Observations of water mass transformation and eddies in the Lofoten basin of the Nordic Seas

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 1735–1756, doi:10.1175/JPO-D-14-0238.1.The Lofoten basin of the Nordic Seas is recognized as a crucial component of the meridional overturning circulation in the North Atlantic because of the large horizontal extent of Atlantic Water and winter surface buoyancy loss. In this study, hydrographic and current measurements collected from a mooring deployed in the Lofoten basin from July 2010 to September 2012 are used to describe water mass transformation and the mesoscale eddy field. Winter mixed layer depths (MLDs) are observed to reach approximately 400 m, with larger MLDs and denser properties resulting from the colder 2010 winter. A heat budget of the upper water column requires lateral input, which balances the net annual heat loss of ~80 W m−2. The lateral flux is a result of mesoscale eddies, which dominate the velocity variability. Eddy velocities are enhanced in the upper 1000 m, with a barotropic component that reaches the bottom. Detailed examination of two eddies, from April and August 2012, highlights the variability of the eddy field and eddy properties. Temperature and salinity properties of the April eddy suggest that it originated from the slope current but was ventilated by surface fluxes. The properties within the eddy were similar to those of the mode water, indicating that convection within the eddies may make an important contribution to water mass transformation. A rough estimate of eddy flux per unit boundary current length suggests that fluxes in the Lofoten basin are larger than in the Labrador Sea because of the enhanced boundary current–interior density difference.The work was supported by NSF OCE 0850416.2015-12-0

A conceptual model of an Arctic sea

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 117 (2012): C06010, doi:10.1029/2011JC007652.We propose a conceptual model for an Arctic sea that is driven by river runoff, atmospheric fluxes, sea ice melt/growth, and winds. The model domain is divided into two areas, the interior and boundary regions, that are coupled through Ekman and eddy fluxes of buoyancy. The model is applied to Hudson and James Bays (HJB, a large inland basin in northeastern Canada) for the period 1979–2007. Several yearlong records from instruments moored within HJB show that the model results are consistent with the real system. The model notably reproduces the seasonal migration of the halocline, the baroclinic boundary current, spatial variability of freshwater content, and the fall maximum in freshwater export. The simulations clarify the important differences in the freshwater balance of the western and eastern sides of HJB. The significant role played by the boundary current in the freshwater budget of the system, and its sensitivity to the wind-forcing, are also highlighted by the simulations and new data analyses. We conclude that the model proposed is useful for the interpretation of observed data from Arctic seas and model outputs from more complex coupled/climate models.We thank NSERC and the Canada Research Chairs program for funding. FS acknowledges support from NSF OCE–0927797 and ONR N00014-08-10490.2012-12-2

Structure and dynamics of a subglacial discharge plume in a Greenlandic fjord

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 8670–8688, doi:10.1002/2016JC011764.Discharge of surface-derived meltwater at the submerged base of Greenland's marine-terminating glaciers creates subglacial discharge plumes that rise along the glacier/ocean interface. These plumes impact submarine melting, calving, and fjord circulation. Observations of plume properties and dynamics are challenging due to their proximity to the calving edge of glaciers. Therefore, to date information on these plumes has been largely derived from models. Here we present temperature, salinity, and velocity data collected in a plume that surfaced at the edge of Saqqarliup Sermia, a midsized Greenlandic glacier. The plume is associated with a narrow core of rising waters approximately 20 m in diameter at the ice edge that spreads to a 200 m by 300 m plume pool as it reaches the surface, before descending to its equilibrium depth. Volume flux estimates indicate that the plume is primarily driven by subglacial discharge and that this has been diluted in a ratio of 1:10 by the time the plume reaches the surface. While highly uncertain, meltwater fluxes are likely 2 orders of magnitude smaller than the subglacial discharge flux. The overall plume characteristics agree with those predicted by theoretical plume models for a convection-driven plume with limited influence from submarine melting.National Science Foundation (NSF) Grant Numbers: PLR-1418256 , OCE-1434041; Woods Hole Oceanographic Institution (WHOI) Ocean and Climate Change Institute (OCCI) Arctic Research Initiative OCCI; National Aeronautics and Space Administration Grant Number: NNX10AN83H2017-06-1

The impact of glacier geometry on meltwater plume structure and submarine melt in Greenland fjords

