The East Mediterranean Sea (EMS) circulation variability has previously been characterized as dominated by transient and recurrent mesoscale eddies. We develop nested high resolution EMS numerical simulations to study the circulation pattern and dynamics, particularly for the yet unexplored EMS submesoscale circulation. The EMS cyclonic boundary current is identified as a principle element of the EMS circulation. It generates and advects eddy chains downstream, and sheds mesoscale eddies at several boundary and island wake locations, as well as submesoscale mixed layer spirals and deep submesoscale coherent vortices. During the seasonal fall to winter mixed layer deepening, energetic submesoscale (O(10 km)) eddies, fronts, and filaments emerge, characterized by O(1) Rossby numbers. Mooring data confirms the EMS winter energization of motions with submesoscale times scales. The model-based seasonal submesoscale energization is associated with a k^-2 kinetic energy (KE) wavenumber (k) spectral slope, shallower than the quasigeostrophic-like k^-3 slope diagnosed at summer, and indicative of local control of material dispersion. We diagnose a seasonal inverse (forward) KE cascade above (below) 30 km scales due to rotational (divergent) motions, and show that these commence after completion of the fall submesosacle energization. Additionally, the mooring data and model solutions show intense internal gravity waves (IGWs) activity, dominated by near-inertial waves. Beneath the pycnocline, Helmholtz decomposition demonstrates that the circulation variability is dominated by IGWs, and the associated model KE spectrum also exhibits k^-2 slope. These rich circulation components coexist in the semi-enclosed and topographically-complex EMS, providing ideal environments for studying multi-scale interactions.

It is now well established that changes in the zonal wind stress over the ACC do not lead to changes in its baroclinicity nor baroclinic transport, a phenomenon referred to as “eddy saturation”. Previous studies provide contrasting dynamical mechanisms for this phenomenon: on one extreme, changes in the winds lead to changes the efficiency with which transient eddies transfer momentum to the sea floor; on the other, structural adjustments of the ACC’s standing meanders increase the efficiency of momentum transfer. In this study the authors investigate the relative importance of these mechanisms using an idealized, isopycnal channel model of the ACC. Via separate diagnoses of the model’s time-mean flow and eddy diffusivity, the authors decompose the model’s response to changes in wind stress into contributions from transient eddies and the mean flow. A key result is that holding the transient eddy diffusivity constant while varying the mean flow very closely compensates changes in the wind stress, whereas holding the mean flow constant and varying the eddy diffusivity does not. This implies that “eddy saturation” primarily occurs due to adjustments in the ACC’s standing waves/meanders, rather than due to adjustments of transient eddy behavior. The authors derive a quasi-geostrophic theory for ACC transport saturation by standing waves, in which the transient eddy diffusivity is held fixed, and thus provides dynamical insights into standing wave adjustment to wind changes. These findings imply that representing eddy saturation in global models requires adequate resolution of the ACC’s standing meanders, with wind-responsive parameterizations of the transient eddies being of secondary importance.

 

 

Antarctic Bottom Water (AABW), which fills the global ocean abyss, is derived from dense water which 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 of blending between AABW sourced from distinct source regions 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. North-wards 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.

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 E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.

Previous 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, non-local BPT and thus non-local 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 non-negligible role in structuring the gyre circulation. Non-local 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 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.

Long-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 spin-down of eddies initialized in idealized topographic bowls. In experiments with 1 or 2 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 ε ≲ 1, but for moderate values (0.5 ≲ ε ≲ 1) partial PV segregation allows the AC to reside at finite distances from the center of the bowl. For higher ε ≳ 1, 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.

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.

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.

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.

Tropical atmospheric intra-seasonal phenomena such as the Madden-Julian oscillation (MJO) and Westerly Wind Bursts (WWB) significantly influence climate at various time scales within and outside of the tropics. WWBs have contributed to the occurrence of every major El-Nino event over the past few decades, and the intraseasonal variability has been observed to have a 2-way interaction with El Nino. Observations of these phenomena have been accumulating for decades, and various theoretical models have been proposed for their mechanism. It has been suggested that “Wind Induced Surface Heat Exchange” (WISHE) plays a significant role in the creation and sustenance of these phenomena.

We study the atmosphere’s first baroclinic mode response to nonlinear WISHE forcing in the tropics using a simple shallow water atmospheric model. This simple model produces an interestingly rich behavior with intraseasonal time scales and some characteristics reminiscent of MJO observations.

In a wide range of parameters the model develops a slow, eastward propagating signal, comprised of significant westerly wind anomaly in proximity to the equator, and coherent with a Yanai wave group which travels at the same velocity. The slow signal is not consistent with any free linear wave in the model. A mechanism for the generation of the signal is suggested, where the Yanai wave group forces a slow, off-resonance, Kelvin wave through the nonlinear WISHE term. The slow eastward propagating signal has some variable, wide-band characteristics, and it is shown that its slow dynamics (i.e. envelope) is the suggested forced Kelvin wave. These results may have implications for observed tropical WISHE-related atmospheric intra-seasonal phenomena.