Description: Sub-Antarctic islands represent critical breeding habitats for land-based top predators that dominate Southern Ocean food webs. Reproduction and molting incur high energetic demands that are sustained at the sub-Antarctic Prince Edward Islands (PEIs) by both inshore (phytoplankton blooms; "island mass effect"; autochthonous) and offshore (allochthonous) productivity. As the relative contributions of these sustenance pathways are, in turn, affected by oceanographic conditions around the PEIs, we address the consequences of climatically driven changes in the physical environment on this island ecosystem. We show that there has been a measurable long-term shift in the carbon isotope signatures of the benthos inhabiting the shallow shelf region of the PEIs, most likely reflecting a long-term decline in enhanced phytoplankton productivity at the islands in response to a climate-driven shift in the position of the sub-Antarctic Front. Our results indicate that regional climate change has affected the balance between allochthonous and autochthonous productivity at the PEIs. Over the last three decades, inshore-feeding top predators at the islands have shown a marked decrease in their population sizes. Conversely, population sizes of off-shore-feeding predators that forage over great distances from the islands have remained stable or increased, with one exception. Population decline of predators that rely heavily on organisms inhabiting the inshore region strongly suggest changes in prey availability, which are likely driven by factors such as fisheries impacts on some prey populations and shifts in competitive interactions among predators. In addition to these local factors, our analysis indicates that changes in prey availability may also result indirectly through regional climate change effects on the islands' marine ecosystem. Most importantly, our results indicate that a fundamental shift in the balance between allochthonous and autochthonous trophic pathways within this island ecosystem may be detected throughout the food web, demonstrating that the most powerful effects of climate change on marine systems may be indirect.

Description: A maximum in the strength of Agulhas leakage has been registered at the interface between the Indian and South Atlantic oceans during glacial Termination II (T-II). This presumably transported the salt and heat necessary for maintaining the Atlantic circulation at rates similar to the present day. However, it was never shown whether these waters were effectively incorporated into the South Atlantic gyre, or whether they retroflected into the Indian and/or Southern oceans. To resolve this question, we investigate the presence of paleo Agulhas rings from a sediment core on the central Walvis Ridge, almost 1800 km farther into the Atlantic Basin than previously studied. Analysis of a 60 yr data set from the global-nested INALT01 model allows us to relate density perturbations at the depth of the thermocline to the passage of individual rings over the core site. Using this relation from the numerical model as the basis for a proxy, we generate a time series of variability of individual Globorotalia truncatulinoides delta O-18. We reveal high levels of pycnocline depth variability at the site, suggesting enhanced numbers of Agulhas rings moving into the South Atlantic Gyre around T-II. Our record closely follows the published quantifications of Agulhas leakage from the east of the Cape Basin, and thus shows that Indian Ocean waters entered the South Atlantic circulation. This provides crucial support for the view of a prominent role of the Agulhas leakage in the shift from a glacial to an interglacial mode of the Atlantic circulation.

Description: 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 idealised anomalies, designed to geometrically modulate zonal trade wind stress, are applied across the Indian Ocean basin. The imposed wind stress changes exceed ±2 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 be considered in the context of potentially transient forcing regimes.

Description: The interoceanic transfer of seawater between the Indian Ocean and the Atlantic, ‘Agulhas leakage’, forms a choke point for the overturning circulation in the global ocean. Here, by combining output from a series of high-resolution ocean and climate models with in situ and satellite observations, we construct a time series of Agulhas leakage for the period 1870–2014. The time series demonstrates the impact of Southern Hemisphere westerlies on decadal timescales. Agulhas leakage shows a correlation with the Atlantic Multi-decadal Oscillation on multi-decadal timescales; the former leading by 15 years. This is relevant for climate in the North Atlantic

Description: If unstructured meshes are refined to locally represent eddy dynamics in ocean circulation models, a practical question arises on how to vary the resolution and where to deploy the refinement. We propose to use the observed sea surface height variability as the refinement criterion. We explore the utility of this method (i) in a suite of idealized experiments simulating a wind-driven double gyre flow in a stratified circular basin and (ii) in simulations of global ocean circulation performed with FESOM. Two practical approaches of mesh refinement are compared. In the first approach the uniform refinement is confined within the areas where the observed variability exceeds a given threshold. In the second one the refinement varies linearly following the observed variability. The resolution is fixed in time. For the double gyre case it is shown that the variability obtained in a high-resolution reference run can be well captured on variable-resolution meshes if they are refined where the variability is high and additionally upstream the jet separation point. The second approach of mesh refinement proves to be more beneficial in terms of improvement downstream the midlatitude jet. Similarly, in global ocean simulations the mesh refinement based on the observed variability helps the model to simulate high variability at correct locations. The refinement also leads to a reduced bias in the upper-ocean temperature

Description: 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 to 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 300m 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 parts originate from the Agulhas, leading to higher SSTs in the tropics

Description: Mesoscale eddies and meanders have been shown to be one of the dominant sources of flow variability in the world's ocean. One example of an isolated eddy hotspot is the South-West Indian Ridge (SWIR). Several investigations have shown that the SWIR and the corresponding planetary potential vorticity field (f/H) exert a strong influence on the location and dynamics of the Antarctic Circumpolar Current (ACC), resulting in substantial fragmentation of the jets downstream of the ridge. The easterly extension of this eddy corridor appears to be restricted to the deep channel separating the Conrad Rise from the Del Cano and Crozet Plateau. However, while the fate of eddies formed at the SWIR has been widely investigated and the frontal character of this eastward extension is well known, the zone of diminishing variability that extends southwards to approximately 60°S remains poorly sampled. Using a combination of Argo, AVISO and NCEP/NCAR datasets, the character of this eddy corridor as a conduit for warm core eddies to move across the ACC into the Antarctic zone is investigated. In this study, we track a single warm-core eddy as it moves southwards from an original position of 31°E, 50°20'S to where it dissipates 10 months later in the Enderby Basin at 56°20'S. An Argo float entrained within the eddy confirms that its water masses are consistent with water found within the Antarctic Polar Frontal Zone north of the APF. Latent and sensible heat fluxes are on average 8W/m2 and 10W/m2 greater over the eddy than directly east of this feature. It is estimated that the eddy lost an average of 5W/m2 of latent heat and 5W/m2 of sensible heat over a 1-year period, an amount capable of melting approximately 0.92m of sea ice. In addition, using an eddy tracking algorithm a total of 28 eddies are identified propagating southwards, 25 of which are anti-cyclonic in rotation. Based on the new Argo float data, combined with AVISO and NCEP/NCAR datasets, these results suggest that the southward passage of warm-core eddies act as vehicles transporting heat, salt and biota southwards across the ACC and into the eastern boundary of the Weddell gyre.