ALBERT

Description: Sustaining phytoplankton primary production requires the physical supply of nutrients to the thermocline and the mixed layer. In the extensive downwelling regions of the subtropical gyres, the pathways of this nutrient supply remain unclear. Global estimates of mesoscale eddy mixing and internal-wave driven diapycnal mixing and climatological nitrate, salinity, and temperature data are combined to explore the relative importance of microscale turbulence and eddy stirring in supplying nutrients to phytoplankton in the North Atlantic subtropical gyre. The nutrient fluxes by eddy stirring and microscale turbulence are estimated in terms of their advective and diffusive transfers associated with the evolution of a nutrient. Mesoscale eddy stirring and microscale turbulence together supply an average of 0.10 mol N m〈sup〉-2〈/sup〉 yr〈sup〉-1〈/sup〉 to the mixed layer over the gyre. There is substantial spatial variation in the relative importance of the eddy- and turbulence-driven supplies across the gyre, where eddy stirring dominates over microscale turbulence in the northern and south-western gyre and vice versa in the south-eastern gyre. Given the combined nitrate supply by eddy stirring and microscale turbulence to the mixed layer falls short of nitrate export over 90% of the gyre, we suggest that mesoscale eddy stirring and microscale turbulence does not sustain phytoplankton primary production in the subtropical gyres alone. Other mixing mechanisms such as time-varying eddy circulations and double diffusion by salt fingers are expected to be important.

Description: Transports across the continental shelf edge enhance shelf-sea production, remove atmospheric carbon and imply an active boundary to ocean circulation. Overall transports across the varied shelf edge from south-west of Britain to north of Scotland are estimated (from a variety of measurements and models) as several m〈sup〉2〈/sup〉s〈sup〉−1〈/sup〉. This large value results from variable strong wind-forced and tidal currents and along-slope flow.Even a globally typical 1 m〈sup〉2〈/sup〉s〈sup〉−1〈/sup〉 across an estimated 5x10〈sup〉5〈/sup〉 km of shelf edge amounts to 500 Sv; large compared with oceanic transports and potentially important to shelf-sea and adjacent oceanic budgets. However, exchanges with periods ∼ one day or less may be effective only for water properties that evolve on such short time-scales. Thus transports’ significance depends on distinctive properties of the water, or its contents, and on internal shelf-sea circulation affecting further transport. Transports across the NW European shelf edge enable its disproportionately strong CO〈sub〉2〈/sub〉 “pump”.The complex context, and small scales of numerous processes enabling cross-slope transports, imply a need for models. Measurements remain limited in extent and duration, but widely varied contexts, particular conditions, events, processes and behaviours are now available for model validation. Variability still renders observations insufficient for stable estimates of transports and exchanges, especially if partitioned by sector and season; indeed, there may be significant inter-annual differences. Validated fine-resolution models give the best prospect of spatial and temporal coverage and of estimating shelf-sea sensitivities to the adjacent ocean.

Description: The ocean's subtropical gyres account for about half of global ocean biological carbon fixation. Their productivity is limited by the rate of supply of essential nutrients to the sunlit surface water, which is continuously depleted by gravitational sinking of organic particles and large-scale, wind-driven downwelling. To address this conundrum of how productivity is sustained in subtropical gyres, we conduct two complementary studies: 1. We conducted a field campaign in in the North Atlantic subtropical gyre, consisting of transects over and off the mid-Atlantic ridge including measurements of turbulent kinetic energy dissipation and nitrate. Diapycnal mixing provides a nutrient supply within the euphotic zone, but a loss of nutrients within the upper thermocline. Eddy stirring augments, and is comparable to, the diapycnal transfer of nutrients within the summertime upper thermocline, while also acting to replenish nutrients within the deeper parts of the thermocline. 2. We diagnose an eddy-permitting simulation of ocean physics, biogeochemistry and ecology over the North Pacific subtropical gyre. Near the base of the sunlit zone, small-scale diapycnal mixing and adiabatic excursions of nutrient-rich density surfaces deliver nutrients into the euphotic zone, depleting the layers immediately below. These sub-euphotic layers are replenished by a combination of the remineralization of sinking particles and along-isopycnal eddy stirring. In summary, the nutrient supply to the euphotic zone is achieved via a multi-stage mechanism: a diapycnal transfer of nutrients by small-scale turbulence to the euphotic zone, and an isopycnal stirring of nutrients by mesoscale eddies replenishing nutrients in the upper thermocline.

