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Joke F. Lübbecke, Jonathan V. Durgadoo, and Arne Biastoch

Abstract

The upper tropical Atlantic Ocean has markedly warmed since the 1960s. It has been shown that this warming was not due to local heat fluxes and that the trade winds that drive the coastal and equatorial upwelling have intensified rather than weakened. Remote forcing might thus have played an important role. Here, model experiments are used to investigate the contribution from an increased inflow of warm Indian Ocean water through Agulhas leakage. A high-resolution hindcast experiment with interannually varying forcing for the time period 1948–2007, in which Agulhas leakage increases by about 45% from the 1960s to the early 2000s, reproduces the observed warming trend. To tease out the role of Agulhas leakage, a sensitivity experiment designed to only increase Agulhas leakage is used. Compared to a control simulation, it shows a pronounced warming in the upper tropical Atlantic Ocean. A Lagrangian trajectory analysis confirms that a significant portion of Agulhas leakage water reaches the upper 300 m of the tropical Atlantic Ocean within two decades and that the tropical Atlantic warming in the sensitivity experiment is mainly due to water of Agulhas origin. Therefore, it is suggested that the increased trade winds since the 1960s favor upwelling of warmer subsurface waters, which in part originate from the Agulhas, leading to higher SSTs in the tropics.

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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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Siren Rühs, Victor Zhurbas, Inga M. Koszalka, Jonathan V. Durgadoo, and Arne Biastoch

Abstract

The Lagrangian analysis of sets of particles advected with the flow fields of ocean models is used to study connectivity, that is, exchange pathways, time scales, and volume transports, between distinct oceanic regions. One important factor influencing the dispersion of fluid particles and, hence, connectivity is the Lagrangian eddy diffusivity, which quantifies the influence of turbulent processes on the rate of particle dispersal. Because of spatial and temporal discretization, turbulence is not fully resolved in modeled velocities, and the concept of eddy diffusivity is used to parameterize the impact of unresolved processes. However, the relations between observation- and model-based Lagrangian eddy diffusivity estimates, as well as eddy parameterizations, are not clear. This study presents an analysis of the spatially variable near-surface lateral eddy diffusivity estimates obtained from Lagrangian trajectories simulated with 5-day mean velocities from an eddy-resolving ocean model (INALT01) for the Agulhas system. INALT01 features diffusive regimes for dynamically different regions, some of which exhibit strong suppression of eddy mixing by mean flow, and it is consistent with the pattern and magnitude of drifter-based eddy diffusivity estimates. Using monthly mean velocities decreases the estimated diffusivities less than eddy kinetic energy, supporting the idea that large and persistent eddy features dominate eddy diffusivities. For a noneddying ocean model (ORCA05), Lagrangian eddy diffusivities are greatly reduced, particularly when the Gent and McWilliams parameterization of mesoscale eddies is employed.

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