Mark Inall

Water mass modification in an Arctic fjord through cross-shelf exchange: The seasonal hydrography of Kongsfjorden, Svalbard

[1] Kongsfjorden and the West Spitsbergen Shelf is a region whose seasonal hydrography is dominated by the balance of Atlantic Water, Arctic waters, and glacial melt. Regional seasonality and the cross-shelf exchange processes have been investigated using conductivity-temperature-depth (CTD) observations from 2000–2003 and a 5-month mooring deployment through the spring and summer of 2002. Modeling of shelf-fjord dynamics was performed with the Bergen Ocean Model. Observations show a rapid and overwhelming intrusion of Atlantic Water across the shelf and into the fjord during midsummer giving rise to intense seasonality. Pockets of Atlantic Water, from the West Spitsbergen Current, form through barotropic instabilities at the shelf front. These leak onto the shelf and propagate as topographically steered features toward the fjord. Model results indicate that such cross-front exchange is enhanced by north winds. Normally, Atlantic Water penetration into the fjord is inhibited by a density front at the fjord mouth. This geostrophic control mechanism is found to be more important than the hydraulic control common to many fjords. Slow modification of the fjord water during spring reduces the effectiveness of geostrophic control, and by midsummer, Atlantic Water intrudes into the fjord, switching from being Arctic dominant to Atlantic dominant. Atlantic Water continues to intrude throughout the summer and by September reaches some quasi steady state condition. The fjord adopts a ‘‘cold’’ or ‘‘warm’’ mode according to the degree of Atlantic Water occupation. Horizontal exchange across the shelf may be an important process causing seasonal variability in the northward heat transport to the Arctic.

Does Presence of a Mid-Ocean Ridge Enhance Biomass and Biodiversity?

In contrast to generally sparse biological communities in open-ocean settings, seamounts and ridges are perceived as areas of elevated productivity and biodiversity capable of supporting commercial fisheries. We investigated the origin of this apparent biological enhancement over a segment of the North Mid-Atlantic Ridge (MAR) using sonar, corers, trawls, traps, and a remotely operated vehicle to survey habitat, biomass, and biodiversity. Satellite remote sensing provided information on flow patterns, thermal fronts, and primary production, while sediment traps measured export flux during 2007–2010. The MAR, 3,704,404 km2 in area, accounts for 44.7% lower bathyal habitat (800–3500 m depth) in the North Atlantic and is dominated by fine soft sediment substrate (95% of area) on a series of flat terraces with intervening slopes either side of the ridge axis contributing to habitat heterogeneity. The MAR fauna comprises mainly species known from continental margins with no evidence of greater biodiversity. Primary production and export flux over the MAR were not enhanced compared with a nearby reference station over the Porcupine Abyssal Plain. Biomasses of benthic macrofauna and megafauna were similar to global averages at the same depths totalling an estimated 258.9 kt C over the entire lower bathyal north MAR. A hypothetical flat plain at 3500 m depth in place of the MAR would contain 85.6 kt C, implying an increase of 173.3 kt C attributable to the presence of the Ridge. This is approximately equal to 167 kt C of estimated pelagic biomass displaced by the volume of the MAR. There is no enhancement of biological productivity over the MAR; oceanic bathypelagic species are replaced by benthic fauna otherwise unable to survive in the mid ocean. We propose that globally sea floor elevation has no effect on deep sea biomass; pelagic plus benthic biomass is constant within a given surface productivity regime.

Measurement of the Rates of Production and Dissipation of Turbulent Kinetic Energy in an Energetic Tidal Flow: Red Wharf Bay Revisited

Simultaneous measurements of the rates of turbulent kinetic energy (TKE) dissipation («) and production (P) have been made over a period of 24 h at a tidally energetic site in the northern Irish Sea in water of 25-m depth. Some « profiles from;5 m below the surface to 15 cm above the seabed were obtained using a fast light yoyo (FLY) microstructure profiler, while P profiles were determined from a bottom-mounted high-frequency acoustic Doppler current profiler (ADCP) using the variance method. In homogeneous flow of the kind observed, the turbulence regime should approximate to local equilibrium so that, with no buoyancy forces involved, « and P are expected to covary with mean values that are equal. The results show a close tracking of « and P for most of the observational period. For the second tidal cycle, when there was no significant surface wave activity, a mean ratio of «/P . 0.63 6 0.17 was obtained. Although this is a significant deviation from unity, it is within the range of uncertainty previously reported for the « measurements. A marked phase lag of between 5 and 20 min between the maximum P and the maximum « is interpreted using a simple model in terms of the decay rate of TKE. Consideration of inherent instrument noise has enabled an estimate of the lowest P threshold measurable using the variance technique. For the chosen averaging parameters a value of Pmin ; 7 3 1025 W m23 is estimated. Two other significant differences between the two sets of measurements are attributed to errors in the stress estimate. The first is a bias in the estimate of stress resulting from a combination of instrument tilt (18‐3.58) and surface wave activity. The second are anomalously high stress estimates, covering nearly one-half of the water column at times, which are thought to be due to instrument noise associated with the large wave orbital velocities.

Impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break

The vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the Malin Shelf has been determined using a combination of the free-falling light yo-yo profiler and acoustic doppler current profilers. Two distinct internal wave regimes were encountered: period I in which large-amplitude high-frequency nonlinear internal waves (NIWs) occurred (around neap tides) and period II in which the internal wave spectral continuum was not dominated by any particular frequency band (around spring tides). Empirical orthogonal function analysis shows that for the low-frequency waves, 76% of the variance was described by mode 1, rising to 95% for the high-frequency waves. During period I the dissipation and vertical mixing were characterized by the NIWs, and 70% of the dissipation occurred in the bottom boundary layer. During period II the depth-integrated dissipation was more evenly distributed throughout the tidal cycle, whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification. During both periods I and II ∼30% of the total measured dissipation occurred within the thermocline when averaged over 12.4 hours; the remainder occurred within the bottom boundary layer(BBL). Tidal average values for depth-integrated dissipation and vertical eddy diffusivity for period I (II) were 1.1×10−2 W m−2 (4.0×10−2 W m−2) and 5 cm2 s−1 (12 cm2 s−1), respectively. Decay rates and internal damping are discussed, and vertical heat fluxes are estimated. Observed dissipation rates are compared with a simple model for BBL dissipation.

Calving rates at tidewater glaciers vary strongly with ocean temperature

Rates of ice mass loss at the calving margins of tidewater glaciers (frontal ablation rates) are a key uncertainty in sea level rise projections. Measurements are difficult because mass lost is replaced by ice flow at variable rates, and frontal ablation incorporates sub-aerial calving, and submarine melt and calving. Here we derive frontal ablation rates for three dynamically contrasting glaciers in Svalbard from an unusually dense series of satellite images. We combine ocean data, ice-front position and terminus velocity to investigate controls on frontal ablation. We find that frontal ablation is not dependent on ice dynamics, nor reduced by glacier surface freeze-up, but varies strongly with sub-surface water temperature. We conclude that calving proceeds by melt undercutting and ice-front collapse, a process that may dominate frontal ablation where submarine melt can outpace ice flow. Our findings illustrate the potential for deriving simple models of tidewater glacier response to oceanographic forcing.

Vertical structure, turbulent mixing and fluxes during Lagrangian observations of an upwelling filament system off Northwest Iberia

Abstract In August 1998, a recurrent filament located near 42°N off Galicia was sampled as part of the OMEX-II project. Lagrangian and other observations were made on the shelf where the filament arose and offshore in the filament itself under upwelling favourable but fluctuating winds. The shelf drift experiment monitored a change from southward to weak northward net flow as the winds decreased to zero. Shipborne ADCP measurements showed that the shelf was supplying decreasing volumes of water to the filament as the wind speeds decreased. At the shelf edge the internal tide was larger than can be explained by local forcing and there were many unusually large high frequency internal waves with a quasi-sinusoidal form. Turbulence observations revealed enhanced dissipation rates and vertical eddy diffusion coefficients within the shelf thermocline (of order 1 cm 2 s −1 ), which appeared to be caused by the breaking of internal wave. A second Lagrangian experiment was executed in the filament some 120 km offshore, which again coincided with a period of wind relaxation. Cross-sections revealed a double cold core and that the offshore flow was limited to a thin surface layer. Substantial onshore flow occurred below 50 m in the centre of the filament, while the strongest and deepest offshore flow coincided with its northern boundary. Turbulent kinetic energy dissipation rate measurements showed very weak mixing below 15 m in the filament core, but enhanced mixing at its boundaries. Four mixed layer drifters released in the filament initially indicated convergence at its southern boundary, marked by strong temperature and salinity contrasts. After the wind became more favourable for upwelling, the drifters accelerated. One drifter traced the full extent of the filament, while the other three escaped from it and began to circulate cyclonically over 28 days in a 100 km diameter loop back towards their release point. Although strong mesoscale activity linked the shelf and ocean regimes, offshore transport in the filament was weak at the time of the experiment and vertical and horizontal re-circulations on a variety of time scales were important. There was sufficient vertical mixing in the thermocline to cause it to thicken and draw some heat into the lower layers during the summer months on the shelf. The amount of heat involved was too little to have a significant impact on the development of a filament over a typical lifetime of a week.

Two Lagrangian experiments in the Iberian upwelling system: tracking an upwelling event and an off-shore filament

