Juliet Hermes

On the role of the Agulhas system in ocean circulation and climate

The Atlantic Ocean receives warm, saline water from the Indo-Pacific Ocean through Agulhas leakage around the southern tip of Africa. Recent findings suggest that Agulhas leakage is a crucial component of the climate system and that ongoing increases in leakage under anthropogenic warming could strengthen the Atlantic overturning circulation at a time when warming and accelerated meltwater input in the North Atlantic is predicted to weaken it. Yet in comparison with processes in the North Atlantic, the overall Agulhas system is largely overlooked as a potential climate trigger or feedback mechanism. Detailed modelling experiments—backed by palaeoceanographic and sustained modern observations—are required to establish firmly the role of the Agulhas system in a warming climate.

Inter-ocean fluxes south of Africa in an eddy-permitting model

Abstract Exchanges of water south of Africa between the South Indian Ocean and the South Atlantic Ocean are an important component of the global thermohaline circulation. Evidence exists that the variability in these exchanges, on both meso- and longer time scales, may significantly influence weather and climate patterns in the southern African region and the significance of these regional ocean–atmosphere interactions is discussed. Observations of the inter-ocean exchange are limited and it is necessary to augment these with estimates derived from models. As a first step in this direction, this study uses an eddy-permitting model to investigate the heat and volume transport in the oceanic region south of Africa and its variability on meso, seasonal and inter-annual time scales. On the annual mean, about 0.84 PW (standard deviation 0.13 PW ) of heat flows west into the South Atlantic across 20°E (longitude of Cape Agulhas, the southernmost point of Africa), with just over 1 PW (standard deviation 0.4 PW ) flowing north into the South Atlantic across 35°S. The seasonal variations in this transport are about 10% at 35°S in the South Atlantic and around 20% through 20°E; the model value of 0.66 PW for summer (standard deviation ranging from 0.13 PW in January to 0.17 PW in March) appears consistent with respective estimates of 0.51 and 0.60 PW derived from two WOCE summer cruises southwest of Cape Town to 45°S in 1990 and 1993. Volume transports of the Agulhas Current section through 35°S in the SW Indian Ocean range from 58 to 59 Sv in summer/autumn to 64– 65 Sv in winter/spring. The model results suggest that the inter-ocean exchange south of Africa is highly variable on seasonal through to interannual scales. If this variability is also the case in the real ocean (and the limited observations suggest that this is so), then there are likely to be significant implications for climate.

Agulhas Current Meanders Facilitate Shelf-Slope Exchange on the Eastern Agulhas Bank: MEANDERS ON THE AGULHAS SHELF

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.