Physical oceanography

Abstract:

Although the surface eddy kinetic energy (EKE) has been well studied using satellite altimeter and surface drifter observations, our knowledge of EKE in the ocean interior is much more limited due to the sparsity of subsurface current measurements. Here we develop a new approach for estimating EKE over the full depth of the global ocean by combining 20 years of satellite altimeter and Argo float data to infer the vertical profile of eddies. The inferred eddy profiles are surface-intensified at low latitudes and deep-reaching at mid- and high latitudes. They compare favorably to the first empirical orthogonal function obtained from current meter velocities. The global-integrated EKE estimated from the inferred profiles is about 3.1 × 10¹⁸ J, which is close to that estimated from the surface mode (3.0 × 10¹⁸ J) but about 30% smaller than that estimated from the traditional flat bottom modes (4.6 × 10¹⁸ J).

Abstract:

The Southern Ocean greatly contributes to the regulation of the global climate by controlling important heat and carbon exchanges between the atmosphere and the ocean. Rates of climate change on decadal timescales are therefore impacted by oceanic processes taking place in the Southern Ocean, yet too little is known about these processes. Limitations come both from the lack of observations in this extreme environment and its inherent sensitivity to intermittent processes at scales that are not well captured in current Earth system models. The Southern Ocean Carbon and Heat Impact on Climate programme was launched to address this knowledge gap, with the overall objective to understand and quantify variability of heat and carbon budgets in the Southern Ocean through an investigation of the key physical processes controlling exchanges between the atmosphere, ocean and sea ice using a combination of observational and modelling approaches. Here, we provide a brief overview of the programme, as well as a summary of some of the scientific progress achieved during its first half. Advances range from new evidence of the importance of specific processes in Southern Ocean ventilation rate (e.g. storm-induced turbulence, sea–ice meltwater fronts, wind-induced gyre circulation, dense shelf water formation and abyssal mixing) to refined descriptions of the physical changes currently ongoing in the Southern Ocean and of their link with global climate.

This article is part of a discussion meeting issue ‘Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities’.

Abstract:

Diapycnal mixing shapes the distribution of climatically important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation (AMOC). The rate of water mass transformation in the Atlantic Ocean's interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW, neutral density γn ≃ 27.6–28.15), with a diapycnal circulation of 0.5–8 Sv between 48°N and 32°S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to a vertical displacement of hundreds of meters in the vertical. This result, confirmed with a zonally averaged numerical model of the AMOC, indicates that mixing can alter where tracers upwell in the Southern Ocean, ultimately affecting their global pathways and ventilation timescales. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.

Abstract:

Abstract:

Density staircases are observed in an idealised model of a deep western boundary current upon crossing the equator. We propose that the staircases are generated by the excitement of symmetric instability as the current crosses the equator. The latitude at which symmetric instability is excited can be predicted using simple scaling arguments. Symmetric instability generates overturning cells which, in turn, cause the inhomogenous mixing of waters with different densities. The mixing barriers and well mixed regions in density profiles coincide, respectively, with the boundaries and centres of the overturning cells generated by the symmetric instability. This new mechanism for producing density staircases may require us to re-evaluate the origins of some of the density staircases observed in the Tropical Atlantic.