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The characteristics of cold core eddies and their effects on the hydrodynamics and biological production in the Andaman seas were studied using in-situ and satellite observations. The existence and spatial extent of the eddy are revealed by the precise structure and patterns of the temperature-salinity (T–S) profiles, nutrients, and chl a. A good illustration of this is the anomaly in sea surface height (SSHA). The Cyclonic Eddies (CE1 and CE2) are monitored using the Okubo-Weiss parameter (− 2 × 10–11/s2) calculated from the satellite SSHA and a geostrophic current centered at 8°N and 92°E and 13°N and 93°E, respectively. At the eastern flank of CE1, measurements are taken in-situ along 8°N and 92.5°–93.5°E. Vertical currents measured with Acoustic Doppler Current Profiles (ADCP) at 0.3 m/s show northward flow along the track. A significant northward flow (0.3 m/s) can be seen in the vertical currents recorded with ADCP, yet a weak southern flow can be seen over the western margin. In addition, the SSHA acquired from altimetry demonstrates the spatial extent, supporting the occurrence of cyclonic eddies. The vertical shear in the horizontal flow is the main contributor to baroclinic instability (Ri 0.0001) in the water column, according to an analysis of the factors that could result in the formation of an eddy. The T–S profiles show that the area contains Bay of Bengal (BoB) water, and there are semi-annual Rossby waves there as well. However, the wind stress curl did not provide a reliable indication of divergence in the region. The region's biological production (chl a) and nutrient distribution (NO2, NO3, PO4, and SiO4) were impacted by the eddy. CE2 is connected to convective mixing processes that take place along the northwest coast of the Andaman Islands as a result of the primarily cold, dry continental air flowing from the north east. [ABSTRACT FROM AUTHOR]

Although El Niño‐Southern Oscillation (ENSO) and its global impacts through teleconnection have been known for decades, if and how the mean currents and mesoscale eddies in the Caribbean Sea are linked to ENSO remains an open question. Here, by analyzing satellite observations and an ocean reanalysis product, we found a close connection between mean currents, eddies in the Caribbean Sea and ENSO on interannual timescales. Strong El Niño events result in enhanced north‐south sea surface height differences and consequently stronger mean currents in the Caribbean Sea, and the opposite happens during La Niña events. The eddy kinetic energy responds to ENSO via eddy‐mean flow interaction, primarily through baroclinic instability, which releases the available potential energy stored in the mean currents to mesoscale eddies. Our results suggest some predictability of the mean currents and eddies in the Caribbean Sea, particularly during strong El Niño and La Niña events. Plain Language Summary: We explored the potential impacts of El Niño‐Southern Oscillation (ENSO) on the circulation and mesoscale eddies in the Caribbean Sea. We found ENSO‐related synchronized changes in mean currents and eddies across the entire Caribbean Sea. The connection between mean currents and ENSO is established through ENSO's impact on the north‐south sea surface height (SSH) difference in the Caribbean Sea, which determines the strength of the geostrophic jet. During strong El Niño events, the easterly wind anomalies will increase the north‐south SSH difference through Ekman transport, and consequently generate stronger mean currents. During strong La Niña events, the opposite happens. Through baroclinic instability, available potential energy stored in the mean currents will be transferred to eddies and results in ENSO‐modified interannual variations of eddy kinetic energy. Our results suggest that interannual variations of mean currents and eddies in the Caribbean Sea might be predictable, particularly during strong El Niño and La Niña events. Key Points: Interannual variations of mean currents and eddies in the Caribbean Sea are linked to El Niño‐Southern Oscillation (ENSO)ENSO‐induced wind pattern changes modulate the north‐south sea surface height differences and hence the mean currents in the Caribbean SeaInterannual variation of eddy kinetic energy in the Caribbean Sea is controlled by baroclinic instability [ABSTRACT FROM AUTHOR]

