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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Climate dynamics 8 (1993), S. 151-160 
    ISSN: 1432-0894
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The influences of horizontal advection and horizontal diffusion on the variability of sea surface salinity in stochastically forced systems are investigated. Basic ideas are developed using a two dimensional box model and then extended to a more realistic three dimensional ocean general circulation model. It is shown that, in the absence of advection and diffusion, the ocean response is essentially that predicted by Taylor's random walk model. Advection becomes important when the advective time scale is less than the response time of the mixed layer to the stochastic forcing. Advection of parcels from regions of upwelling into regions of downwelling limits their exposure time to the stochastic forcing and thus the maximum attainable variance in the system (variance increases linearly with time). Regions of upwelling and downwelling may be introduced through the thermohaline overturning circulation or by the wind driven Ekman transport, depending on the specific model configuration. Horizontal diffusion is found to be important when the diffusive time scale is less than the mixed layer response time. The primary role of diffusion is to reduce the effective stochastic forcing through rapid mixing of uncorrelated surface forcing events. Because sea surface salinity does not have a negative feedback with the atmosphere, it is more strongly influenced by weak horizontal processes than sea surface temperature (SST). Accurate knowledge of the stochastic forcing amplitude, decorrelation time, and length scale and distribution are critical to model the variance of sea surface salinity. Aspects of the ocean model which strongly influence the variability of sea surface salinity include the surface velocity, horizontal diffusivity, and the mixed layer depth. Implications on modeling of the ocean and coupled ocean-atmosphere systems are discussed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Open-ocean deep convection, one of the processes by which deep waters of the world's oceans are formed, is restricted to a small number of locations (for example, the Mediterranean and Labrador seas). Recently, the southwest Irminger Sea has been suggested as an additional location for ...
    Type of Medium: Electronic Resource
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  • 3
    Publication Date: 2015-09-23
    Description: The Denmark Strait overflow water is the largest dense water plume from the Nordic seas to feed the lower limb of the Atlantic Meridional Overturning Circulation. Its primary source is commonly thought to be the East Greenland Current. However, the recent discovery of the North Icelandic Jet—a deep-reaching current that flows along the continental slope of Iceland—has called this view into question. Here we present high-resolution measurements of hydrography and velocity north of Iceland, taken during two shipboard surveys in October 2008 and August 2009. We find that the North Icelandic Jet advects overflow water into the Denmark Strait and constitutes a pathway that is distinct from the East Greenland Current. We estimate that the jet supplies about half of the total overflow transport, and infer that it is the primary source of the densest overflow water. Simulations with an ocean general circulation model suggest that the import of warm, salty water from the North Icelandic Irminger Current and water-mass transformation in the interior Iceland Sea are critical to the formation of the jet. We surmise that the timescale for the renewal of the deepest water in the meridional overturning cell, and its sensitivity to changes in climate, could be different than presently envisaged.
    Type: Article , PeerReviewed
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  • 4
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    Sears Foundation of Marine Research
    In:  Journal of Marine Research, 52 (6). pp. 1051-1080.
    Publication Date: 2018-06-15
    Description: The forcing of abyssal recirculation gyres by cross-isopycnal mixing and wave fluxes near the deep western boundary is investigated. A three-layer isopycnal primitive equation model is applied in a series of experiments to an idealized basin with bottom topography. In the absence of deep western boundary current instabilities, cross-isopycnal mixing forces a cyclonic recirculation gyre, modified by topography, which is consistent with the traditional Stommel-Arons model. Instabilities of the boundary current fundamentally alter the mean basin-scale deep flow from a cyclonic recirculation to an anticyclonic recirculation. Bottom topography plays a key role in destabilizing the mean flow. The forcing mechanism for the interior recirculation is the horizontal divergence of momentum and potential vorticity fluxes carried by topographic waves that are forced by the boundary current instabilities. The strength of the recirculation gyre is linearly proportional to the kinetic energy of the waves, which is controlled in the present model by bottom drag, and well predicted by a simple scale analysis. This is essentially an adiabatic process. The addition of cross-isopycnal mixing forces the large-scale interior recirculation toward the pole, partially into boundary currents, through linear vorticity dynamics. Vorticity budgets reveal three dynamical regimes for the eddy-driven flows, the western boundary current, the recirculation region, and the interior. Similarities and differences between the mean flow and recent observations in the Brazil Basin are discussed.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2017-01-05
    Description: Author Posting. © American Meteorological Society, 2008. 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 38 (2008): 2294-2307, doi:10.1175/2008JPO3853.1.