Meltwater from the Greenland Ice Sheet often drains subglacially into fjords, driving upwelling plumes at glacier termini. Ocean models and observations of submarine termini suggest that plumes enhance melt and undercutting, leading to calving and potential glacier destabilization. Here we systematically evaluate how simulated plume structure and submarine melt during summer months depends on realistic ranges of subglacial discharge, glacier depth, and ocean stratification from 12 Greenland fjords. Our results show that grounding line depth is a strong control on plume-induced submarine melt: deep glaciers produce warm, salty subsurface plumes that undercut termini, and shallow glaciers produce cold, fresh surface-trapped plumes that can overcut termini. Due to sustained upwelling velocities, plumes in cold, shallow fjords can induce equivalent depth-averaged melt rates compared to warm, deep fjords. These results detail a direct ocean-ice feedback that can affect the Greenland Ice Sheet

The role of wave dynamics and small-scale topography for downslope wind events in Southeast Greenland

In Ammassalik, in southeast Greenland, downslope winds can reach hurricane intensity and represent a hazard for the local population and environment. They advect cold air down the ice sheet and over the Irminger Sea, where they drive large ocean–atmosphere heat fluxes over an important ocean convection region. Earlier studies have found them to be associated with a strong katabatic acceleration over the steep coastal slopes, flow convergence inside the valley of Ammassalik, and—in one instance—mountain wave breaking. Yet, for the general occurrence of strong downslope wind events, the importance of mesoscale processes is largely unknown. Here, two wind events—one weak and one strong—are simulated with the atmospheric Weather Research and Forecasting (WRF) Model with different model and topography resolutions, ranging from 1.67 to 60 km. For both events, but especially for the strong one, it is found that lower resolutions underestimate the wind speed because they misrepresent the steepness of the topography and do not account for the underlying wave dynamics. If a 5-km model instead of a 60-km model resolution in Ammassalik is used, the flow associated with the strong wind event is faster by up to 20 m s−1. The effects extend far downstream over the Irminger Sea, resulting in a diverging spatial distribution and temporal evolution of the heat fluxes. Local differences in the heat fluxes amount to 20%, with potential implications for ocean convection

Large spatial variations in the flux balance along the front of a Greenland tidewater glacier

The frontal flux balance of a medium-sized tidewater glacier in western Greenland in the summer is assessed by quantifying the individual components (ice flux, retreat, calving, and submarine melting) through a combination of data and models. Ice flux and retreat are obtained from satellite data. Submarine melting is derived using a high-resolution ocean model informed by near-ice observations, and calving is estimated using a record of calving events along the ice front. All terms exhibit large spatial variability along the  ∼ 5&thinsp;km wide ice front. It is found that submarine melting accounts for much of the frontal ablation in small regions where two subglacial discharge plumes emerge at the ice front. Away from the subglacial plumes, the estimated melting accounts for a small fraction of frontal ablation. Glacier-wide, these estimates suggest that mass loss is largely controlled by calving. This result, however, is at odds with the limited presence of icebergs at this calving front – suggesting that melt rates in regions outside of the subglacial plumes may be underestimated. Finally, we argue that localized melt incisions into the glacier front can be significant drivers of calving. Our results suggest a complex interplay of melting and calving marked by high spatial variability along the glacier front.</p

Strong downslope wind events in Ammassalik, Southeast Greenland

Ammassalik in southeast Greenland is known for strong wind events that can reach hurricane intensity and cause severe destruction in the local town. Yet, these winds and their impact on the nearby fjord and shelf region have not been studied in detail. Here, data from two meteorological stations and the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim) are used to identify and characterize these strong downslope wind events, which are especially pronounced at a major east Greenland fjord, Sermilik Fjord, within Ammassalik. Their local and regional characteristics, their dynamics and their impacts on the regional sea ice cover, and air–sea fluxes are described. Based on a composite of the events it is concluded that wind events last for approximately a day, and seven to eight events occur each winter. Downslope wind events are associated with a deep synoptic-scale cyclone between Iceland and Greenland. During the events, cold dry air is advected down the ice sheet. The downslope flow is accelerated by gravitational acceleration, flow convergence inside the Ammassalik valley, and near the coast by an additional thermal and synoptic-scale pressure gradient acceleration. Wind events are associated with a large buoyancy loss over the Irminger Sea, and it is estimated that they drive one-fifth of the net wintertime loss. Also, the extreme winds drive sea ice out of the fjord and away from the shelf