The ocean circulation dynamics can be represented as a high‐dimensional multi‐scale nonlinear system with inhomogenous meanfields and topography. Numerical simulations of the ocean dynamics resolve the large scales of motion on a computational grid, with the unresolved subgrid interactions parameterized. These simulations are highly dependent upon the grid resolution, unless the subgrid terms are appropriately parameterized. There are five fundamental classes of subgrid interactions: eddy‐eddy; eddy‐topographic; eddy‐meanfield; meanfield‐meanfield; and meanfield‐topographic. Scale dependent parameterizations representing each of these interaction classes are presented here in oceanic flows for the first time. Subgrid parameterizations are calculated and validated in baroclinic quasi‐geostrophic simulations of idealized Antarctic Circumpolar Current flows with representative mean currents and ocean floor topography. The parameterization coefficients are derived from the coarse grained statistics of high resolution reference simulations in spectral space. Stochastic and deterministic parameterizations are developed for the eddy‐eddy interactions, and deterministic forms for the remaining classes. The kinetic energy spectra and meanfield resulting from large eddy simulations (LES) adopting these coefficients accurately replicate those of the reference simulation. The eddy‐eddy interactions are dominant, but all classes need to be parameterized for best results. For LES where baroclinic instability is explicitly resolved the stochastic variants out‐perform the deterministic ones across all scales. When baroclinic instability is not explicitly resolved, the stochastic variants out‐perform the deterministic cases at the large scales, but introduce some distortions at the smallest resolved scales. This study provides an assessment of the relevant strengths of the subgrid interaction classes in an idealized yet representative ocean. Plain Language Summary: Numerical simulation of oceanic circulations is incredibly challenging due in part to the large range of scales. There are oscillations on the size of entire ocean basins, through to fine scale centimeter sized turbulence, and everything in between. Global simulations of the ocean typically capture the large scale features on a discrete grid. To ensure these simulations are physically representative, the influence of the missing smaller scales needs to be captured accurately. Here we present a first calculation of parameterizations governing the interactions between the resolved dynamics and the missing small scale eddies, meanfield and topographic features. Reduced resolution simulations incorporating these parameterizations without tuning have statistics in very good agreement with the high resolution simulations from which they were derived. Key Points: Subgrid models of all interaction classes between eddies, meanfield and topography are calculated from reference global ocean simulationsInter‐eddy interactions are modeled both stochastically and deterministically, with deterministic models used for the other interactionsCoarse simulations using these subgrid coefficients without tuning are in very good agreement with those of the high‐resolution simulation [ABSTRACT FROM AUTHOR]

An analysis is made of unstable perturbations of a geostrophic current of finite transverse scale with a vertical parabolic velocity profile of a general form (with linear and constant velocity shear) in a vertically bounded layer. The model is based on the potential vortex equation in the quasi-geostrophic approximation taking into account the vertical diffusion of momentum and mass. The equation and boundary conditions are reduced to a spectral eigenvalue problem of the Orr–Sommerfeld type. To search for eigenfunctions and eigenvalues, a high-precision analytic–numerical method was used. Particular attention is paid to the study of unstable perturbations with a phase velocity exceeding the maximum flow velocity. Such instability should be distinguished from baroclinic instability and critical layer instability. It is found that the indicated instability can develop in ocean currents when the problem parameters vary in a wide range of values. It is also obtained that, at an increase in the Prandtl number, the phase velocity of such perturbations increases and can significantly exceed the maximum flow velocity. However, the occurrence of such unstable perturbations is possible only when the maximum flow velocity is located in the inner region of the layer (but not necessarily in its center). It has also been found that narrow currents (the transverse scale is equal to or smaller than the Rossby radius) with a parabolic vertical profile can be unstable. The most unstable perturbations have approximately equal scales along and across the flow; that is, they are circular perturbations. A discussion of various types of geostrophic current instability with a vertical parabolic velocity profile as applied to the ocean is presented. [ABSTRACT FROM AUTHOR]

We describe an idealized primitive-equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the Equator and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally include diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence. [ABSTRACT FROM AUTHOR]

