The response of the ventilation of mode and intermediate waters to abrupt changes in the Southern Annular Mode (SAM) is examined by analyzing the ideal age in a global ocean-sea ice model. The age response is shown to differ between the central Pacific Ocean and other basins. In the central Pacific there are large decreases in the age of subtropical mode and intermediate waters associated with a more positive SAM, contrasting only small age changes in the Atlantic and Indian Oceans, except near where intermediate water density surfaces outcrop. These interbasin differences hold for simulations at different horizontal resolutions, and can be explained by the combination of zonal variations in wind stress changes associated with the SAM, and differences in the age response to an increase or shift in the wind stress. These results suggest that the carbon and heat uptake associated with the SAM will likely vary between ocean basins.

The positive trend of the Southern Annular Mode (SAM) will impact the Southern Ocean’s role in Earth’s climate, however the details of the Southern Ocean’s response remain uncertain. We introduce a methodology to examine the influence of SAM on the Southern Ocean, and apply this method to a global ocean–sea-ice model run at three resolutions (1$^{\circ}$, 1/4$^{\circ}$ and 1/10$^{\circ}$). Our methodology drives perturbation simulations with realistic atmospheric forcing of extreme SAM conditions. The thermal response agrees with previous studies; positive SAM perturbations warm the upper ocean north of the windspeed maximum and cool it to the south, with the opposite response for negative SAM. The overturning circulation exhibits a rapid response that increases/decreases for positive/negative SAM perturbations and is insensitive to model resolution. The longer term adjustment of the overturning circulation, however, depends on the representation of eddies, and is faster at higher resolutions.

We introduce a novel wind-derived metric to quantify variability in the Southern Ocean overturning circulation. This metric, which we call the Ekman streamfunction, integrates the Ekman pumping vertical velocity zonally and northwards from the Antarctic coastline to a given latitude. To evaluate the relationship between the Ekman streamfunction and Southern Ocean overturning circulation, we use a global 0.1 ocean–sea-ice model driven with interannual forcing (1958-2018). Throughout much of the Southern Ocean, strong correlations (r>0.9) exist between the Ekman streamfunction and the Southern Ocean overturning circulation on monthly and annual timescales. A regression analysis identifies regions where Ekman streamfunction variability coincides with >4Sv changes in the overturning; one such location is where the wind stress curl changes sign and the Ekman pumping velocity is highly variable. This study offers a new approach to infer recent changes in the Southern Ocean overturning circulation from existing datasets of wind stress.

Motivated by recent advances in mapping mesoscale eddy tracer mixing in the ocean we evaluate the sensitivity of a coarse-resolution global ocean model to a spatially variable neutral diffusion coefficient $\kappa_n(x,y,z)$. We gradually introduce physically-motivated models for the horizontal (mixing length theory) and vertical (surface mode theory) structure of $\kappa_n$ along with suppression of mixing by mean flows. Each structural feature influences the ocean’s hydrography and circulation to varying extents, with the suppression of mixing by mean flows being the most important factor and the vertical structure being relatively unimportant. When utilizing the full theory (experiment “FULL’) the interhemispheric overturning cell is strengthened by $2$ Sv at $26^\circ$N (a $\sim20\%$ increase), bringing it into better agreement with observations. Zonal mean tracer biases are also reduced in FULL. Neutral diffusion impacts circulation through surface temperature-induced changes in surface buoyancy fluxes and non-linear equation of state effects. Surface buoyancy forcing anomalies are largest in the Southern Ocean where decreased neutral diffusion in FULL leads to surface cooling and enhanced dense-to-light surface watermass transformation, reinforced by reductions in cabbeling and thermobaricity. The increased watermass transformation leads to enhanced mid-latitude stratification and interhemispheric overturning. The spatial structure for $\kappa_n$ in FULL is important as it enhances the interhemispheric cell without degrading the Antarctic bottom water cell, unlike a spatially-uniform reduction in $\kappa_n$. These results highlight the sensitivity of modeled circulation to $\kappa_n$ and motivate the use of physics-based models for its structure.

The meandering jet streams of the Northern Hemisphere influence the weather for more than half of Earth’s population, so it is imperative that we improve our understanding of their behaviour and how they might respond to climate change. Here we describe a novel laboratory model for a meandering zonal jet. This model comprises a large rotating annulus with a series of topographic ridges, and an imposed radial vorticity flux. Flow interactions with the topographic ridges operate to concentrate the zonal transport into a narrow jet, which supports the development and propagation of Rossby waves. We investigate the dynamics of the jet for a range of rotation rates, imposed radial vorticity fluxes, and topographic ridge configurations. The circulations are classified into two distinct regimes: predominantly zonal, or predominantly meandering. The flow regime can be quantified by the ratio of the Ekman dissipation and jet advection timescales, which gives an indication of whether disturbances arising from the flow-topography interaction are dissipated faster than the time taken to circuit the annulus; if not, these disturbances will re-encounter the topography, and thus be reinforced and amplified. For predominantly zonal flows, the radial vorticity flux is mainly performed by transient eddies. For predominantly meandering flows, standing meanders perform 81+/-14% of the radial vorticity flux, with 15+/-16% accommodated by the transient eddies. Our experiments indicate that the Arctic amplification associated with climate change will tend to favour predominantly zonal flow conditions, suggesting a reduced occurrence of atmospheric blocking events caused by the jet streams.

The generation of topographic internal waves (IWs) by the sum of an oscillatory and a steady flow is investigated experimentally and with a linear model. The two forcing flows represent the combination of a tidal constituent and a weaker quasi-steady flow interacting with an abyssal hill. The combined forcings cause a coupling between internal tides and lee waves that impacts their dynamics of internal waves as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream internal wave beams due to a Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a super-buoyancy (ω>N) downstream beam and an evanescent upstream beam are measured. Energy fluxes are measured and compared with the linear model, that has been extended to account for the coupling mechanism. The structure and amplitude of energy fluxes match well in most regimes, showing the relevance of the linear prediction for IW wave energy budgets, while the energy flux toward IW beams is limited by the generation of periodic vortices in a particular experiment. The upstream-bias energy flux - and consequently net horizontal momentum - described in Shakespeare [2020] is measured in the experiments. The coupling mechanism plays an important role in the pathway to IW induced mixing, that has previously been quantified independently for lee waves and internal tides. Hence, future parameterizations of IW processes ought to include the coupling mechanism to quantify its impact on the global distribution of mixing.