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  • Mesoscale processes  (21)
  • Topographic effects  (19)
  • American Meteorological Society  (39)
  • Springer Science + Business Media
  • 2020-2023  (16)
  • 2015-2019  (23)
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  • American Meteorological Society  (39)
  • Springer Science + Business Media
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  • 1
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    Formation of intrathermocline lenses by eddy–wind interaction (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: 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): 606–612, doi:10.1175/JPO-D-14-0221.1.
    Description: Mesoscale intrathermocline lenses are observed throughout the World Ocean and are commonly attributed to water mass anomalies advected from a distant origin. An alternative mechanism of local generation is offered herein, in which eddy–wind interaction can create lens-shaped disturbances in the thermocline. Numerical simulations illustrate how eddy–wind-driven upwelling in anticyclones can yield a convex lens reminiscent of a mode water eddy, whereas eddy–wind-driven downwelling in cyclones produces a concave lens that thins the mode water layer (a cyclonic “thinny”). Such transformations should be observable with long-term time series in the interiors of mesoscale eddies.
    Description: Support of this research by the National Science Foundation and National Aeronautics and Space Administration is gratefully acknowledged.
    Description: 2015-08-01
    Keywords: Circulation/ Dynamics ; Eddies ; Ekman pumping/transport ; Mesoscale processes ; Models and modeling ; Ocean models ; Primitive equations model
    Repository Name: Woods Hole Open Access Server
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  • 2
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    Mixing variability in the Southern Ocean (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: 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): 966–987, doi:10.1175/JPO-D-14-0110.1.
    Description: A key remaining challenge in oceanography is the understanding and parameterization of small-scale mixing. Evidence suggests that topographic features play a significant role in enhancing mixing in the Southern Ocean. This study uses 914 high-resolution hydrographic profiles from novel EM-APEX profiling floats to investigate turbulent mixing north of the Kerguelen Plateau, a major topographic feature in the Southern Ocean. A shear–strain finescale parameterization is applied to estimate diapycnal diffusivity in the upper 1600 m of the ocean. The indirect estimates of mixing match direct microstructure profiler observations made simultaneously. It is found that mixing intensities have strong spatial and temporal variability, ranging from O(10−6) to O(10−3) m2 s−1. This study identifies topographic roughness, current speed, and wind speed as the main factors controlling mixing intensity. Additionally, the authors find strong regional variability in mixing dynamics and enhanced mixing in the Antarctic Circumpolar Current frontal region. This enhanced mixing is attributed to dissipating internal waves generated by the interaction of the Antarctic Circumpolar Current and the topography of the Kerguelen Plateau. Extending the mixing observations from the Kerguelen region to the entire Southern Ocean, this study infers a large water mass transformation rate of 17 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) across the boundary of Antarctic Intermediate Water and Upper Circumpolar Deep Water in the Antarctic Circumpolar Current. This work suggests that the contribution of mixing to the Southern Ocean overturning circulation budget is particularly significant in fronts.
    Description: AM was supported by the joint CSIRO–University of Tasmania Quantitative Marine Science (QMS) program and the 2009 CSIRO Wealth from Ocean Flagship Collaborative Fund. BMS was supported by the Australian Climate Change Science Program, jointly funded by the Department of the Environment and CSIRO. KLPs salary support was provided by Woods Hole Oceanographic Institution bridge support funds.
    Description: 2015-10-01
    Keywords: Geographic location/entity ; Southern Ocean ; Circulation/ Dynamics ; Diapycnal mixing ; Fronts ; Ocean circulation ; Topographic effects ; Atm/Ocean Structure/ Phenomena ; Mixing
    Repository Name: Woods Hole Open Access Server
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  • 3
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    The role of wave dynamics and small-scale topography for downslope wind events in southeast Greenland (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: 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 the Atmospheric Sciences 72 (2015): 2786–2805, doi:10.1175/JAS-D-14-0257.1.
    Description: 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.
    Description: This study was supported by grants of the National Science Foundation (OCE- 0751554 and OCE-1130008) as well as the Natural Sciences and Engineering Research Council of Canada.
    Description: 2016-01-01
    Keywords: Katabatic winds ; Severe storms ; Air-sea interaction ; Mesoscale processes ; Orographic effects ; Model evaluation/performance
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Length scale of the finite-amplitude meanders of shelfbreak fronts (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: 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): 2598–2620, doi:10.1175/JPO-D-14-0249.1.
