Sutherland
David A.
Sutherland
David A.
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ArticleEstuarine exchange flow variability in a seasonal, segmented estuary(American Meteorological Society, 2020-02-26) Conroy, Ted ; Sutherland, David A. ; Ralston, David K.Small estuaries in Mediterranean climates display pronounced salinity variability at seasonal and event time scales. Here, we use a hydrodynamic model of the Coos Estuary, Oregon, to examine the seasonal variability of the salinity dynamics and estuarine exchange flow. The exchange flow is primarily driven by tidal processes, varying with the spring–neap cycle rather than discharge or the salinity gradient. The salinity distribution is rarely in equilibrium with discharge conditions because during the wet season the response time scale is longer than discharge events, while during low flow it is longer than the entire dry season. Consequently, the salt field is rarely fully adjusted to the forcing and common power-law relations between the salinity intrusion and discharge do not apply. Further complicating the salinity dynamics is the estuarine geometry that consists of multiple branching channel segments with distinct freshwater sources. These channel segments act as subestuaries that import both higher- and lower-salinity water and export intermediate salinities. Throughout the estuary, tidal dispersion scales with tidal velocity squared, and likely includes jet–sink flow at the mouth, lateral shear dispersion, and tidal trapping in branching channel segments inside the estuary. While the estuarine inflow is strongly correlated with tidal amplitude, the outflow, stratification, and total mixing in the estuary are dependent on the seasonal variation in river discharge, which is similar to estuaries that are dominated by subtidal exchange flow.
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PreprintObservations of fresh, anticyclonic eddies in the Hudson Strait outflow( 2010-07) Sutherland, David A. ; Straneo, Fiamma ; Lentz, Steven J. ; St-Laurent, PierreThe waters that flow out through Hudson Strait, a coastal system that connects Hudson Bay with the Labrador Sea, constitute the third largest freshwater contribution to the northern North Atlantic. Recent studies have documented the mean structure and transport of the outflow, as well as highlighting significant variability on synoptic scales (days–week). This study examines the outflow’s variability on these synoptic scales through the use of observations collected by a mooring array from 2005-2006. We focus on the mechanisms that cause the freshwater export to be concentrated in a series of discrete pulses during the fall/winter season. We find that the pulses occur once every 4.4 days on average and are associated with anticyclonic, surface-trapped eddies propagated through the strait by the mean outflow. Their occurrence is related to the passage of storms across Hudson Bay, although local instability processes also play a role in their formation. The eddies are responsible for approximately 40% of the mean volume transport and 50% of the mean freshwater transport out of the strait. We discuss the implications of this freshwater release mechanism on the delivery of nutrient-rich and highly stratified waters to the Labrador shelf, a productive region south of Hudson Strait.
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ArticleMoored observations of synoptic and seasonal variability in the East Greenland Coastal Current(John Wiley & Sons, 2014-12-23) Harden, Benjamin E. ; Straneo, Fiamma ; Sutherland, David A.We present a year-round assessment of the hydrographic variability within the East Greenland Coastal Current on the Greenland shelf from five synoptic crossings and 4 years of moored hydrographic data. From the five synoptic sections the current is observed as a robust, surface intensified flow with a total volume transport of 0.66 ± 0.18 Sv and a freshwater transport of 42 ± 12 mSv. The moorings showed heretofore unobserved variability in the abundance of Polar and Atlantic water masses in the current on synoptic scales. This is exhibited as large vertical displacement of isotherms (often greater than 100 m). Seasonally, the current is hemmed into the coast during the fall by a full depth Atlantic Water layer that has penetrated onto the inner shelf. The Polar Water layer in the current then thickens through the winter and spring seasons increasing the freshwater content in the current; the timing implies that this is probably driven by the seasonally varying export of freshwater from the Arctic and not the local runoff from Greenland. The measured synoptic variability is enhanced during the winter and spring period due to a lower halocline and a concurrent enhancement in the along-coast wind speed. The local winds force much of the high-frequency variability in a manner consistent with downwelling, but variability distinct from downwelling is also visible.
