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  • 1
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    A Dynamically Consistent Closure for Zonally Averaged Ocean Models (2011)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 41 (11). pp. 2242-2258.
    Details
    Publication Date: 2018-04-12
    Description: Simple idealized layered models and primitive equation models show that the meridional gradient of the zonally averaged pressure has no direct relation with the meridional flow. This demonstrates a contradiction in an often-used parameterization in zonally averaged models. The failure of this parameterization reflects the inconsistency between the model of Stommel and Arons and the box model of Stommel, as previously pointed out by Straub. A new closure is proposed. The ocean is divided in two dynamically different regimes: a narrow western boundary layer and an interior ocean; zonally averaged quantities over these regions are considered. In the averaged equations three unknowns appear: the interior zonal pressure difference Delta p(i), the zonal pressure difference Delta p(b) of the boundary layer, and the zonal velocity us at the interface between the two regions. Here Delta p(i) is parameterized using a frictionless vorticity balance, Delta p(b), by the difference of the mean pressure in the interior and western boundary, and u(delta) by the mean zonal velocity of the western boundary layer. Zonally resolved models, a layer model, and a primitive equation model validate the new parameterization by comparing with the respective zonally averaged counterparts. It turns out that the zonally averaged models reproduce well the buoyancy distribution and the meridional flow in the zonally resolved model versions with respect to the mean and time changes.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-11-021.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Stability analysis of the Labrador Current (2014)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 44 (2). pp. 445-463.
    Details
    Publication Date: 2020-08-04
    Description: Mooring observations and model simulations point to an instability of the Labrador Current (LC) during winter, with enhanced eddy kinetic energy (EKE) at periods between 2 to 5 days, and much less EKE during other seasons. Linear stability analysis using vertical shear and stratification from the model reveals three dominant modes of instability in the LC: - a balanced interior mode with along-flow wavelengths of about 30–45 km, phase velocities of 0.3 m/s, maximal growth rates of 1 d−1 and surface intensified, but deep reaching amplitudes, - a balanced shallow mode with along-flow wavelengths of about 0.3–1.5 km, about three times larger phase speeds and growth rates, but amplitudes confined to the mixed layer (ML), - and an unbalanced symmetric mode with largest growth rates, vanishing phase speeds and along-flow structure, and very small cross-flow wavelengths, also confined to the ML. Both balanced modes are akin to baroclinic instability, but operate at moderate to small Richardson numbers Ri with much larger growth rates as for the quasi-geostrophic limit of Ri ≫ 1. The interior mode is found to be responsible for the instability of the LC during winter. Weak stratification and enhanced vertical shear due to local buoyancy loss and the advection of convective water masses from the interior result in small Ri within the LC, and to three times larger growth rates of the interior mode in March compared to summer and fall conditions. Both the shallow and the symmetric mode are not resolved by the model, but it is suggested that they might also play an important role for the instability in the LC and for lateral mixing.
    Type: Article , PeerReviewed
    Format: text
    Format: text
    DOI: 10.1175/JPO-D-13-0121.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    On the Driving Mechanism of the Annual Cycle of the Florida Current Transport (2012)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 42 (5). pp. 824-839.
    Details
    Publication Date: 2018-04-12
    Description: The mechanisms involved in setting the annual cycle of the Florida Current transport are revisited using an adjoint model approach. Adjoint sensitivities of the Florida Current transport to wind stress reproduce a realistic seasonal cycle with an amplitude of ~1.2 Sv (1 Sv ≡ 106 m3 s−1). The annual cycle is predominantly determined by wind stress forcing and related coastal upwelling (downwelling) north of the Florida Strait along the shelf off the North American coast. Fast barotropic waves propagate these anomalies southward and reach the Florida Strait within a month, causing an amplitude of ~1 Sv. Long baroclinic planetary Rossby waves originating from the interior are responsible for an amplitude of ~0.8 Sv but have a different phase. The sensitivities corresponding to the first baroclinic mode propagate westward and are highly influenced by topography. Considerable sensitivities are only found west of the Mid-Atlantic Ridge, with maximum values at the western shelf edge. The second baroclinic mode also has an impact on the Florida Current variability, but only when a mean flow is present. A second-mode wave train propagates southwestward from the ocean bottom on the western side of the Mid-Atlantic Ridge between ~36° and 46°N and at Flemish Cap, where the mean flow interacts with topography, to the surface. Other processes such as baroclinic waves along the shelf and local forcing within the Florida Strait are of minor importance.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-11-0109.1
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  • 4
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    Interpreting eddy fluxes (2007)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 37 . pp. 1282-1296.
