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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Article
  • Peer Reviewed
UCLA Previously Published Works (2021)
AbstractLong-lived anticyclonic eddies (ACs) have been repeatedly observed over several North Atlantic basins characterized by bowl-like topographic depressions. Motivated by these previous findings, the authors conduct numerical simulations of the spindown of eddies initialized in idealized topographic bowls. In experiments with one or two isopycnal layers, it is found that a bowl-trapped AC is an emergent circulation pattern under a wide range of parameters. The trapped AC, often formed by repeated mergers of ACs over the bowl interior, is characterized by anomalously low potential vorticity (PV). Several PV segregation mechanisms that can contribute to the AC formation are examined. In one-layer experiments, the dynamics of the AC are largely determined by a nonlinearity parameter ϵ that quantifies the vorticity of the AC relative to the bowl’s topographic PV gradient. The AC is trapped in the bowl for low , but for moderate values () partial PV segregation allows the AC to reside at finite distances from the center of the bowl. For higher , eddies freely cross the topography and the AC is not confined to the bowl. These regimes are characterized across a suite of model experiments using ϵ and a PV homogenization parameter. Two-layer experiments show that the trapped AC can be top or bottom intensified, as determined by the domain-mean initial vertical energy distribution. These findings contrast with previous theories of mesoscale turbulence over topography that predict the formation of a prograde slope current, but do not predict a trapped AC.
    Cover page: Formation of Anticyclones above Topographic DepressionsCreative Commons 'BY' version 4.0 license
    • Thesis
    • Peer Reviewed
    UCLA Electronic Theses and Dissertations (2020)

    The trajectories and stability of boundary currents, of mesoscale vortices, and of recirculations, are often largely imposed by ocean bottom topography. Here several related questions in the influence of topography on mesoscale ocean circulation are investigated, largely motivated by observed circulation features in the sub-polar North Atlantic ocean.

    Observations show that boundary currents tend to become highly variable and shed material near sharp topographic variations, such as peninsula edges or corners of underwater capes. Baroclinic instability is understood to be one of the main causes of internal variability of large scale ocean circulation. Therefore the influence of horizontally curving topography on baroclinic instability is studied, under the hypothesis that the curvature may cause a higher tendency towards instability. That is done within a minimum complexity model, a two-layer quasi-geostrophic model, and compared with the classic rectilinear model. First necessary conditions for instability as well as growth rate bounds are derived. Growth rates are calculated analytically or numerically for several flow and topography profiles. The growth rate in uniform azimuthal flow is similar to that in uniform rectilinear azimuthal flow, but decreases with increasing depth-averaged flow component amplitude. That is recognized as a generalization of the so called “barotropic governor” effect. Instability growth rate is nonetheless higher with uniform azimuthal flow when isopycnal slope is similar to the topographic slope magnitude, a common scenario in the ocean. Non-normal instability is studied as well, and is generally intensified with uniform azimuthal flow. Thus a complex picture emerges as to the influence of horizontal curvature on baroclinic instability.

    The Deep Western Boundary Current (DWBC) carries water masses formed in deep convection sites southward, as part of the Atlantic Overturning Meridional Circulation (AMOC), a circulation pattern of climatic importance. Observations show that the DWBC “leaks” material at an anomalously high rate in its path along two underwater capes in the Newfoundland Basin. The leakiness, resulting in water masses dilution, and in AMOC alternative (interior) pathways southward, has not been studied extensively from a dynamical perspective before. A high-resolution realistic regional numerical model configuration and a particle advection model are developed for this purpose. The numerical results, as well as two datasets of ocean float trajectories, are analyzed to determine the dynamical causes of leakiness and its phenomenology. It is found that leakiness is concentrated in three “hotspots”, in which topography turns and steepens. Mean Lagrangian velocity is offshore at these locations, showing that leakiness occurs by mean separation. The mean velocity does not have a substantial eddy-rectified component at the two northern hotspots, where most of the mean leakiness happens. Likewise, energetic analysis shows eddies do not locally force the mean offshore flow. Furthermore, potential vorticity is not diluted substantially by eddies along mean separating streamlines. These results are consistent with mean leakiness occurring by inertial separation. A scaling analysis also suggests that bathymetric conditions near the leakiness hotspots are supportive of inertial separation. Eddy processes also contribute substantially to leakiness, partially through chaotic advection.

