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The ocean transports vast amounts of heat around the planet, helping to regulate regional climate. One important component of this heat transport is the movement of warm water from equatorial regions toward the poles, with colder water flowing in return. Here, we introduce a framework relating meridional heat transport to the diabatic processes of surface forcing and turbulent mixing that move heat across temperature classes. Applied to a (1/4)° global ocean model the framework highlights the role of the tropical Indo‐Pacific in the global ocean heat transport. A large fraction of the northward heat transport in the Atlantic is ultimately sourced from heat uptake in the eastern tropical Pacific. Turbulent mixing moves heat from the warm, shallow Indo‐Pacific circulation to the cold deeper‐reaching Atlantic circulation. Our results underscore a renewed focus on the tropical oceans and their role in global circulation pathways.

The eddy heat flux generated by the statistically equilibrated baroclinic instability of a uniform, horizontal temperature gradient is studied using a two- mode quasigeostrophic model. An overview of the dependence of the eddy diffusivity of heat D[tau] on the planetary potential vorticity gradient [beta], the bottom friction [kappa], the deformation radius [lambda], the vertical shear of the large-scale flow 2U and the domain size L is provided at 70 numerical simulations with [beta]=0 (f-plane) and 110 simulations with [beta] != 0 ([beta]-plane). Strong, axisymmetric, well-separated baroclinic vortices dominate the equilibrated barotropic vorticity and temperature fields of f-plane turbulence. The heat flux arises from a systematic northward (southward) migration of anti-cyclonic (cyclonic) eddies with warm (cold) fluid trapped in the cores. Zonal jets form spontaneously on the [beta]-plane, and stationary, isotropic, jet-scale eddies align within the strong eastward-flowing regions of the jets. In both studies, the vortices and jets give rise to a strong anti-correlation between the barotropic vorticity [zeta] and the temperature field [tau]. The baroclinic mode is also an important contributor to dissipation by bottom friction and energizes the barotropic mode at scales larger than [lambda]. This in part explains why previous parameterizations for the eddy heat flux based on Kolmogorovian cascade theories are found to be unreliable. In a separate study, temperature and salinity profiles obtained with expendable conductivity, temperature and depth (XCTD) probes within Drake Passage, Southern Ocean are used to analyze the turbulent diapycnal eddy diffusivity [kappa][rho] to a depth of 1000 meters. The Polar Front separates two dynamically different regions with strong, surface-intensified mixing north of the Front. South of the Polar Front mixing is weaker and peaks at a depth of approximately 500 m, near the local temperature maximum. Peak values of [kappa][rho] are found to exceed 10^-3 m^2 s^-1. Wind-driven near-inertial waves, mesoscale eddies and thermohaline intrusions are discussed as possible factors contributing to observed mixing patterns

The goal of this project was to develop the first algorithms that allow a heterogeneous group of oceanic robots to autonomously determine and implement sampling strategies with the help of numerical ocean forecasts and remotely-sensed observations. Two-way feedback with shore-based numerical models, tested in the field, had not previously been attempted. New planning algorithms were tested during two field programs in Monterey Bay during a 12-month period using three different types of autonomous vehicles.

The dense water outflow from the Antarctic continental shelf is closely associated with the strength and position of the Antarctic Slope Front. This study explores the short-term and spatial variability of the Antarctic Slope Front system and the mechanisms that regulate cross-slope exchange using highly temporally and spatially resolved measurements from three ocean gliders deployed in 2012. The 22 sections along the eastern Antarctic Peninsula and west of the South Orkney Islands are grouped regionally and composited by isobaths. There is consistency in the front position around the Powell Basin, varying mostly between the 500- and 800-m isobaths. In most of the study area the flow is bottom intensified. The along-slope transport of the Antarctic Slope Current (upper 1000 m) varies between 0.2 and 5.9 Sv (1 Sv ≡ 10^6 m^3 s^(−1)) and does not exhibit a regional pattern. The magnitude of the velocity field shows substantial variability, up to twice its mean value. Higher eddy kinetic energy (0.003 m^2 s^(−2)) is observed in sections with dense water, possibly because of baroclinic instabilities in the bottom layer. Distributions of potential vorticity show an increase toward the shelf along isopycnals and also in the dense water layer. Glider sections located west of the South Orkney Islands indicate a northward direction of the flow associated with the Weddell Front, which differs from previous estimates of the mean circulation. This study provides some of the first observational confirmation of the high-frequency variability associated with an active eddy field that has been suggested by recent numerical simulations in this region.