    Description: A linear stability analysis of a meridional boundary current on the beta plane is presented. The boundary current is idealized as a constant-speed meridional jet adjacent to a semi-infinite motionless far field. The far-field region can be situated either on the eastern or the western side of the jet, representing a western or an eastern boundary current, respectively. It is found that when unstable, the meridional boundary current generates temporally growing propagating waves that transport energy away from the locally unstable region toward the neutral far field. This is the so-called radiating instability and is found in both barotropic and two-layer baroclinic configurations. A second but important conclusion concerns the differences in the stability properties of eastern and western boundary currents. An eastern boundary current supports a greater number of radiating modes over a wider range of meridional wavenumbers. It generates waves with amplitude envelopes that decay slowly with distance from the current. The radiating waves tend to have an asymmetrical horizontal structure—they are much longer in the zonal direction than in the meridional, a consequence of which is that unstable eastern boundary currents, unlike western boundary currents, have the potential to act as a source of zonal jets for the interior of the ocean.
    Description: This work was supported by the National Science Foundation through Grants OCE- 0423975 (MS, HH) and OCE-9901654 (JP). HH would like to thank her thesis committee as well as the MIT– WHOI Joint Program for partial financial support.
    Keywords: Instability ; Boundary currents
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2007. 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 20 (2007): 3785–3801, doi:10.1175/JCLI4234.1
    Description: The influences of strong gradients in sea surface temperature on near-surface cross-front winds are explored in a series of idealized numerical modeling experiments. The atmospheric model is the Naval Research Laboratory Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model, which is fully coupled to the Regional Ocean Modeling System (ROMS) ocean model. A series of idealized, two-dimensional model calculations is carried out in which the wind blows from the warm-to-cold side or the cold-to-warm side of an initially prescribed ocean front. The evolution of the near-surface winds, boundary layer, and thermal structure is described, and the balances in the momentum equation are diagnosed. The changes in surface winds across the front are consistent with previous models and observations, showing a strong positive correlation with the sea surface temperature and boundary layer thickness. The coupling arises mainly as a result of changes in the flux Richardson number across the front, and the strength of the coupling coefficient grows quadratically with the strength of the cross-front geostrophic wind. The acceleration of the winds over warm water results primarily from the rapid change in turbulent mixing and the resulting unbalanced Coriolis force in the vicinity of the front. Much of the loss/gain of momentum perpendicular to the front in the upper and lower boundary layer results from acceleration/deceleration of the flow parallel to the front via the Coriolis term. This mechanism is different from the previously suggested processes of downward mixing of momentum and adjustment to the horizontal pressure gradient, and is active for flows off the equator with sufficiently strong winds. Although the main focus of this work is on the midlatitude, strong wind regime, calculations at low latitudes and with weak winds show that the pressure gradient and turbulent mixing terms dominate the cross-front momentum budget, consistent with previous work.
    Description: This work was supported by the Office of Naval Research Grant N00014-05-1-0300.
    Keywords: Fronts ; Sea surface temperature ; Wind stress ; Coupled models ; Boundary layer
    Repository Name: Woods Hole Open Access Server
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  • 7
    Publication Date: 2017-01-04
    Description: Author Posting. © Sears Foundation for Marine Research, 2010. 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 68 (2010): 215-236, doi:10.1357/002224010793721424.
    Description: Using a fully-implicit high-resolution two-layer quasi-geostrophic model combined with pseudo-arclength continuation methods, we perform a bifurcation analysis of double-gyre ocean flows to study their initial oscillatory instabilities. In this model, both wind- and thermally-forced flows can be represented. We demonstrate that on the branch of anti-symmetric steady-state solutions the ratio, Ω, of the flow advective speed to the long internal Rossby wave speed determines the type of oscillatory modes to first become unstable. This is the same nondimensional parameter that controls the shape of the geostrophic contours in the linear limit of the circulation. For large values of Ω, the first Hopf bifurcations correspond to the classical baroclinic modes with inter-monthly time periods arising from shear instability of the flow. For small values of Ω, the first Hopf bifurcations correspond instead to barotropic Rossby modes with shorter, monthly periods arising from mixed barotropic-baroclinic instability of the flow. By considering both a wind-forced and a thermally-forced ocean, we show that this is a robust feature that does not depend on the type of forcing driving the circulation.
    Description: NSF Grant OCE-0423975, NSF Grants OCE-042975 and OCE-0850416
    Repository Name: Woods Hole Open Access Server
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  • 8
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2011. 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 24 (2011): 4844–4858, doi:10.1175/2011JCLI4130.1.
