Agulhas Current Meanders Facilitate Shelf‐Slope Exchange on the Eastern Agulhas Bank

Abstract

Large solitary meanders are arguably the dominant mode of variability in the Agulhas Current. Observational studies have shown that these large meanders are associated with strong upwelling velocities and affect the shelf circulation for over 100 days per year. Here 10‐year time series from two ocean general circulation models are used to create a composite picture of the Agulhas Current and its interactions with the shelf circulation in meandering and nonmeandering modes. Both models show good agreement with the size, propagation speed, and frequency of observed meanders. These composite meanders are then used to examine the response of shelf waters to the onset of large meanders, with the use of model output enabling the dynamics at depth to be explored. Results show a composite mean warming of up to 3°C of depth‐averaged temperature along the shelf edge associated with an intrusion of the current jet onto the shelf driven by an intensification of the flow along the leading edge of large meanders. However, this intensification of flow results in cooling of bottom waters, driving cold events at the shelf break of <10°C at 100 m. Thus, the intensification of the current jet associated with large meander events appears to drive strong up and downwelling events across the inshore front of the Agulhas Current, facilitating shelf‐slope exchange.

Multidecadal Wind Variability Drives Temperature Shifts on the Agulhas Bank

“…A depth of 100 m was chosen in order to minimize the impact of the wind‐driven component of the flow. To generate the time series, using a modification of the method established in Malan et al (), the intensity of the current is taken to be the maximum scalar speed at 100 m along the longitude of 24°E between 34°S and 36°S. Both the position and speed show considerable interannual variability, but not the significant multidecadal variability seen in both temperature and zonal wind stress.…”

“…Figure 7 shows the intensity and position of the Agulhas Current's core at 24 • E. A depth of 100 m was chosen in order to minimize the impact of the wind-driven component of the flow. To generate the time series, using a modification of the method established in Malan et al (2018), the intensity of the current is taken to be the maximum scalar speed at 100 m along the longitude of 24 • E between 34 • S and 36 • S. Both the position and speed show considerable interannual variability, but not the significant multidecadal variability seen in both temperature and zonal wind stress. The largest decadal shifts in intensity take place in the late 1960s and the late 1980s but are far smaller then the shifts seen in temperature and wind stress, while position shows interannual, but little decadal variability.…”

Environmental controls on marine productivity near Cape St. Francis, South Africa

“…1a and b. Sharp gradients in the mean SST field were evident: inshore waters are 17-19 • C, while offshore waters are 22-24 • C. Shelf-edge upwelling next to the warm Agulhas Current divided the two regimes (Lutjeharms et al, 2000;Malan et al, 2018). Over the cool inshore waters, the sensible heat flux was low (Fig.…”

“…The southern coast of South Africa is swept by the prevailing warm Agulhas Current (Lutjeharms et al, 2000). Shelfedge upwelling is induced by cyclonic shear of the current and downstream widening of the shelf (Schumann, 1986, 1987, Lutjeharms, 2006Goschen et al, 2015;Malan et al, 2018). Coastal winds average 7 m s −1 (Schumann and Martin, 1991) and tend to blow from east in summer and from the west in winter.…”

The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations

“…The predecessor of these configurations is INALT01 (Durgadoo et al, 2013), a well-established 1/10 • model configuration. INALT01 has been widely used to understand AC dynamics (Rühs et al, 2017;Malan et al, 2018), the effect of the Southern Hemisphere wind systems on the AC (Durgadoo et al, 2013;Loveday et al, 2014), the impact of Agulhas leakage on the Atlantic circulation and hydrography (Biastoch et al, 2015;Lübbecke et al, 2015), as well as local phenomena in the (greater) Agulhas region (Cronin et al, 2013;Malan et al, 2019), and has been utilized for a range of interdisciplinary applications (Scussolini et al, 2013;Steinhardt et al, 2014;. The set of configurations described here updates INALT01 with respect to the code version.…”