modes of the southern extension of the East Madagascar Current

Abstract

[1] Data sets from satellite observations and a nested high-resolution model are used to study a source region of the Agulhas Current. Altimeter-derived geostrophic surface currents are averaged over varying periods, providing evidence of the persistence of flow patterns in the extension of the southern branch of the East Madagascar Current (SEMC). South of Madagascar, the SEMC separates into one branch toward the Agulhas Current and into a second branch retroflecting and connecting to the Subtropical Indian Ocean Countercurrent (SICC). Good agreement is found between long-term mean patterns of observational and model dynamic heights. Two basic modes are identified in the SEMC extension, with anticyclonic motion favoring retroflection in the northern Mozambique Basin when the extension is in a southwestward direction and cyclonic motion occurring in the case of the SEMC flowing westward along the southern Madagascar slope. A cross-correlation sequence between model SEMC transports and the modal changes in the extension region displays a correlation at about 1-month lag which agrees with eddy propagation time from the SEMC to the outflow region. Mean model SEMC transports are determined using floats released at 21°S, and the contribution of the SEMC to the SICC is obtained using floats injected at 55°E with the model running backward. Almost half of the SEMC volume transport contributes to the Agulhas system, and about 40% of SICC water originates from the SEMC.

Evaluating two numerical advection schemes in HYCOM for eddy-resolving modelling of the Agulhas Current

“…The mean SSH streamlines retroflect in an anticyclonic direction to flow eastward into the South Indian Ocean. Higher levels of mesoscale variability are encountered due to this retroflecting current, with occurrences of both cyclonic and anticyclonic eddies as reported by Siedler et al (2009). The variability in the O4 simulation is slightly broader and more energetic but remains lower than observed from altimetry.…”

“…The two modes of the southern extension of East Madagascar Current (SEMC) described by Siedler et al (2009) are clearly evident in the skewness field. The current consistently retroflects southwest of Madagascar near 42 • E, with anticyclonic motion (and eddies) favoured during a southwestward extension of the SEMC, while cyclonic motion (and eddies) occurs during westward flow along the south Madagascar continental shelf edge.…”

“…8b). The method of distinguishing between cyclonic and anticyclonic rotation by calculating the fraction from total EKE was used in the study of the southern extension of the East Madagascar Current (Siedler et al, 2009), but here was applied to the entire greater Agulhas Current domain.…”

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

“…Anticyclonic (Agulhas rings) and cyclonic vortices, shed at the retroflection, transport warm and salty water into the South Atlantic, which subsequently affects the Atlantic meridional overturning circulation (Biastoch et al 2008b). Mesoscale perturbations, seen as eddies in the Mozambique Channel and east of Madagascar (Siedler et al 2009), drift toward the Agulhas Current, introducing offshore meanders into the core of the current, Natal pulses (Lutjeharms and Roberts 1988). These Natal pulses rapidly propagate southward along the current, causing upstream retroflections (Lutjeharms and van Ballegooyen 1988) and triggering (van Leeuwen et al 2000) the shedding and thus the characteristics of Agulhas rings (Biastoch et al 2008c).…”

Variations in Remotely-Sensed Phytoplankton Size Structure of a Cyclonic Eddy in the Southwest Indian Ocean

“…Previous studies have suggested that the southern branch of the East Madagascar Current (SEMC) bifurcates, with a portion of the flow retroflecting and persisting east [4,5], and the remainder of the flow continuing west across the northern part of the Mozambique Basin (MB) and feeding into the Agulhas Current system (Figure 1), mainly in the form of mesoscale eddies [6][7][8][9][10]. In contrast, a more recent investigation has shown that the SEMC flow dissolves into a series of cyclonic and anticyclonic dipole eddy pairs (Figure 1), and that there is no retroflection or westward jet [3].…”