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ArticleQuantifying flow regimes in a Greenland glacial fjord using iceberg drifters(John Wiley & Sons, 2014-12-11) Sutherland, David A. ; Roth, George E. ; Hamilton, Gordon S. ; Mernild, Sebastian H. ; Stearns, Leigh A. ; Straneo, FiammaLarge, deep-keeled icebergs are ubiquitous in Greenland's outlet glacial fjords. Here we use the movement of these icebergs to quantify flow variability in Sermilik Fjord, southeast Greenland, from the ice mélange through the fjord to the shelf. In the ice mélange, a proglacial mixture of sea ice and icebergs, we find that icebergs consistently track the glacier speed, with slightly faster speeds near terminus and episodic increases due to calving events. In the fjord, icebergs accurately capture synoptic circulation driven by both along-fjord and along-shelf winds. Recirculation and in-/out-fjord variations occur throughout the fjord more frequently than previously reported, suggesting that across-fjord velocity gradients cannot be ignored. Once on the shelf, icebergs move southeastward in the East Greenland Coastal Current, providing wintertime observations of this freshwater pathway.
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DatasetPhysical data (sal, temp, wind, wave height) collected from January-March of 2014, 2015, and 2016 at the OIMB pier located in the Coos Bay entrance of the Oregon Coast(Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2020-01-21) Emlet, Richard ; Shanks, Alan L. ; Sutherland, David A.Physical data (sal, temp, wind, wave height) measured at the Oregon Institute of Marine Biology (OIMB) pier during 2014-2016. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/682521
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ArticleFreshwater composition of the waters off southeast Greenland and their link to the Arctic Ocean(American Geophysical Union, 2009-05-27) Sutherland, David A. ; Pickart, Robert S. ; Jones, E. Peter ; Azetsu-Scott, Kumiko ; Eert, A. Jane ; Olafsson, JonThe freshwater composition of waters on the southeast Greenland shelf and slope are described using a set of high-resolution transects occupied in summer 2004, which included hydrographic, velocity, nutrient, and chemical tracer measurements. The nutrient and tracer data are used to quantify the fractions of Pacific Water, sea ice melt, and meteoric water present in the upper layers of the East Greenland Current (EGC) and East Greenland Coastal Current (EGCC). The EGC/EGCC system dominates the circulation of this region and strongly influences the observed distribution of the three freshwater types. Sea ice melt and meteoric water fractions are surface intensified, reflecting their sources, and generally increase southward from Denmark Strait to Cape Farewell, as well as shoreward. Significant fractions of Pacific Water are found in the subsurface layers of the EGCC, supporting the idea that this inner shelf branch is directly linked to the EGC and thus to the Arctic Ocean. A set of historical sections is examined to investigate the variability of Pacific Water content in the EGC and EGCC from 1984 to 2004 in the vicinity of Denmark Strait. The fraction of Pacific Water increased substantially in the late 1990s and subsequently declined to low levels in 2002 and 2004, mirroring the reduction in Pacific Water content reported previously at Fram Strait. This variability is found to correlate significantly with the Arctic Oscillation index, lagged by 9 years, suggesting that the Arctic Ocean circulation patterns bring varying amounts of Pacific Water to the North Atlantic via the EGC/EGCC.
