The causes of full ocean depth interannual variability in Drake Passage

In recent years a number of large scale modes of Southern Hemisphere climate variability have been observed, most notably the Southern Annular Mode (SAM, e.g. Thompson and Solomon, 2002), the Pacific South American modes (PSA, e.g. Mo and Peagle, 2001), the Antarctic Dipole (e.g. Martinson and Ianuzzi, 2003), the Antarctic Circumpolar Wave (e.g. White and Peterson, 1996), and of course the El Niño Southern Oscillation (ENSO). All have pronounced effects over or in the Southern Ocean, and may be expected to account for a significant part of the interannual variability observed there. Most studies analyse these phenomena from a large-scale point of view, often by extracting modes from Southern Hemisphere atmospheric and oceanic fields using various mathematical techniques. In this study we have taken an alternative approach, and tried to understand the causes of the full ocean depth variability in Drake Passage observed in the WOCE SR1b repeat hydrographic sections (Cunningham et al. 2003)

Internal lee wave closures: Parameter sensitivity and comparison to observations

This is the final version. Available from AGU via the DOI in this recordThe SOFine and DIMES data analyzed in this paper can be obtained through the British Oceanographic Data Centre (BODC) by navigating the following links, respectively: http://archive.noc.ac.uk/SOFINE/and http://dimes.ucsd.edu/en/data/This paper examines two internal lee wave closures that have been used together with ocean models to predict the time‐averaged global energy conversion rate into lee waves and dissipation rate associated with lee waves and topographic blocking: the Garner (2005) scheme and the Bell (1975) theory. The closure predictions in two Southern Ocean regions where geostrophic flows dominate over tides are examined and compared to microstructure profiler observations of the turbulent kinetic energy dissipation rate, where the latter are assumed to reflect the dissipation associated with topographic blocking and generated lee wave energy. It is shown that when applied to these Southern Ocean regions, the two closures differ most in their treatment of topographic blocking. For several reasons, pointwise validation of the closures is not possible using existing observations, but horizontally averaged comparisons between closure predictions and observations are made. When anisotropy of the underlying topography is accounted for, the two horizontally averaged closure predictions near the seafloor are approximately equal. The dissipation associated with topographic blocking is predicted by the Garner (2005) scheme to account for the majority of the depth‐integrated dissipation over the bottom 1000 m of the water column, where the horizontally averaged predictions lie well within the spatial variability of the horizontally averaged observations. Simplifications made by the Garner (2005) scheme that are inappropriate for the oceanic context, together with imperfect observational information, can partially account for the prediction‐observation disagreement, particularly in the upper water column.D. S. Trossman and B. K. Arbic gratefully acknowledge support from National Science Foundation (NSF) grant OCE‐0960820 and Office of Naval Research (ONR) grant N00014‐11‐1‐0487. S. Waterman gratefully acknowledges support from the Australian Research Council (grants DE120102927 and CE110001028) and the National Science and Engineering Research Council of Canada (grant 22R23085)

Modification of turbulent dissipation rates by a deep Southern Ocean eddy

This is the final version. Available from AGU via the DOI in this recordAll data used in this study are available by communication with the author and will be archived at British Oceanographic Data CentreThe impact of a mesoscale eddy on the magnitude and spatial distribution of diapycnal ocean mixing is investigated using a set of hydrographic and microstructure measurements collected in the Southern Ocean. These data sampled a baroclinic, middepth eddy formed during the disintegration of a deep boundary current. Turbulent dissipation is suppressed within the eddy but is elevated by up to an order of magnitude along the upper and lower eddy boundaries. A ray tracing approximation is employed as a heuristic device to elucidate how the internal wave field evolves in the ambient velocity and stratification conditions accompanying the eddy. These calculations are consistent with the observations, suggesting reflection of internal wave energy from the eddy center and enhanced breaking through critical layer processes along the eddy boundaries. These results have important implications for understanding where and how internal wave energy is dissipated in the presence of energetic deep geostrophic flows.DIMES is supported by the Natural Environment Research Council (NERC) grants NE/E007058/1 and NE/E005667/1 and U.S. National Science Foundation grants OCE‐1231803, OCE‐0927583, and OCE‐1030309. K.L.S. and J.A.B. are supported by NERC

Stabilization of dense Antarctic water supply to the Atlantic Ocean overturning circulation

The lower limb of the Atlantic overturning circulation is resupplied by the sinking of dense Antarctic Bottom Water (AABW) that forms via intense air–sea–ice interactions next to Antarctica, especially in the Weddell Sea. In the last three decades, AABW has warmed, freshened and declined in volume across the Atlantic Ocean and elsewhere, suggesting an ongoing major reorganization of oceanic overturning. However, the future contributions of AABW to the Atlantic overturning circulation are unclear. Here, using observations of AABW in the Scotia Sea, the most direct pathway from the Weddell Sea to the Atlantic Ocean, we show a recent cessation in the decline of the AABW supply to the Atlantic overturning circulation. The strongest decline was observed in the volume of the densest layers in the AABW throughflow from the early 1990s to 2014; since then, it has stabilized and partially recovered. We link these changes to variability in the densest classes of abyssal waters upstream. Our findings indicate that the previously observed decline in the supply of dense water to the Atlantic Ocean abyss may be stabilizing or reversing and thus call for a reassessment of Antarctic influences on overturning circulation, sea level, planetary-scale heat distribution and global climate