In this study, the upper-ocean absolute geostrophic currents in the southern Indian Ocean are constructed using Argo temperature and salinity data from the middepth (1000 m) zonal velocity derived from the Argo float trajectory. The results reveal alternating quasi-zonal striation-like structures of middepth zonal velocity in the equatorial and southern tropical Indian Ocean. Specifically, the eastward time-mean flows are located at the equator and 2°, 5°, 8°, 13°, 16°, 18°–19°, and 21°–22°S, with a meridional scale of ∼300 km. The generation mechanisms of the striation-like zonal velocity structure differ between the near-equatorial and off-equatorial regions. The triad of baroclinic Rossby wave instability plays a significant role in near-equatorial striations. In the south, the high potential vorticity (PV) of Antarctic intermediate water and low PV of southern Indian Ocean Subantarctic Mode Water lead to strong baroclinic instability, which increases the eddy kinetic energy in the middepth layer, thus contributing to a turbulent PV gradient. The convergence/divergence of the eddy PV flux generates the quasi-zonal striations. The meridional scale of the striations is controlled by the most unstable wavelength of baroclinic instability, which explains the observations. Significance Statement: The middepth zonal velocity resembles a system of eastward/westward jets with a considerably smaller width than the larger-scale ocean surface circulation. Such a phenomenon always occurs in a turbulent ocean that presents eddy or eddy–mean flow interactions. This study used float observations to reveal a robust middepth zonal velocity in the southern tropical Indian Ocean, where the width of the eastward time-mean flows is approximately 300 km. Smaller eddies drive the zonal currents with a smaller width, and the energy of the eddies is released from the unstable vertical structure at middepths. This study provides new insights into the generation mechanism of small-width zonal current structures in the deep ocean. [ABSTRACT FROM AUTHOR]

The instability of a surface-trapped jet over rough bottom topography is examined using a linearized quasigeostrophic model. The jet is laterally sheared and thus susceptible to both barotropic and baroclinic instability. The relative magnitude of the two depends on the jet width and on the spectral characteristics and amplitude of the bathymetry. The most unstable eddies in the upper layer are typically smaller over bathymetry than with a flat bottom. Topography also alters momentum flux convergence in the upper layer and causes the perturbations to resemble eddies in a 1.5-layer flow. But as long as the jet is wider than the deformation radius, baroclinic instability is present, yielding deep eddies that are phase-locked to those at the surface. In addition, topography facilitates scattering of energy at depth to other scales. So, instability over rough topography could be an efficient, and largely overlooked, means of transferring mesoscale energy to the dissipative scales. Significance Statement: This study investigates the effects of bottom roughness on large-scale ocean currents and their associated eddies. Roughness affects the eddy size and speed and how they exchange energy with the mean flow. Roughness also facilitates energy transfer to smaller scales where it can be dissipated. Thus, instability over rough topography could be an important part of the oceanic energy balance. [ABSTRACT FROM AUTHOR]

The article presents the first evidence of a dual kinetic energy (KE) cascade in the ocean by analyzing in situ velocity measurements from surface drifters. Topics discussed include seasonal modulation of surface KE, seasonality of interscale KE transfers, and statistical metrics and error estimates.

In idealized simulations of moist baroclinic instability on a sphere, the most unstable mode transitions from a periodic wave to an isolated vortex in sufficiently warm climates. The vortex mode is maintained through latent heating and shows the principal characteristics of a diabatic Rossby vortex (DRV) that has been found in a range of different simulations and observations of the current climate. Currently, there is no analytical theory for DRVs or understanding of the wave–vortex transition that has been found in warmer climates. Here, we introduce a minimal moist two-layer quasigeostrophic model with tilted boundaries capable of producing a DRV mode, and we derive growth rates and length scales for this DRV mode. In the limit of a convectively neutral stratification, the length scale of ascent of the DRV is the same as that of a periodic moist baroclinic wave, but the growth rate of the DRV is 54% faster. We explain the isolated structure of the DRV using a simple potential vorticity (PV) argument, and we create a phase diagram for when the most unstable solution is a periodic wave versus a DRV, with the DRV emerging when the moist static stability and meridional PV gradients are weak. Last, we compare the structure of the DRV mode with DRV storms found in reanalysis and with a DRV storm in a warm-climate simulation. Significance Statement: Past research has identified a special class of midlatitude storm, dubbed the diabatic Rossby vortex (DRV), which derives its energy from the release of latent heat associated with condensation of water vapor and as such goes beyond the traditional understanding of midlatitude storm formation. DRVs have been implicated in extreme and poorly predicted forms of cyclogenesis along the east coast of the United States and the west coast of Europe with significant damage to property and human life. The purpose of this study is to develop a mathematical theory for the intensification rate and length scale of DRVs to gain a deeper understanding of the dynamics of these storms in current and future climates. [ABSTRACT FROM AUTHOR]