    Description: Through combining analytical arguments and numerical models, this study investigates the finite-amplitude meanders of shelfbreak fronts characterized by sloping isopycnals outcropping at both the surface and the shelfbreak bottom. The objective is to provide a formula for the meander length scale that can explain observed frontal length scale variability and also be verified with observations. Considering the frontal instability to be a mixture of barotropic and baroclinic instability, the derived along-shelf meander length scale formula is [b1/(1 + a1S1/2)]NH/f, where N is the buoyancy frequency; H is the depth of the front; f is the Coriolis parameter; S is the Burger number measuring the ratio of energy conversion associated with barotropic and baroclinic instability; and a1 and b1 are empirical constants. Initial growth rate of the frontal instability is formulated as [b2(1 + a1S1/2)/(1 + a2αS1/2)]NH/L, where α is the bottom slope at the foot of the front, and a2 and b2 are empirical constants. The formulas are verified using numerical sensitivity simulations, and fitting of the simulated and formulated results gives a1 = 2.69, b1 = 14.65, a2 = 5.1 × 103, and b2 = 6.2 × 10−2. The numerical simulations also show development of fast-growing frontal symmetric instability when the minimum initial potential vorticity is negative. Although frontal symmetric instability leads to faster development of barotropic and baroclinic instability at later times, it does not significantly influence the meander length scale. The derived meander length scale provides a framework for future studies of the influences of external forces on shelfbreak frontal circulation and cross-frontal exchange.
    Description: WGZ and GGG were supported by the National Science Foundation through Grant OCE-1129125.
    Description: 2016-04-01
    Keywords: Circulation/ Dynamics ; Instability ; Ocean circulation ; Topographic effects ; Atm/Ocean Structure/ Phenomena ; Fronts ; Models and modeling ; Numerical analysis/modeling
    Repository Name: Woods Hole Open Access Server
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  • 5
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    Spreading of Denmark Strait overflow water in the western subpolar North Atlantic : insights from eddy-resolving simulations with a passive tracer (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: 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): 2913–2932, doi:10.1175/JPO-D-14-0179.1.
    Description: The oceanic deep circulation is shared between concentrated deep western boundary currents (DWBCs) and broader interior pathways, a process that is sensitive to seafloor topography. This study investigates the spreading and deepening of Denmark Strait overflow water (DSOW) in the western subpolar North Atlantic using two ° eddy-resolving Atlantic simulations, including a passive tracer injected into the DSOW. The deepest layers of DSOW transit from a narrow DWBC in the southern Irminger Sea into widespread westward flow across the central Labrador Sea, which remerges along the Labrador coast. This abyssal circulation, in contrast to the upper levels of overflow water that remain as a boundary current, blankets the deep Labrador Sea with DSOW. Farther downstream after being steered around the abrupt topography of Orphan Knoll, DSOW again leaves the boundary, forming cyclonic recirculation cells in the deep Newfoundland basin. The deep recirculation, mostly driven by the meandering pathway of the upper North Atlantic Current, leads to accumulation of tracer offshore of Orphan Knoll, precisely where a local maximum of chlorofluorocarbon (CFC) inventory is observed. At Flemish Cap, eddy fluxes carry ~20% of the tracer transport from the boundary current into the interior. Potential vorticity is conserved as the flow of DSOW broadens at the transition from steep to less steep continental rise into the Labrador Sea, while around the abrupt topography of Orphan Knoll, potential vorticity is not conserved and the DSOW deepens significantly.
    Description: This work is supported by ONR Award N00014-09-1-0587, the NSF Physical Oceanography Program, and NASA Ocean Surface Topography Science Team Program.
    Description: 2016-06-01
    Keywords: Circulation/ Dynamics ; Abyssal circulation ; Boundary currents ; Ocean circulation ; Ocean dynamics ; Potential vorticity ; Topographic effects
    Repository Name: Woods Hole Open Access Server
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  • 6
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    Downfront winds over buoyant coastal plumes (2016)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2016. 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 46 (2016): 3139-3154, doi:10.1175/JPO-D-16-0042.1.
