Thermobaricity in the Transition Zones between Alpha and Beta Oceans

The role of the ocean in Earth’s climate is fundamentally influenced by the locally dominant stratifying property (heat or salt), which in turn can be used to categorize the ocean into three classes: alpha, beta, and transition zone oceans. Alpha and beta oceans are regions where the stratification is permanently set by heat and salt, respectively. Transition zone oceans exist between alpha and beta oceans and are regions where the stratification is seasonally or intermittently set by heat or salt. Despite their large ranges of temperature and salinity, transition zone oceans are the most weakly stratified regions of the upper oceans, making them ideal locations for thermobaric effects arising from the nonlinear equation of state of seawater. Here a novel definition and quantification of alpha, beta, and transition zone oceans is presented and used to analyze 4 years (2010–13) of hydrographic data developed from the Argo profiling float array. Two types of thermobaric instabilities are defined and identified in the hydrographic data. The first type arises from the vertical relocation of individual water parcels. The second type is novel and relates to the effect of pressure on the stratification through the pressure dependence of the thermal expansion coefficient; water that is stably stratified for one pressure is not necessarily stable for other pressures. The upper 1500 m of the global ocean is composed of 67% alpha, 15% beta, and 17% transition zone oceans, with 5.7% identified as thermobarically unstable. Over 63% of these thermobarically unstable waters exist in transition zone oceans, suggesting that these are important locations for efficient vertical transport of water-mass properties

The role of turbulent mixing in an overturning circulation maintained by surface buoyancy forcing

The role of externally imposed rates of small-scale mixing in an overturning circulation forced by differential surface buoyancy fluxes is examined in a laboratory experiment. The circulation occupies the full volume and involves a dense turbulent plume against the endwall and a broad upwelling throughout the interior. For strong externally imposed stirring, turbulent diffusion is the primary means of vertical density transport in the flow, and the dependence of the equilibrated circulation on the mixing rate accords with a theoretical model; the overturning rate increases as the 1/ 4 power of the turbulent diffusivity. For weak externally imposed stirring, advection is the dominant mechanism of vertical density transport, and the circulation is independent of the rate of external stirring. The rate of vertical density transport is parameterized as a bulk diffusivity obtained from different methods, including one from a Munk-like advection-diffusion balance and another from the transport of buoyancy across the surface. For strong stirring, the bulk diffusivities returned by the various methods agree with the externally imposed mixing rate. However, the parameterizations implicitly include a nondiffusive component of vertical transport associated with advection of the density field and it is shown that, for weak stirring, the bulk diffusivities exceed the externally imposed mixing rate. For the oceans, results suggest that the primary effect of mixing (with energy sourced from winds, tides, and convection) is to deepen the thermocline, thereby influencing the entrainment and consequent vertical transport of density in the dense sinking regions. It is concluded that this advective transport of density, and not vertical mixing, is crucial for coupling the surface to the abyss

When do marginal seas and topographic sills modify the ocean density structure?

We ask what effect marginal seas at high latitudes have on the abyssal densities and stratification of the oceans. Although marginal seas are not necessary for the formation of dense abyssal waters, topographic sills tend to restrict exchange flows and increase density differences. Laboratory experiments with a steady state large-scale overturning circulation, forced by a gradient in surface temperatures or heat fluxes, show that a marginal sea and topographic sill influence the abyssal density when the sill depth is less than twice the oceanic thermocline depth. The overflow over shallow sills interacts directly with the surrounding thermocline stratification, decreasing the abyssal density relative to an ocean with no marginal sea and increasing the density difference between the sea and the ocean. Experiments show that the effect of the sill depends on the surface boundary conditions and indicate a larger response for conditions approaching a prescribed distribution of temperature rather than of heat flux. Application of the results to the North Atlantic circulation suggests that the Greenland-Scotland Ridge is shallow enough to lead to a significant reduction of the density of North Atlantic Deep Water, and this is consistent with the conclusions from an analysis of water mass properties. The Greenland-Scotland Ridge is therefore likely to have a significant effect on the density structure and heat transport in the global oceans

Asymmetric Internal Tide Generation in the Presence of a Steady Flow

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 IWs as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream IW beams due to a quasi-Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a superbuoyancy (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, https://doi.org/10.1175/JPO-D-19-0179.1) 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.This work was supported partly by theFrench PIA project LorraineUniversité d' Excellence, referenceANR-15-IDEX-04-LUE. Y. D.acknowledges support from theEmbassy of France in Australia. C. J. S.acknowledges support from an ARCDiscovery Early Career ResearcherAward DE180100087 and ANU Futures Scheme awar

