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

Abstract

The Agulhas Current system has been analyzed in a nested high-resolution ocean model and compared to observations. The model shows good performance in the western boundary current structure and the transports off the South African coast. This includes the simulation of the northward-flowing Agulhas Undercurrent. It is demonstrated that fluctuations of the Agulhas Current and Undercurrent around 50-70 days are due to Natal pulses and Mozambique eddies propagating downstream. A sensitivity experiment that excludes those upstream perturbations significantly reduces the variability as well as the mean transport of the undercurrent. Although the model simulates undercurrents in the Mozambique Channel and east of Madagascar, there is no direct connection between those and the Agulhas Undercurrent. Virtual float releases demonstrate that topography is effectively blocking the flow toward the north.

Ocean currents generate large footprints in marine palaeoclimate proxies

“…The model is forced using National Centers for Environmental Prediction (NCEP) wind and flux fields. Results from both models have been shown to be consistent with important observed features of the modern ocean circulation, including among others the trajectories of surface buoys 29 and the deep currents in the North Atlantic 30 , the South Atlantic 31 and the Agulhas region 17,32 .…”

Pathways of the Agulhas waters poleward of 29°S

“…Bryden et al [2005] estimated the 267 day averaged southward volume transport across 328S to be 69.7 6 21.5 Sv. SOSE has a northward undercurrent transport of 2.5 6 2.1 Sv (Figure 3b), similar to the 2.7 6 2.6 Sv Agulhas undercurrent in a 1/108 resolution nesteddomain simulation by Biastoch et al [2009b]. These model undercurrent transports are consistent but weaker than the 4.2 6 2.9 Sv in Bryden et al [2005] or the 4.2 6 5.2 Sv in Beal [2009].…”

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“…A portion of the Agulhas waters invade the South Atlantic through the Agulhas rings (Arhan et al, 2011) that transport between 5-20 Sv of warm, saline water from the Indian Ocean to the South Atlanticthe so-called interocean "warm-water route" of the global oceanic thermohaline circulation (THC). Ocean models and paleoceanographic data strongly suggest that the "leakage" of saline Agulhas water into the South Atlantic stimulates regional buoyancy anomalies that ultimately impact convective activity in the northern North Atlantic, plausibly acting as a contributor to the basin-wide Atlantic Meridional Overturning Circulation (AMOC) ( Figure F1) (Weijer, 2000;Weijer et al, 2002;Knorr and Lohmann, 2003;Van Sebille et al, 2009;Biastoch et al, 2008Biastoch et al, , 2009a. The numerical simulations specifically point to Agulhas leakage as a potential modulator of the AMOC and a rheostat for the ventilation of the deep ocean.…”

“…These studies suggest that (1) this exchange affects the buoyancy of Atlantic thermocline waters, potentially influencing NADW formation on multidecadal timescales (Weijer et al, 2002;Knorr and Lohmann, 2003;Biastoch et al, 2009a) and (2) perturbations of planetary waves by mesoscale eddies influence decadal variability of the AMOC (Van Sebille and van Leeuwen, 2007;Biastoch et al, 2008).…”

“…strongly barotropic solitary meanders (Bryden et al, 2005;Biastoch et al, 2009a;Tsugawa and Hasumi, 2010) destabilize the retroflection to the extent that four to six up to 400 km wide anticyclonic eddies (Agulhas rings) form per year and spin off the retroflection area carrying between 2 and 20 Sv of thermocline water to the South Atlantic (de Ruijter et al, 1999a;Richardson, 2007). Some researchers have suggested that Agulhas leakage is linked with interannual modes of Indian and Pacific variability, thus proposing a connection with the IOD and Pacific La Niña/El Niño phases (Schouten et al, 2002b;Palastanga et al, 2006).…”