On the effect of a sill on dense water formation in a marginal sea

Author Posting. © Sears Foundation for Marine Research, 2008. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 66 (2008): 325-345, doi:10.1357/002224008786176016.The properties of water mass transformation in a semi-enclosed basin, separated from the open ocean by a sill and subject to surface cooling, are analyzed both theoretically and numerically using an ocean general circulation model. This study extends previous studies of convection in a marginal sea to the case with a sill. The sill has a strong impact on both the properties of the dense water formed in the interior and on those of the waters flowing out the marginal sea. It results in a colder interior and colder outflow compared to the case with no sill. Dynamically, this is explained by considering that the sill limits the geostrophic contours over which the open ocean/marginal sea exchange can occur. The impact of the sill, however, is not simply limited to a topographic constriction; instead the sill also decreases the stability of the boundary current, which, in turn, results in relatively large heat flux into the interior and colder outflow. The theories that relate the properties of the dense waters formed in the interior, and those of the outflow, are modified to include the impact of the sill. These are found to compare well with the numerical simulations and provide a useful tool for the interpretation of these results. These idealized simulations capture the basic features of the water mass transformation processes in the Nordic Seas and, in particular, provide a dynamical explanation for the difference between the dense waters formed and the source of the overflows water.DI was supported by the Polar Ocean Climate Processes (ProClim) project funded by the Norwegian Research Council. FS was supported by a visiting scientist fellowship from the Bjerknes Centre for Climate Research (Bergen, Norway) and by NSF Ocean Sciences Grant 0525929. Support for MAS was provided by NSF Office of Polar Programs Grant 0421904 and NSF Ocean Sciences Grant 0423975

Mechanisms of variability in a convective basin

Author Posting. © Sears Foundation for Marine Research, 2009. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 67 (2009): 273-303, doi:10.1357/002224009789954757.An idealized model for a convective basin is used to investigate the mechanisms of variability of the formation and export of dense water. In this model, which consists of two isopycnic layers, dense water formation is induced by surface buoyancy loss in the interior, which is at rest. Newly formed dense water is transmitted to the surrounding boundary current through parameterized eddy fluxes. Variability in the formation and export of dense water is due to changes in the two main drivers: variations in the surface buoyancy fluxes and variations in the large-scale wind via a barotropic boundary current. Numerical integrations of the nonlinear model, with parameters and forcings corresponding to the Labrador Sea, show that the rate of dense water formation in the interior of the basin is strongly affected by changes in the buoyancy forcing, but not significantly affected by seasonal to interannual changes in the wind-driven barotropic boundary current. The basin tends to integrate the buoyancy forcing variability with a memory time scale set by eddies, which is decadal for the Labrador Sea. Variability in dense water export, on the contrary, is strongly affected by changes in the wind-driven barotropic boundary current but hardly affected by changes in buoyancy forcing. Indeed changes in the transport of dense water at the basin outflow are dominated by those at the basin inflow, which, in this model, are directly related to fluctuations in the wind-driven barotropic boundary current. These results, which are consistent with analytical solutions of the linear model, suggest that fluctuations in the surface buoyancy fluxes in the interior Labrador Sea have little impact on the interannual variability of the dense water transport by the Deep Western Boundary Current at the outflow of the Labrador Sea, which is dominated by fluctuations in the wind-driven North Atlantic subpolar gyre, but influence the formation and export of recently ventilated waters.Support for JD from the NOAA Office of Hydrologic Development through a scientific appointment administered by UCAR is gratefully acknowledged. Support for FS was provided by NSF grant OCE−0525929. Support for MAS was provided by NSF grant OCE−0423975

Moored observations of synoptic and seasonal variability in the East Greenland Coastal Current

Author Posting. © American Geophysical Union, 2014. 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 119 (2014): 8838–8857, doi:10.1002/2014JC010134.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.This work was funded by the National Science Foundation grant OCE-1130008, NASA grant NNX13AK88G, and the Ocean and Climate Change Institute at the Woods Hole Oceanographic Institution.2015-06-2

Mechanisms behind the temporary shutdown of deep convection in the Labrador Sea : lessons from the Great Salinity Anomaly years 1968–71

Author Posting. © American Meteorological Society, 2012. 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 Climate 25 (2012): 6743–6755, doi:10.1175/JCLI-D-11-00549.1.From 1969 to 1971 convection in the Labrador Sea shut down, thus interrupting the formation of the intermediate/dense water masses. The shutdown has been attributed to the surface freshening induced by the Great Salinity Anomaly (GSA), a freshwater anomaly in the subpolar North Atlantic. The abrupt resumption of convection in 1972, in contrast, is attributed to the extreme atmospheric forcing of that winter. Here oceanic and atmospheric data collected in the Labrador Sea at Ocean Weather Station Bravo and a one-dimensional mixed layer model are used to examine the causes of the shutdown and resumption of convection in detail. These results highlight the tight coupling of the ocean and atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean. They are also relevant to present-day conditions given the increased ice melt in the Arctic Ocean and from the Greenland Ice Sheet. The analysis herein shows that the shutdown was initiated by the GSA-induced freshening as well as the mild 1968/69 winter. After the shutdown had begun, however, the continuing lateral freshwater flux as well as two positive feedbacks [both associated with the sea surface temperature (SST) decrease due to lack of convective mixing with warmer subsurface water] further inhibited convection. First, the SST decrease reduced the heat flux to the atmosphere by reducing the air–sea temperature gradient. Second, it further reduced the surface buoyancy loss by reducing the thermal expansion coefficient of the surface water. In 1972 convection resumed because of both the extreme atmospheric forcing and advection of saltier waters into the convection region.This research was funded by a grant from the NWO/SRON User Support Programme Space Research. FS acknowledges support from OCE- 0850416 and NOAA NA08OAR4310569.2013-04-0