    Description: The factors that determine the heat transport and overturning circulation in marginal seas subject to wind forcing and heat loss to the atmosphere are explored using a combination of a high-resolution ocean circulation model and a simple conceptual model. The study is motivated by the exchange between the subpolar North Atlantic Ocean and the Nordic Seas, a region that is of central importance to the oceanic thermohaline circulation. It is shown that mesoscale eddies formed in the marginal sea play a major role in determining the mean meridional heat transport and meridional overturning circulation across the sill. The balance between the oceanic eddy heat flux and atmospheric cooling, as characterized by a nondimensional number, is shown to be the primary factor in determining the properties of the exchange. Results from a series of eddy-resolving primitive equation model calculations for the meridional heat transport, overturning circulation, density of convective waters, and density of exported waters compare well with predictions from the conceptual model over a wide range of parameter space. Scaling and model results indicate that wind effects are small and the mean exchange is primarily buoyancy forced. These results imply that one must accurately resolve or parameterize eddy fluxes in order to properly represent the mean exchange between the North Atlantic and the Nordic Seas, and thus between the Nordic Seas and the atmosphere, in climate models.
    Description: This study was supported by the National Science Foundation under Grants OCE-0726339 and OCE-0850416.
    Keywords: Eddies ; Forcing ; Meridional overturning circulation ; Transport ; North Atlantic Ocean ; Seas/gulfs/bays
    Repository Name: Woods Hole Open Access Server
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  • 9
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    American Meteorological Society
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 1028–1041, doi:10.1175/JPO-D-12-0159.1.
    Description: The circulation induced by the interaction of surface Ekman transport with an island is considered using both numerical models and linear theory. The basic response is similar to that found for the interaction of Ekman layers and an infinite boundary, namely downwelling (upwelling) in narrow boundary layers and deformation-scale baroclinic boundary layers with associated strong geostrophic flows. The presence of the island boundary, however, allows the pressure signal to propagate around the island so that the regions of upwelling and downwelling are dynamically connected. In the absence of stratification the island acts as an effective barrier to the Ekman transport. The presence of stratification supports baroclinic boundary currents that provide an advective pathway from one side of the island to the other. The resulting steady circulation is quite complex. Near the island, both geostrophic and ageostrophic velocity components are typically large. The density anomaly is maximum below the surface and, for positive wind stress, exhibits an anticyclonic phase rotation with depth (direction of Kelvin wave propagation) such that anomalously warm water can lie below regions of Ekman upwelling. The horizontal and vertical velocities exhibit similar phase changes with depth. The addition of a sloping bottom can act to shield the deep return flow from interacting with the island and providing mass transport into/out of the surface Ekman layer. In these cases, the required transport is provided by a pair of recirculation gyres that connect the narrow upwelling/downwelling boundary layers on the eastern and western sides of the island, thus directly connecting the Ekman transport across the island.
    Description: This study was supported by the National Science Foundation under Grants OCE-0826656 and OCE-0959381 (MAS), and OCE-0925061 (JP).
    Description: 2013-11-01
    Keywords: Coastal flows ; Ekman pumping/transport ; Ocean dynamics
    Repository Name: Woods Hole Open Access Server
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  • 10
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    American Meteorological Society
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2013. 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 43 (2013): 2352–2371, doi:10.1175/JPO-D-13-079.1.
    Description: An idealized eddy-resolving numerical model and an analytic three-layer model are used to develop ideas about what controls the circulation of Atlantic Water in the Arctic Ocean. The numerical model is forced with a surface heat flux, uniform winds, and a source of low-salinity water near the surface around the perimeter of an Arctic basin. Despite this idealized configuration, the model is able to reproduce many general aspects of the Arctic Ocean circulation and hydrography, including exchange through Fram Strait, circulation of Atlantic Water, a halocline, ice cover and transport, surface heat flux, and a Beaufort Gyre. The analytic model depends on a nondimensional number, and provides theoretical estimates of the halocline depth, stratification, freshwater content, and baroclinic shear in the boundary current. An empirical relationship between freshwater content and sea surface height allows for a prediction of the transport of Atlantic Water in the cyclonic boundary current. Parameters typical of the Arctic Ocean produce a cyclonic boundary current of Atlantic Water of O(1 − 2 Sv; where 1 Sv ≡ 106 m3 s−1) and a halocline depth of O(200 m), in reasonable agreement with observations. The theory compares well with a series of numerical model calculations in which mixing and environmental parameters are varied, thus lending credibility to the dynamics of the analytic model. In these models, lateral eddy fluxes from the boundary and vertical diffusion in the interior are important drivers of the halocline and the circulation of Atlantic Water in the Arctic Ocean.
    Description: This study was supported by the National Science Foundation under Grants OCE- 0850416, OCE-0959381, and OCE-1232389.
    Description: 2014-05-01
    Keywords: Arctic
    Repository Name: Woods Hole Open Access Server
    Type: Article
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