    In several North Atlantic basins semi-stationary anticyclonic vortices (ACs) have been repeatedly observed for decades, within areas with bowl-like topography. These basins play significant parts in AMOC transport and transformations, and previous evidence suggests these ACs contribute to these processes. Therefore the formation processes of ACs above topographic bowls is studied here using idealized free evolution simulations in one or two isopycnal layers. It is demonstrated that ACs readily form under different (bowl-like) topographies and initial conditions. A non-dimensional nonlinearity parameter (epsilon ~ ratio of vorticity to bowl PV gradient), or a potential vorticity (PV) inhomogeneity (PVI) parameter, largely determine if a trapped AC is formed from random mesoscale-like initial conditions. Trapped ACs form and stay close to bowl-center for epsilon <~0.5 (PVI ~ 1). For epsilon >~ 1 (PVI ~ 0) vortices freely cross the topography by mutual interactions. For intermediate epsilon or PVI values, trapped ACs can form at different bowl radii since the PV gradient is nullified by the presence of a slope current. Trapped ACs generally form by repeated mergers of ACs within the bowl, and have anomalously low PV. Tracer analysis shows that ACs which eventually merge into the trapped AC are sourced from within (outside) the bowl in low (high) energy cases. Two different cross-bowl propagation mechanisms are examined. Monopole beta drift as well as dipole self propagation can both contribute to cross-bowl AC material transport, but the latter appears faster in relevant cases. The vertical structure of the trapped AC is studied as well. It is shown that it is top (bottom) intensified for top (bottom) intensified domain-mean initial conditions. That is consistent with observational structure but in contrast with the common vertical structure in Taylor Caps and of the slope current in our simulations, which remain bottom-intensified in all cases. Scaling laws for vertical structures are suggested in several cases. The robustness of AC formation to topographic complexity is studied, as well as its long-term evolution, and the results are contrasted with topographic turbulence theories, which predict a slope current but not a bowl-trapped AC.

    • 1 supplemental video
    Cover page: Topographic effects on mesoscale ocean circulation
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2021)
    Northward flow of Antarctic Bottom Water (AABW) across the Southern Ocean comprises a key component of the global overturning circulation. Yet AABW transport remains poorly constrained by observations and state estimates, and there is presently no means of directly monitoring any component of the Southern Ocean overturning. However, AABW flow is dynamically linked to Southern Ocean surface circulation via the zonal momentum balance, offering potential routes to indirect monitoring of the transport. Exploiting this dynamical link, this study shows that wind stress (WS) fluctuations drive large AABW transport fluctuations on time scales shorter than (Formula presented.) 2 years, which comprise almost all of the transport variance. This connection occurs due to differing time scales on which topographic and interfacial form stresses respond to wind variability, likely associated with differences in barotropic versus baroclinic Rossby wave propagation. These findings imply that AABW transport variability can largely be reconstructed from the surface WS alone.
