Search

The nonlinear nature of the equation of state of seawater leads to numerous interesting processes in the ocean. Two of the more well-known processes caused by nonlinearities in the equation of state are cabbeling and thermobaricity. Other nonlinearities lead to difficulties in analyzing water-mass transformation on continuous ‘density’ surfaces due to the ill-defined nature of neutral surfaces. This ill-defined nature of neutral surfaces describes the problem of not being able to globally connect neutral tangent planes, i.e. planes which describe the local mixing direction of fluid particles in the absence of diapycnal mixing, to form a well-defined surface in three-dimensional space. All these processes have been known to exist for quite some time but their impact on ocean circulation and its analysis remains elusive. In this work an algorithm is introduced which improves the description of existing density surfaces to ensure that the resulting surfaces are as close to approximating neutral tangent planes as possible. Because of the remaining slope errors between these continuous ‘density’ surfaces and the neutral tangent planes, even in the absence of diapycnal mixing processes fluid trajectories penetrate through any continuous ‘density’ surfaces. This leads to a fictitious diapycnal diffusivity and an extra physical mechanism that achieves mean vertical advection in the ocean through any continuous ‘density’ surface. Using these accurate density surfaces, the effects of cabbeling, thermobaricity and the diapycnal advection due to the ill-defined nature of neutral surfaces are quantified. It is shown that these processes cause a significant downward diapycnal transport on the order of 6 Sv, concentrated in the Southern Ocean. This additional production of dense water has implications on the ocean’s energetics which is discussed in this work. A new form of vertical advection, similar to the diapycnal advection due to the ill-defined nature of neutral surfaces but caused by temporal changes of the ocean’s hydrography instead of spatial changes, is also introduced here. Another consequence of the ill-defined nature of neutral surface is the non-existence of a geostrophic streamfunction. New approximate expressions for the geostrophic streamfunction are also developed and are shown to be significantly more accurate than previously available expressions. All the algorithms described and the used to quantify the diapycnal velocities caused by nonlinearities in the equation of state are implemented in Matlab. This code including a user manual is part of this work.

No description available

Reduced gravity model simulations to look at the opening of the Tasman Seaway and how this affected the path and transport of the ACC.

The early Cenozoic opening of the Tasmanian Gateway (TG) and Drake Passage (DP), alongside the synergistic action of the westerly winds, led to a Southern Ocean transition from large, subpolar gyres to the onset of the Antarctic Circumpolar Current (ACC). However, the impact of changing latitudinal position and strength of the wind stress in altering the early Southern Ocean circulation have been poorly addressed. Here, we use an eddy-permitting ocean model (0.25°) with realistic Late Eocene paleo-bathymetry to investigate the sensitivity of the Southern Ocean to paleo-latitudinal migrations (relative to the gateways) and strengthening of the wind stress. We find that southward wind stress shifts of 5 or 10°, with a shallow TG (300 m), lead to dominance of subtropical waters in the high latitudes and further warming of the Antarctic coast (increase by 2 °C). Southward migrations of wind stress with a deep TG (1500 m) cause the shrinking of the subpolar gyres and cooling of the surface waters in the Southern Ocean (decrease by 3–4 °C). With a 1500 m deep TG, and maximum westerly winds aligning with both the TG and DP, we observe a proto-ACC with a transport of ~47.9 Sv. This impedes the meridional transport of warm subtropical waters to Antarctic coast, thus laying a foundation for thermal isolation of the Antarctic. Intriguingly, proto-ACC flow through the TG is much more sensitive to strengthened wind stress compared to the DP. We suggest that topographic form stress can balance surface wind stress at depth to support the proto-ACC while the sensitivity of the transport is likely associated with the momentum budget between wind stress and near-surface topographic form stress driven by the subtropical gyres. In summary, this study proposes that the thermal isolation of Antarctica is a consequence of a combination of gateway deepening and the alignment of maximum wind stress with both gateways.

Mesoscale eddies play a major role in the transport of tracers in the ocean. Focusing on a sector in the east Pacific, the authors present estimates of eddy diffusivities derived from kinematic tracer simulations using satellite-observed velocity fields. Meridional diffusivities are diagnosed, and how they are related to eddy properties through the mixing length formulation of Ferrari and Nikurashin, which accounts for the suppression of diffusivity due to eddy propagation relative to the mean flow, is shown. The uniqueness of this study is that, through systematically varying the zonal-mean flow, a hypothetical “unsuppressed” diffusivity is diagnosed. At a given latitude, the unsuppressed diffusivity occurs when the zonal-mean flow equals the eddy phase speed. This provides an independent estimate of eddy phase propagation, which agrees well with theoretical arguments. It is also shown that the unsuppressed diffusivity is predicted very well by classical mixing length theory, that is, that it is proportional to the rms eddy velocity times the observed eddy size, with a spatially constant mixing efficiency of 0.35. Then, the suppression factor is estimated and it is shown that it too can be understood quantitatively in terms of easily observed mean flow properties. The authors then extrapolate from these sector experiments to the global scale, making predictions for the global surface eddy diffusivity. Together with a prognostic equation for eddy kinetic energy and a theory explaining observed eddy sizes, these concepts could potentially be used in a closure for eddy diffusivities in coarse-resolution ocean climate models.

Mesoscale eddies mix tracers along isopycnals and horizontally at the sea surface. This paper compares different methods of diagnosing eddy mixing rates in an idealized, eddy-resolving model of a channel flow meant to resemble the Antarctic Circumpolar Current. The first set of methods, the “perfect” diagnostics, are techniques suitable only to numerical models, in which detailed synoptic data is available. The perfect diagnostic include flux-gradient diffusivities of buoyancy, QGPV, and Ertel PV; Nakamura effective diffusivity; and the four-element diffusivity tensor calculated from an ensemble of passive tracers. These diagnostics reveal a consistent picture of isopycnal mixing by eddies, with a pronounced maximum near 1000 m depth. The isopycnal diffusivity differs from the buoyancy diffusivity, a.k.a. the Gent–McWilliams transfer coefficient, which is weaker and peaks near the surface and bottom. The second set of methods are observationally “practical” diagnostics. They involve monitoring the spreading of tracers or Lagrangian particles in ways that are plausible in the field. We show how, with sufficient ensemble size, the practical diagnostics agree with the perfect diagnostics in an average sense. Some implications for eddy parameterization are discussed.

International audience