    Details
    Publication Date: 2020-08-04
    Description: A generalization of the transformed Eulerian and temporal residual means is presented. The new formulation uses rotational fluxes of buoyancy, and the full hierarchy of statistical density moments, to reduce the cross-isopycnal eddy flux to the physically relevant component associated with the averaged water mass properties. The resulting eddy-induced diapycnal diffusivity vanishes for adiabatic, statistically steady flow, and is related to either the growth or decay of mesoscale density variance and/or the covariance between small-scale forcing (mixing) and density fluctuations, such as that associated with the irreversible removal of density variance by dissipation. The relationship between the new formulation and previous approaches is described and is illustrated using results from an eddying channel model. The formalism is quite general and applies to all kinds of averaging and to any tracer (not just density).
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO3050.1
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  • 5
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    Evaluating Different Parameterizations for Mixed Layer Eddy Fluxes induced by Baroclinic Instability (2014)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 44 (9). pp. 2524-2546.
    Details
    Publication Date: 2015-05-28
    Description: In this study, the authors discuss two different parameterizations for the effect of mixed layer eddies, one based on ageostrophic linear stability analysis (ALS) and the other one based on a scaling of the potential energy release by eddies (PER). Both parameterizations contradict each other in two aspects. First, they predict different functional relationships between the magnitude of the eddy fluxes and the Richardson number (Ri) related to the background state. Second, they also predict different vertical structure functions for the horizontal eddy fluxes. Numerical simulations for two different configurations and for a large range of different background conditions are used to evaluate the parameterizations. It turns out that PER is better suited to capture the Ri dependency of the magnitude of the eddy fluxes. On the other hand, the vertical structure of the meridional eddy fluxes predicted by ALS is more accurate than that of PER, while the vertical structure of the vertical eddy fluxes is well predicted by both parameterizations. Therefore, this study suggests the use of the magnitude of PER and the vertical structure functions of ALS for an improved parameterization of mixed layer eddy fluxes.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/JPO-D-13-0235.1
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  • 6
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    A model with simplified circulation dynamics for a baroclinic ocean with topograpgy, Part I: Waves and wind-driven circulations (2003)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 33 (12). pp. 2719-2737.
    Details
    Publication Date: 2018-04-10
    Description: A new type of ocean general circulation model with simplified physics is described and tested for various simple wind-driven circulation problems. The model consists of the vorticity balance of the depth-averaged flow and a hierarchy of equations for “vertical moments” of density and baroclinic velocity. The first vertical density moment is the (vertically integrated) potential energy, which is used to describe the predominant link between the barotropic and the baroclinic oceanic flow in the presence of sloping topography. Tendency equations for the vertical moments of density and baroclinic velocity and an appropriate truncation of the coupled hierarchy of moments are derived that, together with the barotropic vorticity balance, yield a closed set of equations describing the barotropic–baroclinic interaction (BARBI) model of the oceanic circulation. Idealized companion experiments with a numerical implementation of the BARBI model and a primitive equation model indicate that wave propagation properties and baroclinic adjustments are correctly represented in BARBI in midlatitudes as well as in equatorial latitudes. Furthermore, a set of experiments with a realistic application to the Atlantic/Southern Ocean system reproduces important aspects that have been previously reported by studies of gyre circulations and circumpolar currents using full primitive equation models
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(2003)033〈2719:ASGCMF〉2.0.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Sources of Eddy Kinetic Energy in the Labrador Sea (2002)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 32 (12). pp. 3346-3363.
    Details
    Publication Date: 2020-08-04
    Description: Experiments with a suite of North Atlantic general circulation models are used to examine the sources of eddy kinetic energy (EKE) in the Labrador Sea. A high-resolution model version (112°) quantitatively reproduces the observed signature. A particular feature of the EKE in the Labrador Sea is its pronounced seasonal cycle, with a maximum intensity in early winter, as already found in earlier studies based on altimeter data. In contrast to a previously advanced hypothesis, the seasonally varying eddy field is not related to a forcing by high-frequency wind variations but can be explained by a seasonally modulated instability of the West Greenland Current (WGC). The main source of EKE in the Labrador Sea is an energy transfer due to Reynolds interaction work (barotropic instability) in a confined region near Cape Desolation where the WGC adjusts to a change in the topographic slope: Geostrophic contours tend to converge upstream of Cape Desolation, such that the topographically guided WGC narrows as well and becomes barotropically unstable. The eddies spawned from the WGC instability area, dominating the EKE in the interior Labrador Sea, are predominantly anticyclonic with warm and saline cores in the upper kilometer of the water column, while the few cyclones originating as well from the instability area show a more depth-independent structure. Companion experiments with a ⅓° model exhibit the strength of the WGC, influenced by either changes in the wind stress or heat flux forcing, as a leading factor determining seasonal to interannual changes of EKE in the Labrador Sea
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(2002)032〈3346:SOEKEI〉2.0.CO;2
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  • 8
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    Atlantic Equatorial Deep Jets: Space–Time Structure and Cross-Equatorial Fluxes (2002)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 32 (3). pp. 891-902.