Seasonal and interannual variability of the West Greenland Current System in the Labrador Sea in 1993–2008

Author Posting. © American Geophysical Union, 2015. 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 120 (2015): 1318–1332, doi:10.1002/2014JC010386.The West Greenland Current System (WGCS) transports heat and freshwater into the Labrador Sea, influencing the formation of Labrador Sea Water, a key component of the Atlantic Meridional Overturning Circulation. Notwithstanding its importance, relatively little is known about the structure and transport of this current system and its seasonal and interannual variability. Here we use historical hydrographic data from 1992 to 2008, combined with AVISO satellite altimetry, to diagnose the mean properties as well as seasonal and interannual variability of the boundary current system. We find that while the surface, fresh, cold West Greenland Current is amplified in summer, the subsurface warm, salty Irminger Current has maximum transport in winter, when its waters are also warmer and saltier. Seasonal changes in the total transport are thus mostly due to changes in the baroclinic structure of the current. By contrast, we find a trend toward warmer/saltier waters and a slowdown of the WGCS, within the period studied. The latter is attributed to changes in the barotropic component of the current. Superimposed on this trend, warm and salty anomalies transit through the system in 1997 and 2003 and are associated with a rapid increase in the transport of the boundary current due to changes in the baroclinic component. The boundary current changes precede similar changes in the interior with a 1–2 year lag, indicating that anomalies advected into the region by the boundary current can play an important role in the modulation of convection in the Labrador Sea.T.R. and F.S. were supported by NSF OCE grants 0525929 and 0850416. A.B. was supported by NSF OCE grant 0623192.2015-08-2

Katabatic wind-driven exchange in fjords

Author Posting. © American Geophysical Union, 2017. 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 122 (2017): 8246–8262, doi:10.1002/2017JC013026.The general issue of katabatic wind-driven exchange in fjords is considered using an idealized numerical model, theory, and observations. Two regimes are identified. For fjords narrower than a viscous boundary layer width, the exchange is limited by a balance between wind and friction in lateral boundary layers. For the nonlinear viscous parameterization used here, this boundary layer thickness depends on the properties of the fjord, such as stratification and length, as well as on the wind stress and numerical parameters such as grid spacing and an empirical constant. For wider fjords typical of east Greenland, the balance is primarily between wind, the along-fjord pressure gradient, and acceleration, in general agreement with previous two-layer nonrotating theories. It is expected that O(10%) of the surface layer will be flushed out of the fjord by a single wind event. Application of the idealized model to a typical katabatic wind event produces outflowing velocities that are in general agreement with observations in Sermilik Fjord, a large glacial fjord in southeast Greenland. The presence of a sill has only a minor influence on the exchange until the sill penetrates over most of the lower layer thickness, in which cases the exchange is reduced. It is concluded that the multiple katabatic wind events per winter that are experienced by the fjords along east Greenland represent an important mechanism of exchange between the fjord and shelf, with implications for the renewal of warm, salty waters at depth and for the export of glacial freshwater in the upper layer.National Science Foundation Grant Numbers: OCE-1533170, OCE-1434041, PLR-1418256; NOAA Climate and Global Change Postdoctoral Fellowship2018-04-2

Characteristics and dynamics of two major Greenland glacial fjords

Author Posting. © American Geophysical Union, 2014. 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 119 (2014): 3767–3791, doi:10.1002/2013JC009786.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.Funding for this work came from National Science Foundation OPP grant 0909373 and OCE grants 1130008 (D.A.S. and F.S.) and 0959381 (R.P.), and the WHOI Arctic Research Initiative (FS).2014-12-1

Connecting the Greenland Ice Sheet and the ocean : a case study of Helheim Glacier and Sermilik Fjord

Author Posting. © The Oceanography Society, 2016. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 29, no. 4 (2016): 34–45, doi:10.5670/oceanog.2016.97.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.This work has been supported directly or indirectly by the National Science Foundation; NASA; the Woods Hole Oceanographic Institution; the universities of Kansas, Maine, and Oregon; the Kerr, Clark, and Haas Foundations; and Greenpeace

Flujo y cuaternario en Bertoldo de Moosburgo y Nicolás de Cusa

The article analyzes passages of Nicholas of Cusa’s De coniecturis and Berthold of Moosburg’s Expositio super Elementationem theologicam Proclii that deal with the Pythagorean-Neoplatonic doctrines of quaternary and flow in order to understand their affi nities and differences. It also offers a hypothesis about the diffusion of these doctrines during the first half of the fifteenth century in the context of German Albertism.El artículo analiza algunos pasajes de De coniecturis de Nicholas de Cusa y de Expositio super Elementationem theologicam Proclii de Berthold of Moosburg en el que se tratan las doctrinas pitagóricas-neoplatónicas de cuaternario y flujo, a fin de comprender afinidades y diferencias entre ellos. También se formula una hipótesis sobre su difusión en la primera mitad del siglo XV en Alemania en el contexto albertista

Iceberg meltwater fluxes dominate the freshwater budget in Greenland's iceberg-congested glacial fjords

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 Geophysical Research Letters 43 (2016): 11,287–11,294, doi:10.1002/2016GL070718.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.2017-05-0