Description: Recent theoretical studies〈sup〉 〈/sup〉have hypothesized that deep-ocean upwelling may be primarily sustained by mixing processes within a thin bottom boundary layer (BBL) adjacent to the seafloor. However, this proposition appears at odds with several decades of observations of ocean turbulence, which suggest a pervasive intensification of mixing with depth conducive to deep-ocean downwelling. Here, we reconcile such intensification with the newly-proposed paradigm of BBL-focussed upwelling in the context of a typical continental-slope canyon, in which very rapid upwelling is observed. We show that upwelling along the canyon stems from episodic cells of convective turbulent mixing up to 250 m in height, generated by tidal currents sweeping up- and down-canyon. As drag against the seabed decelerates the currents’ base, their upper layers convey dense waters over slower-flowing lighter waters below, causing the dense waters to convectively mix, lighten and upwell. We discuss how this upwelling mechanism is likely to be of wider representativeness, lending support to the view that deep-ocean upwelling may predominantly occur along the ocean’s sloping boundaries.

Description: Hydrothermal vents have been identified as a key source of trace elements supporting ocean ecosystems. The density and extent of the early plume control the pathways connecting the plume to the surface. Hydrothermal vents generate buoyant water which rises, entraining deep water, until reaching neutral buoyancy. The prevailing view is that the plume then spreads horizontally a few kilometres until constrained by rotation. Here we present a unique observational data set from three Hydrothermal systems consisting of: high horizontal resolution, 100m, Tow-Yo sections; and microstructure profiles. These observations show plumes with horizontal extents much less than a rotational constraint implying a process dispersing the plume time scales shorter than rotation. We calculate the Ertel potential vorticity from the observations to reveal regions of low PV with susceptibility to sub-mesoscale instabilities. Diagnostics reveal that the plume is primed for all three forms of instability: convective instability in the rising plume leading to vertical exchange; centrifugal instability in the upper core of neutrally buoyant plume leading to horizontal exchange; and symmetric instability on the flanks of the plume leading to slantwise exchange. We then show strongly enhanced turbulent mixing within the plume compared to typical deep ocean values, 1000 times larger than background. Finally, we apply a linear mixing model which shows that the plume fluid mixes as strongly when at neutral depth as during the rising phase. In combination these results, in contrast to previous views, present a picture of an intensely dynamic plume rapidly transferring hydrothermal tracers throughout the region.

Description: Ocean dynamics are characterised by a wide range of space-time scales, from basin scale overturning O(1000km, 10 years) to micro-scale turbulence O(1cm, 1s). This presents an observational challenge to simultaneously capture such a wide range of processes in a sustained and cost effective manner. Whilst existing methods allow sustained observations of large-scale processes, smaller scale processes O(〈1km) are difficult to observe in a sustained manner. Here we present a test case for distributed fibre optic dynamic strain sensing as a solution for these smaller scales. A 2 km long offshore legacy seafloor cable at the Fall of Warness renewable tidal energy site was interrogated from the shore end for 12 hours providing distributed dynamic strain data sampled at 1000 Hz with a horizontal resolution of 2m. A nearby bottom lander equipped with an ADCP provides reference velocity measures. The flow in the region is dominated by M2 period tides with a magnitude of 3 ms〈sup〉-1〈/sup〉. The data collected from the cable shows signals consistent with the tidal velocity taken from the nearby ADCP. This includes high frequency signals driven by the interaction between the tide and bottom topography generating turbulence. Additionally, at lower frequency, the cable data is able to reproduce the direction of the tidal flow relative to the cable, providing both horizontal components at turning points. This highlights the potential of the technology to be applied to a wide range of ocean problems. Open development questions related to the sensitivity and calibration of similar systems will be discussed.