    Description: Downfront, or downwelling favorable, winds are commonly found over buoyant coastal plumes. It is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport, spatial extent, and stability of the plumes. In the present study, the interaction of the Ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets. The approach makes use of an idealized numerical model and scaling theory. It is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near-zero average potential vorticity q. For weak to moderate winds, the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratification. However, for very strong winds the wind stress overwhelms the geostrophic stress and leads to a reduction in q, which is balanced by the vertical mixing term. In the absence of winds, the geostrophic stress dominates mixing and the flow rapidly restratifies. Nonlinearity, extremes of relative vorticity and vertical velocity, and mixing are all enhanced by the presence of a coast. Scaling estimates developed for the eddy buoyancy flux, the surface potential vorticity flux, and the diapycnal mixing rate compare well with results diagnosed from a series of numerical model calculations.
    Description: This study was supported by NSF Grants OCE-1433170 (MAS) and OCE-1459677 (LNT).
    Description: 2017-04-07
    Keywords: Coastal flows ; Ekman pumping/transport ; Mesoscale processes ; Wind stress
    Repository Name: Woods Hole Open Access Server
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  • 7
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    ACC meanders, energy transfer, and barotropic–baroclinic instability (2017)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2017. 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 47 (2017): 1291-1305, doi:10.1175/JPO-D-16-0160.1.
    Description: Along-stream variations in the dynamics of the Antarctic Circumpolar Current (ACC) impact heat and tracer transport, regulate interbasin exchange, and influence closure of the overturning circulation. Topography is primarily responsible for generating deviations from zonal-mean properties, mainly through standing meanders associated with regions of high eddy kinetic energy. Here, an idealized channel model is used to explore the spatial distribution of energy exchange and its relationship to eddy geometry, as characterized by both eddy momentum and eddy buoyancy fluxes. Variations in energy exchange properties occur not only between standing meander and quasi-zonal jet regions, but throughout the meander itself. Both barotropic and baroclinic stability properties, as well as the magnitude of energy exchange terms, undergo abrupt changes along the path of the ACC. These transitions are captured by diagnosing eddy fluxes of energy and by adopting the eddy geometry framework. The latter, typically applied to barotropic stability properties, is applied here in the depth–along-stream plane to include information about both barotropic and baroclinic stability properties of the flow. These simulations reveal that eddy momentum fluxes, and thus barotropic instability, play a leading role in the energy budget within a standing meander. This result suggests that baroclinic instability alone cannot capture the dynamics of ACC standing meanders, a challenge for models where eddy fluxes are parameterized.
    Description: The authors all acknowledge support from NSF OCE-1235488. MKY also acknowledges support from the AMS Graduate Student Fellowship.
    Description: 2017-10-12
    Keywords: Southern Ocean ; Channel flows ; Stability ; Topographic effects ; Eddies ; Mesoscale models
    Repository Name: Woods Hole Open Access Server
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  • 8
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    The interaction of recirculation gyres and a deep boundary current (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 573-590, doi:10.1175/JPO-D-17-0206.1.
    Description: Motivated by the proximity of the Northern Recirculation Gyre and the deep western boundary current in the North Atlantic, an idealized model is used to investigate how recirculation gyres and a deep flow along a topographic slope interact. In this two-layer quasigeostrophic model, an unstable jet imposed in the upper layer generates barotropic recirculation gyres. These are maintained by an eddy-mean balance of potential vorticity (PV) in steady state. The authors show that the topographic slope can constrain the northern recirculation gyre meridionally and that the gyre’s adjustment to the slope leads to increased eddy PV fluxes at the base of the slope. When a deep current is present along the topographic slope in the lower layer, these eddy PV fluxes stir the deep current and recirculation gyre waters. Increased proximity to the slope dampens the eddy growth rate within the unstable jet, altering the geometry of recirculation gyre forcing and leading to a decrease in overall eddy PV fluxes. These mechanisms may shape the circulation in the western North Atlantic, with potential feedbacks on the climate system.
    Description: We gratefully acknowledge an AMS graduate fellowship (IALB) and U.S. National Science Foundation Grants OCE-1332667 and 1332834 (IALB and JMT).