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ArticleShifting sediment dynamics in the Coos Bay Estuary in response to 150 years of modification(American Geophysical Union, 2020-12-14) Eidam, Emily F. ; Sutherland, David A. ; Ralston, David K. ; Conroy, Ted ; Dye, BassEstuaries worldwide have experienced modifications including channel deepening and intertidal reclamation over several centuries, resulting in altered fine sediment routing. Estuaries respond differently based on preexisting geometries, freshwater and sediment supplies, and extents and types of modification. The Coos Bay Estuary in Oregon is a relatively small estuary with complex geometry that has been extensively modified since 1865. A sediment transport model calibrated to modern conditions is used to assess the corresponding changes in sediment dynamics. Over ∼150 years, channel deepening (from ∼6.7 to 11 m), a 12% increase in area, and a 21% increase in volume have led to greater tidal amplitudes, salinity intrusion, and estuarine exchange flow. These changes have reduced current magnitudes, reduced bed stresses, and increased stratification, especially during rainy periods. Historically, fluvially derived sediment was dispersed across broad, deltaic‐style flats and through small tidal channels. Now, river water and sediments are diverted into a dredged navigation channel where an estuarine turbidity maximum (ETM) forms, with modeled concentrations >50 mg/L and measured concentrations >100 mg/L during discharge events. This “new” ETM supplies sediment to proximal embayments in the middle estuary and the shallow flats. Overall, sediment trapping during winter (and high river discharges) has increased more than two‐fold, owing to increased accommodation space, altered pathways of supply, and altered bed stresses and tidal asymmetries. In contrast to funnel‐shaped estuaries with simpler geometries and river‐channel transitions, these results highlight the importance of channel routing together with dredging in enhancing sediment retention and shifting pathways of sediment delivery.
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ArticleEstimating ocean heat transports and submarine melt rates in Sermilik Fjord, Greenland, using lowered acoustic Doppler current profiler (LADCP) velocity profiles(International Glaciological Society, 2012-11-01) Sutherland, David A. ; Straneo, FiammaSubmarine melting at the ice-ocean interface is a significant term in the mass balance of marine-terminating outlet glaciers. However, obtaining direct measurements of the submarine melt rate, or the ocean heat transport towards the glacier that drives this melting, has been difficult due to the scarcity of observations, as well as the complexity of oceanic flows. Here we present a method that uses synoptic velocity and temperature profiles, but accounts for the dominant mode of velocity variability, to obtain representative heat transport estimates. We apply this method to the Sermilik Fjord-Helheim Glacier system in southeastern Greenland. Using lowered acoustic Doppler current profiler (LADCP) and hydrographic data collected in summer 2009, we find a mean heat transport towards the glacier of 29 × 109 W, implying a submarine melt rate at the glacier face of 650 m a-1. The resulting adjusted velocity profile is indicative of a multilayer residual circulation, where the meltwater mixture flows out of the fjord at the surface and at the stratification maximum.
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ArticleConnecting the Greenland Ice Sheet and the ocean : a case study of Helheim Glacier and Sermilik Fjord(The Oceanography Society, 2016-12) Straneo, Fiamma ; Hamilton, Gordon S. ; Stearns, Leigh A. ; Sutherland, David A.The rapid ice loss from the Greenland Ice Sheet that began in the late 1990s sparked an interest in glacier/ocean exchanges both because an increase in submarine melting of the glacier is a potential trigger of glacier retreat and because the increasing freshwater discharge can affect the regional ocean’s circulation and ecosystems. An interdisciplinary field project focused on the Helheim Glacier-Sermilik Fjord system began in 2008 and has continued to date. We found that warm, Atlantic Water flows into the fjord, drives melting of the glacier, and is regularly replenished through shelf-forced and glacier-driven circulations. In summer, the release of surface melt at the base of the glacier has a pronounced impact on local ocean circulation, the properties of the glacier, and its melt rate. Measurements taken in the fjord indicate that it is virtually impossible to derive submarine melt rates from hydrographic (including moored) data due to the fjord’s pronounced water mass variability and uncertain contribution from iceberg melt. Efforts to correlate glacier behavior with ocean forcing on seasonal and interannual time scales yield no straightforward connections, likely because of a dependence on a wider range of parameters, including subglacial discharge and bedrock geometry. This project emphasizes the need for sustained long-term measurements of multiple glacier/ocean/atmosphere systems to understand the different dynamics that control their evolution.