In the Arabian Sea, southeast of the Arabian peninsula, an oceanic dipole, named the Ras Al Hadd (RAH) dipole, is formed each year, lying near the Ras Al Hadd cape. The RAH dipole is the association of a cyclonic eddy (CE) to the northeast, with an anticyclonic eddy (AE) to the southwest. This dipole intensifies in the summer monsoon and disappears during the winter monsoon. This dipole has been described previously, but mostly for its surface expression, and for short time intervals. Here, we describe the 3D structure of this dipole over the 2000–2015 period, by combining colocalized ARGO float profiler data (a total of 7552 profiles inside and outside the RAH dipole) with angular momentum eddy detection and tracking algorithm (AMEDA) surface data. We show first the different water masses in and near the RAH dipole. The presence of the Persian Gulf water (PGW) below 200 m depth is confirmed in both eddies. Arabian Sea high salinity water (ASHSW) is found exclusively in the AE; a layer of fresh and cold water is observed above 100 m depth in both eddies. By analyzing the potential density structures, we show that the CE has a surface-intensified structure while the AE is subsurface-intensified. The sea level anomaly shows a 0.04 m elevation above the AE and a 0.2 m depression over the CE. The CE has a faster geostrophic velocity, (vertical velocity, respectively) 0.6 m s−1 than the AE, 0.15 m s−1 (respectively, 3 m day−1 for the CE and 0.6 m day−1 for the AE). After presenting the vertical structure of the dipole, we show the dominance of the nonlinear Ekman pumping in the CE over the linear pumping affecting the dipole. As a consequence, we explain the CE's longer lifetime by its intensity and shallowness, and by its sensitivity to the interaction with the atmosphere (in particular the wind stress) and with neighboring eddies. We examined the possible (co)existence of symmetric, barotropic, and baroclinic instabilities in both eddies. These instabilities coexist near the surface in both eddies. They are intensified for the CE, which suggests that the CE is unstable and the AE is rather stable or may need a long time to be unstable. [ABSTRACT FROM AUTHOR]

Hundreds of full-depth temperature and salinity profiles collected by Deepglider autonomous underwater vehicles (AUVs) in the North Atlantic reveal robust signals in eddy isopycnal vertical displacement and horizontal current throughout the entire water column. In separate glider missions southeast of Bermuda, subsurface-intensified cold, fresh coherent vortices were observed with velocities exceeding 20 cm s−1 at depths greater than 1000 m. With vertical resolution on the order of 20 m or less, these full-depth glider slant profiles newly permit estimation of scaled vertical wavenumber spectra from the barotropic through the 40th baroclinic mode. Geostrophic turbulence theory predictions of spectral slopes associated with the forward enstrophy cascade and proportional to inverse wavenumber cubed generally agree with glider-derived quasi-universal spectra of potential and kinetic energy found at a variety of locations distinguished by a wide range of mean surface eddy kinetic energy. Water-column average spectral estimates merge at high vertical mode number to established descriptions of internal wave spectra. Among glider mission sites, geographic and seasonal variability implicate bottom drag as a mechanism for dissipation, but also the need for more persistent sampling of the deep ocean. Significance Statement: Relative to upper-ocean measurements of temperature, salinity, and velocity, deep ocean measurements (below 2000 m) are fewer in number and more difficult to collect. Deep measurements are needed, however, to explore the nature of deep ocean circulation contributing to the global redistribution of heat and to determine how upper-ocean behavior impacts or drives deep motions. Understanding of geographic and temporal variability in vertical structures of currents and eddies enables improved description of energy pathways in the ocean driven by turbulent interactions. In this study, we use newly developed autonomous underwater vehicles, capable of diving to the seafloor and back on a near daily basis, to collect high-resolution full ocean depth measurements at various locations in the North Atlantic. These measurements reveal connections between surface and deep motions, and importantly show their time evolution. Results of analyzing these vertical structures reveal the deep ocean to regularly "feel" events in the upper ocean and permit new comparisons to deep motions in climate models. [ABSTRACT FROM AUTHOR]