Localized plumes drive front-wide ocean melting of a Greenlandic tidewater glacier

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in in Geophysical Research Letters, 45(22), (2018): 12350-12358. doi: 10.1029/2018GL080763.Recent acceleration of Greenland's ocean‐terminating glaciers has substantially amplified the ice sheet's contribution to global sea level. Increased oceanic melting of these tidewater glaciers is widely cited as the likely trigger, and is thought to be highest within vigorous plumes driven by freshwater drainage from beneath glaciers. Yet melting of the larger part of calving fronts outside of plumes remains largely unstudied. Here we combine ocean observations collected within 100 m of a tidewater glacier with a numerical model to show that unlike previously assumed, plumes drive an energetic fjord‐wide circulation which enhances melting along the entire calving front. Compared to estimates of melting within plumes alone, this fjord‐wide circulation effectively doubles the glacier‐wide melt rate, and through shaping the calving front has a potential dynamic impact on calving. Our results suggest that melting driven by fjord‐scale circulation should be considered in process‐based projections of Greenland's sea level contribution.Support was provided by the National Science Foundation (NSF) through PLR‐1418256 and PLR‐1744835, and through Woods Hole Oceanographic Institution (WHOI) Ocean and Climate Change Institute (OCCI) and the Clark Foundation. This work was also supported by a UK Natural Environmental Research Council (NERC) PhD studentship (NE/L501566/1) and Scottish Alliance for Geoscience, Environment & Society (SAGES) early career research exchange funding to D. A. S. We thank Hanumant Singh, Laura Stevens, Ken Mankoff, Rebecca Jackson, and Jeff Pietro for useful discussions and data collection.2019-05-1

Trend and interannual variability in southeast Greenland sea ice: Impacts on coastal Greenland climate variability

We describe the recent occurrence of a region of diminished sea ice cover or “notch” offshore of the Kangerdlugssuaq Fiord, the site of the largest tidewater glacier along Greenland's southeast coast. The notch's location is consistent with a topographically forced flux of warm water toward the fiord, and the decrease of the sea ice cover is shown to be associated with a regional warming of the upper ocean that began in the mid‐1990s. Sea ice in the vicinity of the notch also exhibits interannual variability that is shown to be associated with a seesaw in surface temperature and sea ice between southeast and northeast Greenland that is not describable solely in terms of the North Atlantic Oscillation. We therefore argue that other modes of atmospheric variability, including the Lofoten Low, are required to fully document the changes to the climate that are occurring along Greenland's east coast

A coastal current in winter : 2. Wind forcing and cooling of a coastal current east of Cape Cod

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C10014, doi:10.1029/2008JC004750.The combined effect of cooling and wind-driven buoyancy flux (WDBF) on a buoyant coastal current east of Cape Cod is investigated using observations and process-oriented numerical modeling. Theoretical considerations show that with the moderately strong surface density gradients observed in the Outer Cape Cod Coastal Current, WDBF can substantially exceed the buoyancy loss due to cooling, especially during intense winter storms. Evidence of deep convection associated with strong negative WDBF during downwelling-favorable winds is clearly seen in the moored observations. A simplified two-dimensional numerical model is used to illustrate the evolution of wind- and buoyancy-driven cross-shelf overturning circulation in response to surface cooling and episodic storm events. The simulation confirms that WDBF plays an important role in driving subduction of cold surface water at the offshore surface outcrop of the coastal current font. The presence of the coastal current is also shown to block onshore Ekman transport. As a result, the downwelling circulation in a cross-shore plane is predicted to have a complex multicell structure, in which exchange between the inner shelf and midshelf is restricted. The downwelling circulation has a major impact on the cross-shelf origin of cold, dense shelf waters contributing to intermediate layers of the Wilkinson Basin of the Gulf of Maine.This work was supported by the Coastal Ocean Institute of the Woods Hole Oceanographic Institution and the WHOI SeaGrant Office under grant NA06OAR4170021. G.G. was supported by the Office of Naval Research as part of the AWACS program under grant N00014-05-1-0410. A.S. was supported, in part, by WHOI Post-Doctoral Scholarship