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PreprintRapid circulation of warm subtropical waters in a major glacial fjord in East Greenland( 2009-12-17) Straneo, Fiamma ; Hamilton, Gordon S. ; Sutherland, David A. ; Stearns, Leigh A. ; Davidson, Fraser J. M. ; Hammill, Mike O. ; Stenson, Garry B. ; Rosing-Asvid, AqqaluThe recent rapid increase in mass loss from the Greenland Ice Sheet is primarily attributed to an acceleration of outlet glaciers. One possible cause is increased melting at the ice/ocean interface driven by the synchronous warming of subtropical waters offshore of Greenland. This hypothesis is largely untested, however, because of the lack of observations from Greenland’s glacial fjords and our limited understanding of their dynamics. Here, we present new ship-based and moored oceanographic data, collected in Sermilik Fjord, a large glacial fjord in East Greenland, showing that subtropical waters are present throughout the fjord and are continuously replenished via a wind-driven exchange with the shelf, where they occur year-round. The temperature and rapid renewal of these waters suggest that, at present, they drive enhanced submarine melting at the terminus. Key controls on the melting rate are the volume and properties of subtropical waters on the shelf and the patterns of the along-shore winds, suggesting the glaciers’ acceleration was triggered by a combination of atmospheric and oceanic changes. These measurements provide evidence of rapid advective pathway for the transmission of oceanic variability to the ice-sheet margins and highlight an important process that is missing from prognostic ice-sheet models.
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PreprintThe East Greenland Coastal Current : structure, variability, and forcing( 2008-03-12) Sutherland, David A. ; Pickart, Robert S.The subtidal circulation of the southeast Greenland shelf is described using a set of high-resolution hydrographic and velocity transects occupied in summer 2004. The main feature is the East Greenland Coastal Current (EGCC), a low-salinity, high-velocity jet with a wedge-shaped hydrographic structure characteristic of other surface buoyancydriven currents. The EGCC was observed along the entire Greenland shelf south of Denmark Strait, while the transect north of the strait showed only a weak shelf flow. This observation, in conjunction with water mass considerations and other supporting evidence, suggests that the EGCC is an inner branch of the East Greenland Current (EGC) that forms south of Denmark Strait. It is argued that bathymetric steering is the most likely reason why the EGC apparently bifurcates at this location. Repeat sections occupied at Cape Farewell between 1997 and 2004 show that the alongshelf wind stress can have a strong influence on the structure and strength of the EGCC and EGC on timescales of 2-3 days. Accounting for the wind-induced effects, the volume transport of the combined EGCC/EGC system is roughly constant (~2 Sv) over the study domain, from 68°N to Cape Farewell near 60°N. The corresponding freshwater transport increases by roughly 60% over this distance (59 to 96 mSv, referenced to a salinity of 34.8). This trend is consistent with a simple freshwater budget of the EGCC/EGC system that accounts for meltwater runoff, melting sea-ice and icebergs, and net precipitation minus evaporation.
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ArticleLaboratory experiments on the interaction of a buoyant coastal current with a canyon : application to the East Greenland Current(American Meteorological Society, 2009-05) Sutherland, David A. ; Cenedese, ClaudiaThis paper presents a set of laboratory experiments focused on how a buoyant coastal current flowing over a sloping bottom interacts with a canyon and what controls the separation, if any, of the current from the upstream canyon bend. The results show that the separation of a buoyant coastal current depends on the current width W relative to the radius of curvature of the bathymetry ρc. The flow moved across the mouth of the canyon (i.e., separated) for W/ρc > 1, in agreement with previous results. The present study extends previous work by examining both slope-controlled and surface-trapped currents, and using a geometry specific to investigating buoyant current–canyon interaction. The authors find that, although bottom friction is important in setting the position of the buoyant front, the separation process driven by the inertia of the flow could overcome even the strongest bathymetric influence. Application of the laboratory results to the East Greenland Current (EGC), an Arctic-origin buoyant current that is observed to flow in two branches south of Denmark Strait, suggests that the path of the EGC is influenced by the large canyons cutting across the shelf, as the range of W/ρc in the ocean spans those observed in the laboratory. What causes the formation of a two-branched EGC structure downstream of the Kangerdlugssuaq Canyon (68°N, 32°W) is still unclear, but potential mechanisms are discussed.