      Cover page: High‐Frequency Fluctuations in Antarctic Bottom Water Transport Driven by Southern Ocean WindsCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2021)
      AbstractPrevious studies have concluded that the wind-input vorticity in ocean gyres is balanced by bottom pressure torques (BPT), when integrated over latitude bands. However, the BPT must vanish when integrated over any area enclosed by an isobath. This constraint raises ambiguities regarding the regions over which BPT should close the vorticity budget, and implies that BPT generated to balance a local wind stress curl necessitates the generation of a compensating, nonlocal BPT and thus nonlocal circulation. This study aims to clarify the role of BPT in wind-driven gyres using an idealized isopycnal model. Experiments performed with a single-signed wind stress curl in an enclosed, sloped basin reveal that BPT balances the winds only when integrated over latitude bands. Integrating over other, dynamically motivated definitions of the gyre, such as barotropic streamlines, yields a balance between wind stress curl and bottom frictional torques. This implies that bottom friction plays a nonnegligible role in structuring the gyre circulation. Nonlocal bottom pressure torques manifest in the form of along-slope pressure gradients associated with a weak basin-scale circulation, and are associated with a transition to a balance between wind stress and bottom friction around the coasts. Finally, a suite of perturbation experiments is used to investigate the dynamics of BPT. To predict the BPT, the authors extend a previous theory that describes propagation of surface pressure signals from the gyre interior toward the coast along planetary potential vorticity contours. This theory is shown to agree closely with the diagnosed contributions to the vorticity budget across the suite of model experiments.
        Cover page: On the Role of Bottom Pressure Torques in Wind-Driven GyresCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2016)
        Observations and models of deep ocean boundary currents show that they exhibit complex variability, instabilities and eddy shedding, particularly over continental slopes that curve horizontally, for example around coastal peninsulas. In this article the authors investigate the source of this variability by characterizing the properties of baroclinic instability in mean flows over horizontally curved bottom slopes. The classical two-layer quasi-geostrophic solution for linear baroclinic instability over sloping bottom topography is extended to the case of azimuthal mean flow in an annular channel. To facilitate comparison with the classical straight channel instability problem of uniform mean flow, the authors focus on comparatively simple flows in an annulus, namely uniform azimuthal velocity and solid-body rotation. Baroclinic instability in solid-body rotation flow is analytically analogous to the instability in uniform straight channel flow due to several identical properties of the mean flow, including vanishing strain rate and vorticity gradient. The instability of uniform azimuthal flow is numerically similar to straight channel flow instability as long as the mean barotropic azimuthal velocity is zero. Non-zero barotropic flow generally suppresses the instability via horizontal curvature-induced strain and Reynolds stress work. An exception occurs when the ratio of the bathymetric to isopycnal slopes is close to (positive) one, as is often observed in the ocean, in which case the instability is enhanced. A non-vanishing mean barotropic flow component also results in a larger number of growing eigenmodes and in increased non-normal growth. The implications of these findings for variability in deep western boundary currents are discussed.
          Cover page: Baroclinic instability of axially symmetric flow over sloping bathymetry
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2023)
          The oceanic Meridional Overturning Circulation (MOC) plays a key role in the climate system, and monitoring its evolution is a scientific priority. Monitoring arrays have been established at several latitudes in the Atlantic Ocean, but other latitudes and oceans remain unmonitored for logistical reasons. This study explores the possibility of inferring the MOC from globally-available satellite measurements via machine learning (ML) techniques, using the ECCOV4 state estimate as a test bed. The methodological advantages of the present approach include the use purely of available satellite data, its applicability to multiple basins within a single ML framework, and the ML model simplicity (a feed-forward fully connected neural network (NN) with small number of neurons). The ML model exhibits high skill in MOC reconstruction in the Atlantic, Indo-Pacific, and Southern Oceans. The approach achieves a higher skill in predicting the model Southern Ocean abyssal MOC than has previously been achieved via a dynamically-based approach. The skill of the model is quantified as a function of latitude in each ocean basin, and of the time scale of MOC variability. We find that ocean bottom pressure generally has the highest reconstruction skill potential, followed by zonal wind stress. We additionally test which combinations of variables are optimal. Furthermore, ML interpretability techniques are used to show that high reconstruction skill in the Southern Ocean is mainly due to (NN processing of) bottom pressure variability at a few prominent bathymetric ridges. Finally, the potential for reconstructing MOC strength estimates from real satellite measurements is discussed.
