Madagascar: A pacemaker for the Agulhas Current system?

Abstract

Western boundary currents are driven by zonally integrated wind‐stress curl over the width of subtropical basins. This cross‐basin integration is interrupted in the South Indian Ocean where Madagascar presents a formidable barrier. Nevertheless, a western boundary current has been thought to exist in the Mozambique Channel, the Mozambique Current. Recent observations have however shown that no such current exists and that the flow in the channel instead consists of a train of eddies. Is this western boundary anomaly due to the presence of Madagascar? We have used a primitive equations model to investigate the flow in the South West Indian Ocean as if there were no Madagascar. We show that a normal, continuous western boundary current is then formed that constitutes a continuum with the Agulhas Current. The presence of Madagascar is shown to affect the frequency of inter‐ocean exchange events south of Africa.

Abrupt environmental shift associated with changes in the distribution of Cape anchovyEngraulis encrasicolusspawners in the southern Benguela

“…Additional work, outside the scope of this paper, to test our hypothesis of a strengthening of coastal upwelling along the east coast of the Agulhas Bank in 1996 and thereafter, could involve detailed analysis of high-resolution, satellite-derived SST and chlorophyll a images, as well as high-resolution, hydrodynamic modelling using regional models (e.g. Penven et al 2006).…”

Ocean variability over the Agulhas Bank and its dynamical connection with the southern Benguela upwelling system

“…Ocean, and on the deepness of its penetration in the Atlantic Ocean. Upstream variability, possibly induced by Natal pulses or shear edge features with origins as remote as the Mozambique Channel, may also trigger the behavior of the Agulhas Current south of the Agulhas Bank [van Leeuwen et al, 2000;Penven et al, 2006a;Quartly et al, 2006], making its dynamics somewhat unpredictable. Then, though our study could not associate intraseasonal variability in mass transfer with specific wind events, one must keep in mind that regional ocean modeling could still benefit from improved atmospheric forcing fields.…”

“…Among other improvements, the SAfE approach [Penven et al, 2006a[Penven et al, , 2006b] we use in this new study allows better coupling of the coastal dynamics with the surrounding large-scale ocean dynamics (using the AGRIF system), better representation of mesoscale features thanks to a slightly higher spatial resolution, and genuine account for interannual surface wind stress variability (as present in the NCEP reanalysis). The SAfE modeling approach has been shown to produce realistic and potent results for the study of various marine ecosystems along the coast of southern Africa [Penven et al, 2006a[Penven et al, , 2006bVeitch et al, 2009]. Within this framework, large intraseasonal anomalies diagnosed in the transfer of water from the Agulhas Bank to the southern Benguela upwelling system find their origin mostly in the presence and movement of mesoscale structures along the western edge of the Agulhas Bank.…”

“…[5] The parent model corresponds to the Southern Africa Experiment (SAfE) [Penven et al, 2006a[Penven et al, , 2006b]. This ROMS configuration was built using ROMSTOOLS (http:// roms.mpl.ird.fr/ [Penven et al, 2007]) and is designed for the resolution of the major phenomena around southern Africa.…”

“…We put the stress on wind forcing since the seasonal and interannual variability of the Benguela upwelling system depends significantly on wind variability [Blanke et al, 2002[Blanke et al, , 2005, with the appearance of subsequent SST anomalies along the coast and possible long-term spatial reorganization of the marine ecosystem [van der Lingen et al, 2002]. We use the IRD (Institut de Recherche pour le Développe-ment) version of the ROMS-UCLA, free surface, primitive equation ocean model [see Shchepetkin and McWilliams, 2005;Penven et al, 2006a]. Particle trajectories calculated with the ARIANE algorithm [Blanke and Raynaud, 1997;Blanke et al, 1999] are used to depict the advective transport of water from the Agulhas Bank to the southern Benguela upwelling system over 1993 -2006, a period over which the model is forced with National Centers for Environmental Prediction (NCEP) atmospheric surface fluxes.…”