Physical oceanography

Abstract:

The transport of the Southern Ocean’s Antarctic Circumpolar Current, closely linked to the global stratification to the north and in turn the inter-hemispheric overturning circulation, is a key metric for quantifying ocean circulation. Understanding the sensitivity of transport to changes in forcing is important in understanding the role of the Southern Ocean in past, present and future climates. Here, we report on an investigation of a negative sensitivity regime, whereby the circumpolar transport decreases with increasing wind forcing, a phenomenon previously reported in ocean modelling investigations where the residual overturning circulation is oriented opposite to the present-day configuration. The present study finds that this negative sensitivity is a subtle effect resulting from both eddy saturation and a negative residual overturning circulation, the latter referring to a poleward mass flux in the warm surface layers. The work provides an examination and rationalisation of the sensitivities relating to the Southern Ocean circumpolar transport, and additionally touches on a numerical methodology that is particularly adept for the study of equilibrium sensitivities, with implications for analogous explorations in the paleoclimate context.

Abstract:

The global overturning circulation (GOC) is the largest scale component of the ocean circulation, associated with a global redistribution of key tracers such as heat and carbon. The GOC generates decadal to millennial climate variability, and will determine much of the long-term response to anthropogenic climate perturbations. This review aims at providing an overview of the main controls of the GOC. By controls, we mean processes affecting the overturning structure and variability. We distinguish three main controls: mechanical mixing, convection, and wind pumping. Geography provides an additional control on geological timescales. An important emphasis of this review is to present how the different controls interact with each other to produce an overturning flow, making this review relevant to the study of past, present and future climates as well as to exoplanets’ oceans.

Abstract:

A parameterization for barotropic eddy potential vorticity (PV) fluxes is introduced which applies both an energetic and an enstrophetic constraint to a down-gradient PV mixing closure. An eddy kinetic energy budget and an eddy potential enstrophy budget are employed to constrain the parameterized eddy PV fluxes. Through the budgets, the parameterization facilitates a bidirectional exchange of kinetic energy between the parameterized eddies and the large-scale flow, and a conversion of potential enstrophy from the large-scale flow to the parameterized eddies. The parameterization is tested in simulations of barotropic, freely-decaying turbulence in a doubly periodic domain over variable bottom topography. The simulations show that employing the parameterization results in an upscale transfer of kinetic energy emerges on average, consistent with quasigeostrophic theory. Furthermore, the kinetic energy and potential enstrophy budgets employed are sufficient to constrain the large-scale flow in a realistic manner when compared to an eddy-resolving model. As a result, a topography-following flow of the correct magnitude emerges in a coarse-resolution model with parameterized eddy effects. Dissipation in the coarse-resolution simulations is significant, leading to the most significant source of discrepancy between the coarse resolution simulation with parameterized eddy effects and the eddy-resolving simulation. This work constitutes a first step towards the ultimate aim of parameterizing both baroclinic and barotropic turbulence. How this may be achieved by integrating this parameterization with other methods in more realistic ocean simulations is discussed.

Abstract:

The reduced sensitivity of mean Southern Ocean zonal transport with respect to surface wind stress magnitude changes, known as eddy saturation, is studied in an idealized analytical model. The model is based on the assumption of a balance between surface wind stress forcing and bottom dissipation in the planetary geostrophic limit, coupled to the GEOMETRIC form of the Gent–McWilliams eddy parameterization. The assumption of a linear stratification, together with an equation for the parameterized domain integrated total eddy energy, enables the formulation of a two component dynamical system, which reduces to the non‐linear oscillator of Ambaum and Novak (2014, https://doi.org/10.1002/qj.2352) in a Hamiltonian limit. The model suggests an intrinsic oscillatory time scale for the Southern Ocean, associated with a combination of mean shear erosion by eddies and eddy energy generation by the mean shear. For Southern Ocean parameters the model suggests that perturbing the system via stochastic wind forcing may lead to relatively large excursions in eddy energy.