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Lionel Renault, James C. McWilliams, and Pierrick Penven
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Jennifer Veitch, Pierrick Penven, and Frank Shillington

Abstract

The Regional Ocean Modeling System (ROMS) is used to systematically investigate equilibrium conditions and seasonal variations of the Benguela system at a resolution of 9 km, including both the large-scale offshore flow regime and the economically and ecologically important coastal upwelling regime. A shelf-edge poleward flow exists in the northern Benguela region (i.e., north of ∼28°S) and is driven primarily by the wind stress curl via the Sverdrup relation. As such, it is strongly seasonal and is most intense during spring and summer, when the wind stress curl is most negative. The poleward flow deepens as it moves southward; between ∼25° and 27°S, much of it veers offshore because of the nature of the wind stress curl and its interaction with the northwestward path of the Benguela Current, which is influenced by alongshore topographical variations. The Benguela Current is driven by nonlinear interactions of passing Agulhas rings and eddies and does not have a striking seasonal signal. In the mean state, it is characterized by two streams. The more inshore stream is topographically controlled and follows the run of the shelf edge. The meandering nature of the offshore stream is a result of the preferential path of Agulhas rings. The model simulates all seven of the major upwelling cells within its domain. The three southernmost cells have the strongest seasonal signal and experience their greatest upwelling during spring and summer months, whereas the two northernmost cells have less seasonal variability but nevertheless have increased upwelling from autumn to spring (and least upwelling in summer), and the central Benguela upwelling cells experience year-round upwelling. The effect of topography on coastal upwelling was investigated by smoothing alongshore coastline and topography variations, which showed that, in all of the seven major upwelling cells, upwelling is enhanced on the downstream side of capes.

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Lionel Renault, James C. McWilliams, and Pierrick Penven

Abstract

Coupled ocean–atmosphere simulations are carried out for the Mozambique Channel, the Agulhas Current system, and the Benguela upwelling system to assess the ocean surface current feedback to the atmosphere and its impact on the Agulhas Current (AC) retroflection and leakage. Consistent with previous studies, the authors show that the current feedback slows down the oceanic mean circulation and acts as an oceanic eddy killer by modulating the energy transfer between the atmosphere and the ocean, reducing by 25% the mesoscale energy and inducing a pathway of energy transfer from the ocean to the atmosphere. The current feedback, by dampening the eddy kinetic energy (EKE), shifts westward the distribution of the AC retroflection location, reducing the presence of eastern retroflections in the simulations and improving the realism of the AC simulation. By modulating the EKE, the AC retroflection and the Good Hope jet intensity, the current feedback allows a larger AC leakage (by 21%), altering the water masses of the Benguela system. Additionally, the eddy shedding is shifted northward and the Agulhas rings propagate less far north in the Atlantic. The current–wind coupling coefficient s w is not spatially constant: a deeper marine boundary layer induces a weaker s w. Finally the results indicate that the submesoscale currents may also be weakened by the current feedback.

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Benjamin R. Loveday, Jonathan V. Durgadoo, Chris J. C. Reason, Arne Biastoch, and Pierrick Penven

Abstract

The relationship between the Agulhas Current and the Agulhas leakage is not well understood. Here, this is investigated using two basin-scale and two global ocean models of incrementally increasing resolution. The response of the Agulhas Current is evaluated under a series of sensitivity experiments, in which idealized anomalies, designed to geometrically modulate zonal trade wind stress, are applied across the Indian Ocean Basin. The imposed wind stress changes exceed plus or minus two standard deviations from the annual-mean trade winds and, in the case of intensification, are partially representative of recently observed trends. The Agulhas leakage is quantified using complimentary techniques based on Lagrangian virtual floats and Eulerian passive tracer flux. As resolution increases, model behavior converges and the sensitivity of the leakage to Agulhas Current transport anomalies is reduced. In the two eddy-resolving configurations tested, the leakage is insensitive to changes in Agulhas Current transport at 32°S, though substantial eddy kinetic energy anomalies are evident. Consistent with observations, the position of the retroflection remains stable. The decoupling of Agulhas Current variability from the Agulhas leakage suggests that while correlations between the two may exist, they may not have a clear dynamical basis. It is suggested that present and future Agulhas leakage proxies should be considered in the context of potentially transient forcing regimes.

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Jonathan V. Durgadoo, Benjamin R. Loveday, Chris J. C. Reason, Pierrick Penven, and Arne Biastoch

Abstract

The Agulhas Current plays a crucial role in the thermohaline circulation through its leakage into the South Atlantic Ocean. Under both past and present climates, the trade winds and westerlies could have the ability to modulate the amount of Indian–Atlantic inflow. Compelling arguments have been put forward suggesting that trade winds alone have little impact on the magnitude of Agulhas leakage. Here, employing three ocean models for robust analysis—a global coarse-resolution, a regional eddy-permitting, and a nested high-resolution eddy-resolving configuration—and systematically altering the position and intensity of the westerly wind belt in a series of sensitivity experiments, it is shown that the westerlies, in particular their intensity, control the leakage. Leakage responds proportionally to the intensity of westerlies up to a certain point. Beyond this, through the adjustment of the large-scale circulation, energetic interactions occur between the Agulhas Return Current and the Antarctic Circumpolar Current that result in a state where leakage no longer increases. This adjustment takes place within one or two decades. Contrary to previous assertions, these results further show that an equatorward (poleward) shift in westerlies increases (decreases) leakage. This occurs because of the redistribution of momentum input by the winds. It is concluded that the reported present-day leakage increase could therefore reflect an unadjusted oceanic response mainly to the strengthening westerlies over the last few decades.

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