            Cover page: Machine Learning‐Derived Inference of the Meridional Overturning Circulation From Satellite‐Observable Variables in an Ocean State EstimateCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2022)
            Antarctic Bottom Water (AABW), which fills the global ocean abyss, is derived from dense water that forms in several distinct Antarctic shelf regions. Previous modeling studies have reached conflicting conclusions regarding export pathways of AABW across the Southern Ocean and the degree to which AABW originating from distinct source regions are blended during their export. This study addresses these questions using passive tracer deployments in a 61-year global high-resolution (0.1°) ocean/sea-ice simulation. Two distinct export “conduits” are identified: Weddell Sea- and Prydz Bay-sourced AABW are blended together and exported mainly to the Atlantic and Indian Oceans, while Ross Sea- and Adelie Land-sourced AABW are exported mainly to the Pacific Ocean. Northward transport of each tracer occurs almost exclusively (>90%) within a single conduit. These findings imply that regional changes in AABW production may impact the three-dimensional structure of the global overturning circulation.
            • 1 supplemental PDF
            Cover page: How Does Antarctic Bottom Water Cross the Southern Ocean?Creative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2020)
            Abstract: We present in situ and remote observations of a Mississippi plume front in the Louisiana Bight. The plume propagated freely across the bight, rather than as a coastal current. The observed cross-front circulation pattern is typical of density currents, as are the small width (≈100 m) of the plume front and the presence of surface frontal convergence. A comparison of observations with stratified density current theory is conducted. Additionally, subcritical to supercritical transitions of frontal propagation speed relative to internal gravity wave (IGW) speed are demonstrated to occur. That is in part due to IGW speed reduction with decrease in seabed depth during the frontal propagation toward the shore. Theoretical steady-state density current propagation speed is in good agreement with the observations in the critical and supercritical regimes but not in the inherently unsteady subcritical regime. The latter may be due to interaction of IGW with the front, an effect previously demonstrated only in laboratory and numerical experiments. In the critical regime, finite-amplitude IGWs form and remain locked to the front. A critical to supercritical transition eventually occurs as the ambient conditions change during frontal propagation, after which IGWs are not supported at the front. The subcritical (critical) to critical (supercritical) transition is related to Froude number ahead (under) the front, consistently with theory. Finally, we find that the front-locked IGW (critical) regime is itself dependent on significant nonlinear speed enhancement of the IGW by their growth to finite amplitude at the front.
            • 1 supplemental PDF
            Cover page: Observations of shoaling density current regime changes in internal wave interactions Observations of shoaling density current regime changes in internal wave interactionsCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2020)
            The southward-flowing deep limb of the Atlantic meridional overturning circulation is composed of both the deep western boundary current (DWBC) and interior pathways. The latter are fed by ‘‘leakiness’’ from the DWBC in the Newfoundland Basin. However, the cause of this leakiness has not yet been explored mechanistically. Here the statistics and dynamics of the DWBC leakiness in the Newfoundland Basin are explored using two float datasets and a high-resolution numerical model. The float leakiness around Flemish Cap is found to be concentrated in several areas (hot spots) that are collocated with bathymetric curvature and steepening. Numerical particle advection experiments reveal that the Lagrangian mean velocity is offshore at these hot spots, while Lagrangian variability is minimal locally. Furthermore, model Eulerian mean streamlines separate from the DWBC to the interior at the leakiness hot spots. This suggests that the leakiness of Lagrangian particles is primarily accomplished by an Eulerian mean flow across isobaths, though eddies serve to transfer around 50% of the Lagrangian particles to the leakiness hot spots via chaotic advection, and rectified eddy transport accounts for around 50% of the offshore flow along the southern face of Flemish Cap. Analysis of the model’s energy and potential vorticity budgets suggests that the flow is baroclinically unstable after separation, but that the resulting eddies induce modest modifications of the mean potential vorticity along streamlines. These results suggest that mean uncompensated leakiness occurs mostly through inertial separation, for which a scaling analysis is presented. Implications for leakiness of other major boundary current systems are discussed.
            • 1 supplemental PDF
            Cover page: Why Does the DeepWestern Boundary Current “Leak” Around Flemish Cap? Why Does the DeepWestern Boundary Current “Leak” Around Flemish Cap?Creative Commons 'BY' version 4.0 license
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