Abstract Two Lagrangian drift experiments were carried out at the NW Iberian margin. The first tracked a body of nutrient-rich, upwelled water as it moved south along the shelf break over a 5 day period. The second experiment, of similar duration, followed a water mass as it moved into the deep ocean in an off-shelf filament. This paper describes the background to and aims of each experiment. The overall objective was to quantify chemical and biological processes relating to the additional potential for the ocean at the shelf margins to sequester atmospheric CO2 in upwelling regions. The first experiment began at a time of intense wind-driven upwelling; within 2 days, the wind speed had moderated and the system entered a relaxation period with greatly reduced upwelling. The patch of upwelled water was marked by a single buoy array and it moved south along the shelf break. Transport was initially rapid but slowed with reducing wind speed. The temperature–salinity characteristics were consistent with sampling only a single water mass throughout the experiment. A model of particle trajectories showed slight deviation from the actual movement of the marked water mass, but overall the data support the assumption that the experiment was Lagrangian. During a 5 day experimental period, nutrients were utilised with a N:P ratio of 18.3 and N:Si of 4. Nutrient concentrations first reduced in the near-surface but depletion deepened in the water column during the experiment. At the beginning of the experiment, the highest chlorophyll concentrations were in the surface 15m but this was replaced by a subsurface chlorophyll maximum at 30m. There was a shift from a small flagellate and dinoflagellate dominated photosynthetic phytoplankton assemblage to a diatom dominated assemblage. A high biomass of heterotrophic dinoflagellates and ciliates was also present. Canonical correlation analysis between environmental variables and microplankton assemblages, as defined by principal component analysis, suggested that a considerable part of DON production resulted from trophic relationships rather than direct release from phytoplankton. The second experiment followed a water mass marked with 5 Argos drifting buoys for 5 days as the water drifted off shelf in an off-shore filament. This water mass was extremely oligotrophic; nitrate concentrations were typically The data presented in this paper are a general description of the experiments and form the background to the more detailed descriptions given in the individual papers that make up this Special Issue of Progress in Oceanography.

Oceanic heat delivery via Kangerdlugssuaq Fjord to the south‐east Greenland ice sheet

Acceleration of the Greenland Ice Sheet (GrIS) tidewater outlet glaciers has increased the ice sheets contribution to global sea level rise over the last two decades. Coincident increases in atmospheric temperatures around Greenland explain some of the increased ice loss, but warm Atlantic-origin water (AW) is increasingly recognized as contributing to the accelerating ice-mass loss, particularly, via the outlet glaciers of south-east (SE) Greenland. However, there remains a lack of understanding of the variability in heat content of the water masses found to the east of Greenland and how this heat is communicated to the outlet glaciers of the GrIS. Here a new analysis is presented of ocean/GrIS interaction in which the oceanic heat flux toward the ice sheet in Kangerdlugssuaq Fjord (0.26 TW) is an order-of-magnitude greater than that reported for the other major outlet glacier of SE Greenland (Helheim). Heat delivered by AW to the calving front of Kangerdlugssuaq is equivalent to ∼10 m d−1 melt (i.e., 30–60% of the ice flow speed), and thus is highly significant. During the observational campaign in September 2010 warm Polar Surface Water (PSWw) melted a substantial volume of ice within the fjord; equivalent to 25% of the volume melted by AW alone. Satellite-derived sea surface temperatures show large interannual variability in PSWw over the 20 year period 1991–2011. Anomalously warm PSWw was observed within the fjord prior to the well-documented major ice front retreats of May 2004 and November 2010.

The physics of mid-latitude fjords: a review

Abstract A rich and wide variety of fluid dynamic processes occur in fjords. Although a fjord may at one level be simply defined a glacially formed coastal inlet, this simple definition belies a huge range of geomorphological manifestations and environmental forcing conditions. It is the interplay between geomorphology and environmental forcing which defines the relative importance of differing physical fluid processes within a given fjord. In this chapter we present a non-mathematical review of the dominant physical processes which are found to occur in fjordic systems, how their relative importance may depend on geomorphology and forcing, and how, in turn, the dominant physical processes effect circulation and sediment distribution. Our aim is to provide the non-physical oceanographer with an insight into the rich and varied fluid dynamical processes presented to us by the fascinating ‘mini-ocean’ geo-type generically referred to as a fjord.

Overturning in the Subpolar North Atlantic Program: A New International Ocean Observing System

A new ocean observing system has been launched in the North Atlantic in order to understand the linkage between the meridional overturning circulation and deep water formation. For decades oceanographers have understood the Atlantic Meridional Overturning Circulation (AMOC) to be primarily driven by changes in the production of deep water formation in the subpolar and subarctic North Atlantic. Indeed, current IPCC projections of an AMOC slowdown in the 21st century based on climate models are attributed to the inhibition of deep convection in the North Atlantic. However, observational evidence for this linkage has been elusive: there has been no clear demonstration of AMOC variability in response to changes in deep water formation. The motivation for understanding this linkage is compelling since the overturning circulation has been shown to sequester heat and anthropogenic carbon in the deep ocean. Furthermore, AMOC variability is expected to impact this sequestration as well as have consequences for regional and global climates through its effect on the poleward transport of warm water. Motivated by the need for a mechanistic understanding of the AMOC, an international community has assembled an observing system, Overturning in the Subpolar North Atlantic (OSNAP), to provide a continuous record of the trans-basin fluxes of heat, mass and freshwater and to link that record to convective activity and water mass transformation at high latitudes. OSNAP, in conjunction with the RAPID/MOCHA array at 26°N and other observational elements, will provide a comprehensive measure of the three-dimensional AMOC and an understanding of what drives its variability. The OSNAP observing system was fully deployed in the summer of 2014 and the first OSNAP data products are expected in the fall of 2017.