    Description: 2018-09-06
    Keywords: Boundary currents ; Meridional overturning circulation ; Mesoscale processes ; Ocean circulation ; Potential vorticity ; Quasigeostrophic models
    Repository Name: Woods Hole Open Access Server
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  • 9
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    Wind-driven circulation in a shelf valley. Part II : Dynamics of the along-valley velocity and transport (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. 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 48 (2018): 883-904, doi:10.1175/JPO-D-17-0084.1.
    Description: The dynamics controlling the along-valley (cross shelf) flow in idealized shallow shelf valleys with small to moderate Burger number are investigated, and analytical scales of the along-valley flows are derived. This paper follows Part I, which shows that along-shelf winds in the opposite direction to coastal-trapped wave propagation (upwelling regime) force a strong up-valley flow caused by the formation of a lee wave. In contrast, along-shelf winds in the other direction (downwelling regime) do not generate a lee wave and consequently force a relatively weak net down-valley flow. The valley flows in both regimes are cyclostrophic with 0(1) Rossby number. A major difference between the two regimes is the along-shelf length scales of the along-valley flows L. In the upwelling regime Ls, depends on the valley width W, and the wavelength lambda(1w) of the coastal-trapped lee wave arrested by the along-shelf flow U-s. In the downwelling regime L depends on the inertial length scale U-s|'f and W-c. The along-valley velocity scale in the upwelling regime, given by V-u approximate to root pi H-c/H-s integral W-c lambda(1w)/2 pi L-x (1+L-x(2)/L-c(2))(-1) e(-(pi Wc)/(lambda 1w),) is based on potential vorticity (PV) conservation and lee-wave dynamics (Hs and H, are the shelf and valley depth scales, respectively, and fis the Coriolis parameter). The velocity scale in the downwelling regime, given by |v(d)| approximate to (H-s/H-s)[1 + (L-x(2)/L-x(2))](-1) fL, is based on PV conservation. The velocity scales are validated by the numerical sensitivity simulations and can be useful for observational studies of along -valley transports. The work provides a framework for investigating cross -shelf transport induced by irregular shelf bathymetry and calls for future studies of this type under realistic environmental conditions and over a broader parameter space.
    Description: Both WGZ and SJL were supported by the National Science Foundation (NSF) through Grant OCE 1154575.WGZis also supported by the NSF Grant OCE 1634965 and SJL by NSF Grant OCE 1558874.
    Description: 2018-10-16
    Keywords: Ocean circulation ; Topographic effects ; Upwelling/downwelling ; Waves, oceanic ; Wind stress ; Ocean models
    Repository Name: Woods Hole Open Access Server
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  • 10
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    Some dynamical constraints on upstream pathways of the Denmark Strait Overflow (2015)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2014. 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 44 (2014): 3033–3053, doi:10.1175/JPO-D-13-0227.1.
    Description: The East Greenland Current (EGC) had long been considered the main pathway for the Denmark Strait overflow (DSO). Recent observations, however, indicate that the north Icelandic jet (NIJ), which flows westward along the north coast of Iceland, is a major separate pathway for the DSO. In this study a two-layer numerical model and complementary integral constraints are used to examine various pathways that lead to the DSO and to explore plausible mechanisms for the NIJ’s existence. In these simulations, a westward and NIJ-like current emerges as a robust feature and a main pathway for the Denmark Strait overflow. Its existence can be explained through circulation integrals around advantageous contours. One such constraint spells out the consequences of overflow water as a source of low potential vorticity. A stronger constraint can be added when the outflow occurs through two outlets: it takes the form of a circulation integral around the Iceland–Faroe Ridge. In either case, the direction of overall circulation about the contour can be deduced from the required frictional torques. Some effects of wind stress forcing are also examined. The overall positive curl of the wind forces cyclonic gyres in both layers, enhancing the East Greenland Current. The wind stress forcing weakens but does not eliminate the NIJ. It also modifies the sign of the deep circulation in various subbasins and alters the path by which overflow water is brought to the Faroe Bank Channel, all in ways that bring the idealized model more in line with observations. The sequence of numerical experiments separates the effects of wind and buoyancy forcing and shows how each is important.
    Description: This study has been supported by National Science Foundation (OCE0927017 and ARC1107412).
    Description: 2015-06-01
    Keywords: Circulation/ Dynamics ; Boundary currents ; Channel flows ; Meridional overturning circulation ; Ocean circulation ; Topographic effects
    Repository Name: Woods Hole Open Access Server
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