    Details
    Publication Date: 2020-08-04
    Description: The so-called equatorial stacked jets are analyzed with ship-board observations and moored time series from the Atlantic Ocean. The features are identified and isolated by comparing vertical wavenumber spectra at the equator with those a few degrees from the equator. Mode-filtering gives clear views of the jets in meridional sections, the typical extent being ±1° in latitude. The vertical structure can be well described (explaining 82% of the variance) by N−1-stretched cosines, with a Gaussian amplitude tapering in the vertical. The stretched wavelengths are somewhat variable. Fitting jets of a fixed (stretched) wavelength to four moored sensors in the depth range 1300–1900 m, allows one to track the vertical phase of the jets with an rms error of 30°–45°. The resulting fit from a 20-month moored time series shows long periods of unchanging jet conditions and intermittent times of high variability. There is no significant vertical propagation on these timescales nor a seasonal reversal. Using a composite from many different experiments, interannual variability is visible, however. A possible mechanism for the stacked jets is inertial instability, resulting from background meridional shears at the equator. A condition is that the Ertel potential vorticity becomes zero somewhere, due to meridional asymmetries in the zonal flows. The ship-board observations show that this may be approximately fulfilled by the instantaneous zonal low-mode flows at various depths, resulting from an excess of zonal momentum south of the equator most of the time. Inertial instability should act to redistribute this zonal momentum, and our mooring data show indeed persistent northward momentum flux, but not at the depth levels expected. The momentum transport might suggest that the jets can also flux or mix other properties across the equator.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(2002)032〈0891:AEDJST〉2.0.CO;2
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  • 9
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    Adiabatically correcting an eddy-permitting model of the North Atlantic using large-scale hydrographic data: application to the Gulf Stream and the North Atlantic Current (2004)
    AMS (American Meteorological Society)
    In:  Journal of Physical Oceanography, 34 (4). pp. 701-719.
    Details
    Publication Date: 2020-08-04
    Description: This study focuses on an important aspect of air–sea interaction in models, namely, large-scale, spurious heat fluxes due to false pathways of the Gulf Stream and North Atlantic Current (NAC) in the “storm formation region” south and east of Newfoundland. Although high-resolution eddy-resolving models show some improvement in this respect, results are sensitive to poorly understood, subgrid-scale processes for which there is currently no complete, physically based parameterization. A simple method to correct an ocean general circulation model (OGCM), acting as a practical substitute for a physically based parameterization, is explored: the recently proposed “semiprognostic method,” a technique for adiabatically adjusting flow properties of a hydrostatic OGCM. The authors show that application of the method to an eddy-permitting model of the North Atlantic Ocean yields more realistic flow patterns and watermass characteristics in the Gulf Stream and NAC regions; in particular, spurious surface heat fluxes are reduced. Four simple modifications to the method are proposed, and their benefits are demonstrated. The modifications successfully account for three drawbacks of the original method: reduced geostrophic wave speeds, damped mesoscale eddy activity, and spurious interaction with topography. It is argued that use of a corrected (eddy permitting) OGCM in a coupled modeling system for simulating present climate (as now becomes possible because of increasing computer power) should lead to a more realistic simulation in regions of strong air–sea interaction as compared with that obtained with an uncorrected model. The method is also well suited for the simulation of the uptake and transport of passive tracers, such as anthropogenic carbon dioxide or components of ecosystem models.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0485(2004)034〈0701:ACAEMU〉2.0.CO;2
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    North Atlantic Interdecadal Variability: Oceanic response to the North Atlantic Oscillation (1865-1997) (2001)
    AMS (American Meteorological Society)
    In:  Journal of Climate, 14 (5). pp. 676-691.
    Details
    Publication Date: 2019-09-23
    Description: In contrast to the atmosphere, knowledge about interdecadal variability of the North Atlantic circulation is relatively restricted. It is the objective of this study to contribute to understanding how the North Atlantic circulation responds to a forcing by the North Atlantic oscillation (NAO) on interdecadal timescales. For this purpose, the authors analyze observed atmospheric and sea surface temperature (SST) data along with the response of an ocean general circulation model to a realistic monthly surface flux forcing that is solely associated with the NAO for the period 1865–1997. In agreement with previous studies, it is shown that the relationship between the local forcing by the NAO and observed SST anomalies on interdecadal timescales points toward the importance of oceanic dynamics in generating SST anomalies. A comparison between observed and modeled SST anomalies reveals that the model results can be used to assess interdecadal variability of the North Atlantic circulation. The observed/modeled developments of interdecadal SST anomalies during the periods 1915–39 and 1960–84 against the local damping influence from the NAO can be traced back to the lagged response (10–20 yr) of the North Atlantic thermohaline circulation and the subpolar gyre strength to interdecadal variability of the NAO. Additional sensitivity experiments suggest that primarily interdecadal variability in the surface net heat flux forcing associated with the NAO governs interdecadal changes of the North Atlantic circulation
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1175/1520-0442(2001)014〈0676:NAIVOR〉2.0.CO;2
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