Estelle Dumont

AUV observations of mixing in the tidal outflow from a Scottish sea loch

Freshwater outflows from estuarine systems carry nutrients, sediments, larvae and other materials into the coastal ocean where they mix into ambient ocean water. Loch Etive is one such estuary on the west coast of Scotland that is fed by a large rainfall catchment area, and from which the outflow is strongly modulated on a tidal basis. Freshwater leaves the loch on the ebbing tide in a pulse released into the coastal ocean as a thin layer, the leading edge of which develops into an undular bore or a train of internal solitary waves with strong mixing and associated entrainment. A Hydroid 600 m depth-rated AUV equipped with a forward-mounted microstructure sensing package has been used in conjunction with moored measurements to study the dynamics of thin freshwater layers that are regularly released into the coastal ocean. During observations conducted in winter 2009 and spring 2010 we observed turbulent bores and internal solitary waves in Ardmucknish Bay, near the entrance to Loch Etive. The passage of the bores and internal solitary waves of depression were associated with downward vertical movement of near-surface water and turbulent mixing that was enhanced above background by over 2 orders of magnitude.

Cooling of the West Spitsbergen Current: AUV-based turbulence measurements west of Svalbard

The role of turbulence is critical in the redistribution of heat and salt throughout the global ocean, and in boundary currents of the high-latitude oceans in particular. In the Arctic Ocean, warm, saline waters of Atlantic origin, which are carried northwards by the West Spitsbergen Current (WSC), are the largest source of oceanic heat carried into the Arctic Eurasian basin, and may have a significant impact on the rate of sea ice decline. While the rapid down-stream cooling of the WSC around its entry point into the Arctic Ocean is well-known, the processes by which this heat flux occurs are still an area of active research. During a cruise to the Svalbard region in July 2010, a Hydroid REMUS 600m AUV, equipped with a forward-mounted microstructure package, was used to quantify the level of turbulent mixing within and across the shoreward edge of the WSC. The microstructure-equipped AUV was used to measure horizontal profiles of water properties and turbulence parameters while a more-conventional, vertically free-falling microstructure profiler was used to conduct complementary transects of vertical profile measurements across the edge of the WSC. In this study we evaluate the resulting measurements and compare the views of the upper ocean derived from similar sensors mounted on two very different platforms: one view assembled from conventional vertical profiles and the second from horizontal, AUV-based profiles. We discuss the impact of mixing on the cooling of the WSC and the capacity of these platforms to resolve small-scale variability in both the vertical and the horizontal.

Structure and Transport of the North Atlantic Current in the Eastern Subpolar Gyre From Sustained Glider Observations

Repeat glider sections obtained during 2014–2016, as part of the Overturning in the Subpolar North Atlantic Program, are used to quantify the circulation and transport of North Atlantic Current (NAC) branches over the Rockall Plateau. Using 16 glider sections collected along 58∘N and between 21∘W and 15∘W, absolute geostrophic velocities are calculated, and subsequently the horizontal and vertical structure of the transport are characterized. The annual mean northward transport (± standard deviation) is 5.1 ± 3.2 Sv over the Rockall Plateau. During summer (May to October), the mean northward transport is stronger and reaches 6.7 ± 2.6 Sv. This accounts for 43% of the total NAC transport of upper-ocean waters (σO < 27.55 kg/m 3) estimated by Sarafanov et al. (2012, https://doi.org/10.1029/2011JC007572) along 59.5∘N, between the Reykjanes Ridge and Scotland. Two quasi-permanent northward flowing branches of the NAC are identified: (i) the Hatton Bank Jet (6.3 ± 2.1 Sv) over the eastern flank of the Iceland Basin (20.5∘W to 18.5∘W) and (ii) the Rockall Bank Jet (1.5 ± 0.7 Sv) over the eastern flank of the Hatton-Rockall Basin (16∘W to 15∘W). Transport associated with the Rockall Bank Jet is mostly depth independent during summer, while 30% of the Hatton Bank Jet transport is due to vertical geostrophic shear. Uncertainties are estimated for each individual glider section using a Monte Carlo approach, and mean uncertainties of the absolute transport are less than 0.5 Sv. Although comparisons with altimetry-based estimates indicate similar large-scale circulation patterns, altimetry data do not resolve small mesoscale current bands in the Hatton-Rockall Basin which are strongly needed for the right transport estimates. Plain Language Summary There is mounting evidence that heat and freshwater transported by the North Atlantic Current (eastward extension of the Gulf Stream) heavily influences European and global climate. To adequately measure this current and understand its dynamics, underwater gliders navigate over the Rockall Plateau, a remote region of the North Atlantic located more than 400 km off Scotland. These robots collect data up to a kilometer beneath the ocean’s surface and, due to their low energy consumption, can operate over thousands of kilometers for months at a time. This study analyzes 2 years of continuous ocean glider measurements in the North Atlantic and reveals two branches of the North Atlantic Current over the Rockall Plateau that we named the Hatton Bank Jet and the Rockall Bank Jet. For the first time, the monthly variability of these currents was characterized (speed and horizontal/vertical extension). These branches carry a significant portion of the upper-ocean waters transported by the North Atlantic Current (40%). This work highlights the importance of using autonomous underwater vehicles as part of an ocean observatory. In addition to monitoring the state of the North Atlantic Ocean, these underwater robots help us understand its dynamics, which impacts the European and global climate.

AUV observations of surface mixing and bubble entrainment in the Clyde estuary, Scotland

Most of the mixing throughout the world ocean is driven by wind forcing of the ocean surface, which also exerts a controlling influence of the rate of exchange of gases between the atmosphere and the ocean. As part of the UK NERC-funded surface boundary layer consortium “OSMOSIS” we conducted AUV-based observations of near-surface turbulent mixing and bubble entrainment in the Clyde estuary, Scotland, in conjunction with a program of ship-, mooring, and glider-based measurements. A Hydroid REMUS 600m AUV equipped with a forward-mounted microstructure sensing package designed by Rockland Scientific was instrumented for this experiment with an upward-looking 1.1 MHz echo sounder, in order to measure bubbles that are injected into the near surface by breaking waves and often organized into quasi-linear vertical curtains by Langmuir circulation cells. The AUV-based observations reported here were conducted over a seven day period in September, 2011, in a two-day window leading up to, and subsequent three-day window recovering from, a period of force 12 winds. As the wind speed reduced a thin layer of fresher water, previously pinned to the coast by the gale-force winds, rapidly slumped over the survey area. Measurements revealed that wind-generated turbulence was largely confined to this brackish surface layer, a layer too thin to be sampled by conventional means. In this paper we describe the vehicle configuration during this pilot study, the resulting data, and further vehicle modifications that will enable both more robust measurements and more flexible use of the AUV as a test-bed sampling platform.