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ArticleIceberg meltwater fluxes dominate the freshwater budget in Greenland's iceberg-congested glacial fjords(John Wiley & Sons, 2016-11-09) Enderlin, Ellyn ; Hamilton, Gordon S. ; Straneo, Fiamma ; Sutherland, David A.Freshwater fluxes from the Greenland ice sheet have increased over the last two decades due to increases in liquid (i.e., surface and submarine meltwater) and solid ice (i.e., iceberg) fluxes. To predict potential ice sheet-ocean-climate feedbacks, we must know the partitioning of freshwater fluxes from Greenland, including the conversion of icebergs to liquid (i.e., meltwater) fluxes within glacial fjords. Here we use repeat ~0.5 m-resolution satellite images from two major fjords to provide the first observation-based estimates of the meltwater flux from the dense matrix of floating ice called mélange. We find that because of its expansive submerged area (>100 km2) and rapid melt rate (~0.1–0.8 m d−1), the ice mélange meltwater flux can exceed that from glacier surface and submarine melting. Our findings suggest that iceberg melt within the fjords must be taken into account in studies of glacial fjord circulation and the impact of Greenland melt on the ocean.
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ThesisThe East Greenland Coastal Current : its structure variability, and large-scale impact(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2008-02) Sutherland, David A.The subtidal circulation of the southeast Greenland shelf is described using a set of highresolution hydrographic and velocity transects occupied in summer 2004. The main feature present is the East Greenland Coastal Current (EGCC), a low-salinity, highvelocity jet with a wedge-shaped hydrographic structure characteristic of other surface buoyancy-driven currents. The EGCC was observed along the entire Greenland shelf south of Denmark Strait, while the transect north of the strait showed only a weak shelf flow. This observation, combined with evidence from chemical tracer measurements that imply the EGCC contains a significant Pacific Water signal, suggests that the EGCC is an inner branch of the polar-origin East Greenland Current (EGC). A set of idealized laboratory experiments on the interaction of a buoyant current with a submarine canyon also supported this hypothesis, showing that for the observed range of oceanic parameters, a buoyant current such as the EGC could exhibit both flow across the canyon mouth or into the canyon itself, setting the stage for EGCC formation. Repeat sections occupied at Cape Farewell between 1997 and 2004 show that the alongshelf wind stress can also have a strong influence on the structure and strength of the EGCC and EGC on timescales of 2-3 days. Accounting for the wind-induced effects, the volume transport of the combined EGC/EGCC system is found to be roughly constant (~2 Sv) over the study domain, from 68°N to Cape Farewell near 60°N. The corresponding freshwater transport increases by roughly 60% over this distance (59 to 96 mSv, referenced to a salinity of 34.8). This trend is explained by constructing a simple freshwater budget of the EGCC/EGC system that accounts for meltwater runoff, melting sea-ice and icebergs, and net precipitation minus evaporation. Variability on interannual timescales is examined by calculating the Pacific Water content in the EGC/EGCC from 1984-2004 in the vicinity of Denmark Strait. The PW content is found to correlate significantly with the Arctic Oscillation index, lagged by 9 years, suggesting that the Arctic Ocean circulation patterns bring varying amounts of Pacific Water to the North Atlantic via the EGC/EGCC.