The ocean’s global overturning circulation regulates the transport and storage of heat, carbon and nutrients. Upwelling across the Southern Ocean’s Antarctic Circumpolar Current and into the mixed layer, coupled to water mass modification by surface buoyancy forcing, has been highlighted as a key process in the closure of the overturning circulation. Here, using twelve high-resolution hydrographic sections in southern Drake Passage, collected with autonomous ocean gliders, we show that Circumpolar Deep Water originating from the North Atlantic, known as Lower Circumpolar Deep Water, intersects sloping topography in narrow and strong boundary currents. Observations of strong lateral buoyancy gradients, enhanced bottom turbulence, thick bottom mixed layers and modified water masses are consistent with growing evidence that topographically generated submesoscale flows over continental slopes enhance near-bottom mixing, and that cross-density upwelling occurs preferentially over sloping topography. Interactions between narrow frontal currents and topography occur elsewhere along the path of the Antarctic Circumpolar Current, which leads us to propose that such interactions contribute significantly to the closure of the overturning in the Southern Ocean.

The spatial and temporal variability of upwelling into the mixed layer in the Southern Ocean is studied using a 1/10° ocean general circulation model. Virtual drifters are released in a regularly-spaced pattern across the Southern Ocean at depths of 250, 500, and 1000 m during both summer and winter months. The drifters are advected along isopycnals for a period of four years, unless they outcrop into the mixed layer, where lateral advection and a parameterization of vertical mixing is applied. The focus of this study is on the discrete exchange between the model mixed layer and the interior. Localization of interior-mixed layer exchange occurs downstream of major topographic features across the Indian and Pacific basins, creating “hotspots” of outcropping. Minimal outcropping occurs in the Atlantic basin, while 59% of drifters outcrop in the Pacific sector and in Drake Passage (the region from 140° W to 40° W), a disproportionately large amount even when considering the relative basin sizes. Due to spatial and temporal variations in mixed layer depth, the Lagrangian trajectories provide a statistical measure of mixed layer residence times. For each exchange into the mixed layer, the residence time has a Rayleigh distribution with a mean of 30 days; the cumulative residence time of the drifters is 261 ± 194 days, over a period of four years. These results suggest that certain oceanic gas concentrations, such as CO_2 and ^(14)C, will likely not reach equilibrium with the atmosphere before being re-subducted.

A signature of submesoscale flows in the upper ocean is skewness in the distribution of relative vorticity. Expected to result for high Rossby number flows, such skewness has implications for mixing, dissipation, and stratification within the upper ocean. An array of moorings deployed in the Northeast Atlantic for 1 year as part of the experiment of the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) reveals that relative vorticity is positively skewed during winter even though the scale of the Rossby number is less than 0.5. Furthermore, this skewness is reduced to zero during spring and autumn. There is also evidence of modest seasonal variations in the gradient Rossby number. The proposed mechanism by which relative vorticity is skewed is that the ratio of lateral to vertical buoyancy gradients, as summarized by the inverse gradient Richardson number, restricts its range during winter but less so at other times of the year. These results support recent observations and model simulations suggesting that the upper ocean is host to a seasonal cycle in submesoscale turbulence.

This work was supported by a Foster and Coco Stanback Postdoctoral Fellowship and a Marie Curie Career Integration grant (CIG14-631752) awarded to A.B., and an NSF grant awarded to A.F.T. Published reconstructions of radiocarbon in the Atlantic sector of the Southern Ocean indicate that there is a mid-depth maximum in radiocarbon age during the last glacial maximum (LGM). This is in contrast to the modern ocean where intense mixing between water masses results in a relatively homogenous radiocarbon profile. Ferrari et al. [2014] suggested that the extended Antarctic sea ice cover during the LGM necessitated a shallower boundary between the upper and lower branches of the meridional overturning circulation (MOC). This shoaled boundary lay above major topographic features associated with strong diapycnal mixing, isolating dense southern-sourced water in the lower branch of the overturning circulation. This isolation would have allowed radiocarbon to decay, and thus provides a possible explanation for the mid-depth radiocarbon age bulge. We test this hypothesis using an idealized, 2D, residual-mean dynamical model of the global overturning circulation. Concentration distributions of a decaying tracer that is advected by the simulated overturning are compared to published radiocarbon data. We find that a 600 km (~5° of latitude) increase in sea ice extent shoals the boundary between the upper and lower branches of the overturning circulation at 45°S by 600 m, and shoals the depth of North Atlantic Deep Water (NADW) convection at 50°N by 2500 m. This change in circulation configuration alone decreases the radiocarbon content in the mid-depth South Atlantic at 45°S by 40‰, even without an increase in surface radiocarbon age in the source region of deep waters during the LGM. Publisher PDF