On cabbeling and thermobaricity in the surface mixed layer

The surface mixed layer (ML) governs atmosphere–ocean fluxes, and thereby affects Earth’s climate. Accurate representation of ML processes in ocean models remains a challenge, however. The O(100) m deep ML exhibits substantial horizontal thermohaline gradients, despite being near-homogenous vertically, making it an ideal location for processes that result from the nonlinearity of the equation of state, such as cabbeling and thermobaricity. Traditional approaches to investigate these processes focus on their roles in interior water-mass transformation and are ill suited to examine their influence on the ML. However, given the climatic significance of the ML, quantifying the extent to which cabbeling and thermobaricity influence the ML density field offers insight into improving ML representations in ocean models. A recent simplified equation of state of seawater allows the local effects of cabbeling and thermobaric processes in the ML to be expressed analytically as functions of the local temperature gradient and ML depth. These simplified expressions are used to estimate the extent to which cabbeling and thermobaricity contribute to local ML density differences. These estimates compare well with values calculated directly using the complete nonlinear equation of state. Cabbeling and thermobaricity predominantly influence the ML density field poleward of 30°. Mixed layer thermobaricity is basin-scale and winter intensified, while ML cabbeling is perennial and localized to intense, zonally coherent regions associated with strong temperature fronts, such as the Antarctic Circumpolar Current and the Kuroshio and Gulf Stream Extensions. For latitudes between 40° and 50° in both hemispheres, the zonally averaged effects of ML cabbeling and ML thermobaricity can contribute on the order of 10% of the local ML density difference.K.D.S. was supported by Australian Research Council Grant DP140103706; T.W.N.H. was supported by National Science Foundation Grants 1338814 and 1536554, and A.M.H. was supported by Australian Research Council Grant CE11000102

Response of the Southern Ocean Overturning Circulation to Extreme Southern Annular Mode Conditions

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◦, (1/4)◦, and (1/10)◦). 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 wind speed 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.Department of Education and Training | Australian Research Council (ARC). Grant Number: LP16010007

Localized rapid warming of West Antarctic subsurface waters by remote winds

The highest rates of Antarctic glacial ice mass loss are occurring to the west of the Antarctica Peninsula in regions where warming of subsurface continental shelf waters is also largest. However, the physical mechanisms responsible for this warming remain unknown. Here we show how localized changes in coastal winds off East Antarctica can produce significant subsurface temperature anomalies (>2 °C) around much of the continent. We demonstrate how coastal-trapped barotropic Kelvin waves communicate the wind disturbance around the Antarctic coastline. The warming is focused on the western flank of the Antarctic Peninsula because the circulation induced by the coastal-trapped waves is intensified by the steep continental slope there, and because of the presence of pre-existing warm subsurface water offshore. The adjustment to the coastal-trapped waves shoals the subsurface isotherms and brings warm deep water upwards onto the continental shelf and closer to the coast. This result demonstrates the vulnerability of the West Antarctic region to a changing climate.P.S. was supported by an Australian Research Council (ARC) DECRA Fellowship DE150100223, A.M.H. by an ARC Future Fellowship FT120100842 and M.H.E. by an ARC Laureate Fellowship FL100100214 and R.M.H. by an ARC Discovery Project DP150101331

ACCESS-OM2 v1.0: A global ocean-sea ice model at three resolutions

We introduce ACCESS-OM2, a new version of the ocean–sea ice model of the Australian Community Climate and Earth System Simulator. ACCESS-OM2 is driven by a prescribed atmosphere (JRA55-do) but has been designed to form the ocean–sea ice component of the fully coupled (atmosphere–land–ocean–sea ice) ACCESS-CM2 model. Importantly, the model is available at three different horizontal resolutions: a coarse resolution (nominally 1∘ horizontal grid spacing), an eddy-permitting resolution (nominally 0.25∘), and an eddy-rich resolution (0.1∘ with 75 vertical levels); the eddy-rich model is designed to be incorporated into the Bluelink operational ocean prediction and reanalysis system. The different resolutions have been developed simultaneously, both to allow for testing at lower resolutions and to permit comparison across resolutions. In this paper, the model is introduced and the individual components are documented. The model performance is evaluated across the three different resolutions, highlighting the relative advantages and disadvantages of running ocean–sea ice models at higher resolution. We find that higher resolution is an advantage in resolving flow through small straits, the structure of western boundary currents, and the abyssal overturning cell but that there is scope for improvements in sub-grid-scale parameterizations at the highest resolution.This research has been supported by the Australian Research Council (grant nos. LP160100073, CE170100023, FT13101532, DP160103130 and DE170100184), the International Space Science Institute (grant no. 406), and the Australian Antarctic Science (grant nos. 4301 and 4390)

ACCESS-OM2 v1.0: a global ocean-sea ice model at three resolutions

We introduce ACCESS-OM2, a new version of the ocean–sea ice model of the Australian Community Climate and Earth System Simulator. ACCESS-OM2 is driven by a prescribed atmosphere (JRA55-do) but has been designed to form the ocean–sea ice component of the fully coupled (atmosphere–land–ocean–sea ice) ACCESS-CM2 model. Importantly, the model is available at three different horizontal resolutions: a coarse resolution (nominally 1∘ horizontal grid spacing), an eddy-permitting resolution (nominally 0.25∘), and an eddy-rich resolution (0.1∘ with 75 vertical levels); the eddy-rich model is designed to be incorporated into the Bluelink operational ocean prediction and reanalysis system. The different resolutions have been developed simultaneously, both to allow for testing at lower resolutions and to permit comparison across resolutions. In this paper, the model is introduced and the individual components are documented. The model performance is evaluated across the three different resolutions, highlighting the relative advantages and disadvantages of running ocean–sea ice models at higher resolution. We find that higher resolution is an advantage in resolving flow through small straits, the structure of western boundary currents, and the abyssal overturning cell but that there is scope for improvements in sub-grid-scale parameterizations at the highest resolution