Contribution of topographically-generated submesoscale turbulence to Southern Ocean overturning

The ocean’s global overturning circulation regulates the transport and storage of heat, carbon and nutrients. Upwelling across the Southern Ocean’s Antarctic Circumpolar Current and into the mixed layer, coupled to water mass modification by surface buoyancy forcing, has been highlighted as a key process in the closure of the overturning circulation. Here, using twelve high-resolution hydrographic sections in southern Drake Passage, collected with autonomous ocean gliders, we show that Circumpolar Deep Water originating from the North Atlantic, known as Lower Circumpolar Deep Water, intersects sloping topography in narrow and strong boundary currents. Observations of strong lateral buoyancy gradients, enhanced bottom turbulence, thick bottom mixed layers and modified water masses are consistent with growing evidence that topographically generated submesoscale flows over continental slopes enhance near-bottom mixing, and that cross-density upwelling occurs preferentially over sloping topography. Interactions between narrow frontal currents and topography occur elsewhere along the path of the Antarctic Circumpolar Current, which leads us to propose that such interactions contribute significantly to the closure of the overturning in the Southern Ocean

Spatiotemporal characteristics of the near-surface turbulent cascade at the submesoscale in the Drake Passage

Abstract: Submesoscale currents and internal gravity waves achieve an intense turbulent cascade near the ocean surface (0 m – O(100) m depth), which is thought to give rise to significant energy sources and sinks for mesoscale eddies. Here, we characterise the contributions of Non-Wave Currents (NWCs; including eddies and fronts) and Internal Gravity Waves (IGWs; including near-inertial motions, lee waves and the internal wave continuum) to near-surface submesoscale turbulence in the Drake Passage. Using a numerical simulation, we combine Lagrangian filtering and a Helmholtz decomposition to identify NWCs and IGWs and to characterise their dynamics (rotational vs. divergent). We show that NWCs and IGWs contribute in different proportions to the inverse and forward turbulent kinetic energy cascades, based on their dynamics and spatiotemporal scales. Purely rotational NWCs cause most of the inverse cascade, while coupled rotational– divergent components of NWCs and coupled NWC–IGWs cause the forward cascade. The cascade changes direction at a spatial scale at which motions become increasingly divergent. However, the forward cascade is ultimately limited by the motions’ spatiotemporal scales. The bulk of the forward cascade (80 – 95%) is caused by NWCs and IGWs of small spatiotemporal scales (L <10 km; T <6 hours), which are primarily rotational: submesoscale eddies, fronts, and the internal wave continuum. These motions also cause a significant part of the inverse cascade (30%). Our results highlight the requirement for high spatiotemporal resolutions to diagnose the properties and large-scale impacts of near-surface submesoscale turbulence accurately, with significant implications for ocean energy cycle study strategies

Sensitivity of Deep Ocean Mixing to Local Internal Tide Breaking and Mixing Efficiency

There have been recent advancements in the quantification of parameters describing the proportion of internal tide energy being dissipated locally and the “efficiency” of diapycnal mixing, i.e. the ratio of the diapycnal mixing rate to the kinetic energy dissipation rate. We show that oceanic tidal mixing is non‐trivially sensitive to the co‐variation of these parameters. Varying these parameters one at the time can lead to significant errors in the patterns of diapycnal mixing driven upwelling and downwelling, and to the over and under estimation of mixing in such a way that the net rate of globally‐integrated deep circulation appears reasonable. However, the local rates of upwelling and downwelling in the deep ocean are significantly different when both parameters are allowed to co‐vary and be spatially variable. These findings have important implications for the representation of oceanic heat, carbon, nutrients and other tracer budgets in general circulation models. Plain Language Summary Deep ocean basins are filled with dense waters that form at high latitudes and sink to the abyss. The overturning circulation of the ocean, a key regulator of the climate system, is only feasible if such dense waters can resurface. The breaking of internal waves makes such resurfacing possible. In the deep ocean, internal waves are largely generated by the flow of tides over topography. Their breaking mixes dense deep waters with lighter waters above them, bringing them upward. Two key parameters in climate models for modeling such mixing are: (I) the ratio of energy in the wave field that is spent near rough topography due to breaking as opposed to what is radiated away; and (II) the amount of energy from wave breaking that goes to mixing versus what is wasted through dissipation by viscosity of seawater. Both parameters are considered constant in climate models. In this work, we quantify the roles of variations in each of these two parameters in setting the patterns of deep ocean upwelling of dense waters and argue that the two parameters need to be changed realistically and inter‐dependently to avoid significant inaccuracies in the quantification of the mixing‐induced deep branch of ocean circulation

The Flow of Dense Water Plumes in the Western Weddell Sea Simulated with the Finite Element Ocean Model (FEOM)

Ocean simulations performed with the Finite Element Ocean Model (FEOM) were used to show the relevance of the location of the dense water plume source on the western Weddell Sea continental shelf. When the plume starts close to the tip of the Antarctic Peninsula it flows into Bransfield Strait, but if it is found further south it can flow down the slope and contribute to Weddell Sea Deep Water (WSDW). The influence of density on the spreading was also tested indicating that a denser plume reaches greater depths while lighter plumes do not interact with the WSDW