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ArticleIs the Faroe Bank Channel overflow hydraulically controlled?(American Meteorological Society, 2006-12) Girton, James B. ; Pratt, Lawrence J. ; Sutherland, David A. ; Price, James F.The overflow of dense water from the Nordic Seas through the Faroe Bank Channel (FBC) has attributes suggesting hydraulic control—primarily an asymmetry across the sill reminiscent of flow over a dam. However, this aspect has never been confirmed by any quantitative measure, nor is the position of the control section known. This paper presents a comparison of several different techniques for assessing the hydraulic criticality of oceanic overflows applied to data from a set of velocity and hydrographic sections across the FBC. These include 1) the cross-stream variation in the local Froude number, including a modified form that accounts for stratification and vertical shear, 2) rotating hydraulic solutions using a constant potential vorticity layer in a channel of parabolic cross section, and 3) direct computation of shallow water wave speeds from the observed overflow structure. Though differences exist, the three methods give similar answers, suggesting that the FBC is indeed controlled, with a critical section located 20–90 km downstream of the sill crest. Evidence of an upstream control with respect to a potential vorticity wave is also presented. The implications of these results for hydraulic predictions of overflow transport and variability are discussed.
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ArticleCharacteristics and dynamics of two major Greenland glacial fjords(John Wiley & Sons, 2014-06-16) Sutherland, David A. ; Straneo, Fiamma ; Pickart, Robert S.The circulation regimes of two major outlet glacial fjords in southeastern Greenland, Sermilik Fjord (SF) and Kangerdlugssuaq Fjord (KF), are investigated using data collected in summer 2009. The two fjords show similar flow patterns, with a time-dependent, vertically sheared flow structure dominating over the background estuarine flow driven by buoyancy input. We show that this time-dependent flow is consistent with circulation induced by density interface fluctuations at the fjord mouth, often referred to as intermediary circulation. One difference between the fjords is that the hydrographic and velocity structure below a surface modified layer is found to be three layer in KF in summer, compared to two layer in SF. Outside each fjord, large-scale geostrophic currents dictate the stratification at the mouth, although the way in which these large-scale flows impinge on each fjord is distinct. Combining the observations with estimates from existing theories, we find the magnitudes of the estuarine (Qe) and intermediary (Qi) circulation and show that Qi >> Qe, although along-fjord winds can also be significant. We expect that the critical parameter determining Qi/Qe is the sill depth compared to the fjord depth, with shallower sills corresponding to weaker intermediary circulation. Finally, we discuss the implications of strong intermediary circulation on calculating heat transport to the glacier face and its potential feedbacks on the background circulation in these highly stratified estuaries.
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ArticleAtlantic water variability on the SE Greenland continental shelf and its relationship to SST and bathymetry(John Wiley & Sons, 2013-02-20) Sutherland, David A. ; Straneo, Fiamma ; Stenson, Garry B. ; Davidson, Fraser J. M. ; Hammill, Mike O. ; Rosing-Asvid, AqqaluInteraction of warm, Atlantic-origin water (AW) and colder, polar origin water (PW) advecting southward in the East Greenland Current (EGC) influences the heat content of water entering Greenland's outlet glacial fjords. Here we use depth and temperature data derived from deep-diving seals to map out water mass variability across the continental shelf and to augment existing bathymetric products. We compare depths derived from the seal dives with the IBCAO Version 3 bathymetric database over the shelf and find differences up to 300 m near several large submarine canyons. In the vertical temperature structure, we find two dominant modes: a cold mode, with the typical AW/PW layering observed in the EGC, and a warm mode, where AW is present throughout the water column. The prevalence of these modes varies seasonally and spatially across the continental shelf, implying distinct AW pathways. In addition, we find that satellite sea surface temperatures (SST) correlate significantly with temperatures in the upper 50 m (R = 0.54), but this correlation decreases with depth (R = 0.22 at 200 m), and becomes insignificant below 250 m. Thus, care must be taken in using SST as a proxy for heat content, as AW mainly resides in these deeper layers.