Eddies in the southern Mozambique Channel

Abstract

The Agulhas Current system contains one of the world's strongest western boundary currents, and plays an important part in the warm water path of the global thermohaline circulation. However, there have been few surveys of the source regions of the Agulhas Current, and thus little in situ measurement of their variability. Utilizing the more than 5-year record of SeaWiFS data, we examine the eddy activity present in the southern portion of the Mozambique Channel. The two sources of Agulhas input from the central Indian Ocean (southward flow through the Mozambique Channel and westward flow around the southern limit of Madagascar) both show great temporal variability, with no clear seasonal signal. A number of large (-200 km diameter) anticyclonic rings intermittently propagate poleward along the western edge of the channel, sweeping coastal waters into mid channel. Their passage past Maputo appears to affect the circulation of the lee eddy in the Delagoa Bight. The eastern side of the channel is mainly characterized by cyclonic eddies. These are made manifest in the lee of the southern tip of Madagascar, although it is not clear whether many form there or just develop a visible presence due to entrainment of high chlorophyll coastal waters. Several of these cyclonic eddies then appear to move in west-southwesterly direction. The chlorophyll data do reveal the East Madagascar retroflection on occasions, but do not show clear examples of the pinching off of anticyclonic eddies. However surface waters from the East Madagascar Current may reach the African mainland on occasions when no retroflection is present.

Mesoscale eddies influence distribution and aggregation patterns of micronekton in the Mozambique Channel

“…Here, we present evidence from acoustic surveys and satellite data showing that the distribution of micronekton aggregations in the Mozambique Channel (southwestern Indian Ocean) is affected by mesoscale eddies. The circulation in the Mozambique Channel is indeed ruled by an important mesoscale activity: large cyclonic and anticyclonic eddies propagate southwards along the western edge of the channel (de Ruijter et al 2002, Schouten et al 2003, Quartly & Srokosz 2004. The spatial overlap between the foraging areas of great frigatebirds and eddies in the Mozambique Channel have already been suggested to be related to high prey availability (Weimerskirch et al 2004), but a link between the prey of pelagic top predators and mesoscale eddies has not been demonstrated previously.…”

“…Cyclonic eddies can generate upwelling of deepcold-nutrient-rich waters towards upper euphotic surface layers in their centre (McGillicuddy et al 1998, Bakun 2006, enhancing local primary production (Seki et al 2001, Mizobata et al 2002, Benitez-Nelson et al 2007). Anticyclonic eddies can also generate an upward movement of rich waters, but located around their edge, where phytoplankton enrichment has also been observed (Mizobata et al 2002, Quartly & Srokosz 2004. Eddies influence the transport and distribution of nutrients and can thereby spatially struc-ture the biological activity over the different trophic levels of the pelagic food chain.…”

“…Eddies influence the transport and distribution of nutrients and can thereby spatially struc-ture the biological activity over the different trophic levels of the pelagic food chain. Several studies have demonstrated that eddies markedly enhanced geochemical transformations and primary production in the subtropical Pacific Ocean (Seki et al 2001, BenitezNelson et al 2007, in the Atlantic Ocean (McGillicuddy et al 1998, Garçon et al 2001, in the Bering Sea (Mizobata et al 2002) and in the Indian Ocean (Quartly & Srokosz 2004, Waite et al 2007). Mesoscale eddies also affect the upper-trophic-level organisms by influencing the horizontal and vertical distribution of mesozooplankton and fish larvae (Bakun 2006, Muhling et al 2007, affecting movement patterns of turtles (Polovina et al 2004, Lambardi et al 2008, attracting tuna (Young et al 2001) or swordfish (Seki et al 2002), and influencing the foraging strategy of seabirds (Nel et al 2001, Weimerskirch et al 2004, Hyrenbach et al 2006.…”

A link between low‐frequency mesoscale eddy variability around Madagascar and the large‐scale Indian Ocean variability

“…At the southern tip of the island the EMC seems to experience a retroflection, in which eddies are shed from the current [Lutjeharms, 1988]. Westward continuation toward south Africa is another possible regime for the EMC after separation from the coast [Quartly and Srokosz, 2004]. The flow of the SEC north of Madagascar bifurcates again at the African coast where it forms to the north the East African Coastal Current (EACC) and to the south the Mozambique Current.…”

“…[3] South of Madagascar cyclonic and anticyclonic eddies have been observed from altimetry propagating (south) westward toward the Agulhas Current (AC) [Gründlingh, 1995;Schouten et al, 2002a;Quartly and Srokosz, 2004]. Additionally, large pairs of counterrotating eddies or dipoles were discovered in this region and measured hydrographically [de Ruijter et al, 2004].…”