Quasigeostrophic (QG) theory describes the dynamics of synoptic-scale flows in the troposphere that are balanced with respect to both acoustic and internal gravity waves. Within this framework, effects of (turbulent) friction near the ground are usually represented by Ekman layer theory. The troposphere covers roughly the lowest 10 km of the atmosphere while Ekman layer heights are typically just a few hundred meters. However, this two-layer asymptotic theory does not explicitly account for substantial changes of the potential temperature stratification due to diabatic heating associated with cloud formation or with radiative and turbulent heat fluxes which can be significant in about the lowest 3 km and in the middle latitudes. To address this deficiency, this paper extends the classical QG–Ekman layer model by introducing an intermediate dynamically and thermodynamically active layer, called the "diabatic layer" (DL) from here on. The flow in this layer is also in acoustic, hydrostatic, and geostrophic balance but, in contrast to QG flow, variations of potential temperature are not restricted to small deviations from a stable and time-independent background stratification. Instead, within the DL diabatic processes are allowed to affect the leading-order stratification. As a consequence, this layer modifies the pressure field at the top of the Ekman layer, and with it the intensity of Ekman pumping seen by the quasigeostrophic bulk flow. The result is the proposed extended quasigeostrophic three-layer QG–DL–Ekman model for midlatitude dynamics. [ABSTRACT FROM AUTHOR]

A four-dimensional survey by a fleet of seven underwater gliders was used to identify pathways of subduction at the Almeria–Oran Front in the western Mediterranean Sea. The combined glider fleet covered nearly 9000 km over ground while doing over 2500 dives to as deep as 700 m. The gliders had sensors to measure temperature, salinity, velocity, chlorophyll fluorescence, and acoustic backscatter. Data from the gliders were analyzed through objective maps that were functions of across-front distance, alongfront distance, and time on vertical levels separated by 10 m. Geostrophic velocity was inferred using a variational approach, and the quasigeostrophic omega equation was solved for vertical and ageostrophic horizontal velocities. Peak downward vertical velocities were near 25 m day−1 in an event that propagated in the direction of the frontal jet. An examination of an isopycnal surface that outcropped as the front formed showed consistency between the movement of the tracers and the inferred vertical velocity. The vertical velocity tended to be downward on the dense side of the front and upward on the light side so as to flatten the front in the manner of a baroclinic instability. The resulting heat flux approached 80 W m−2 near 100-m depth with a structure that would cause restratification of the front. One glider was used to track an isotherm over a day for a direct measure of vertical velocity as large as 50 m day−1, with a net downward displacement of 15 m over the day. [ABSTRACT FROM AUTHOR]

The dynamics of mesoscale eddies in the Bering Sea is studied by the method for automated eddy identification on the basis of altimetry-derived sea-level anomaly fields (SLA method, AVISO+). For this purpose, a regional database has been created for the Bering Sea (1993–2018) on the basis of the "Mesoscale Eddy Trajectory Atlas Product" global dataset to estimate the polarity (cyclonic/anticyclonic), distribution, radius, intensity, nonlinearity, velocity, and motion trajectories of eddies and determine regions of their formation and decay. The results confirm that mesoscale eddies play a significant role in the Bering Sea. About 592 eddies with a minimum lifetime of 90 days were identified (361 anticyclonic and 231 cyclonic). Anticyclonic and cyclonic eddies had average radii of 53.2 and 63.5 km in the Bering Sea. The average orbital velocities of geostrophic currents in the outer closed loops of the eddies were 14.0 and 10.8 cm/s, respectively. The mean speed of anticyclones and cyclones along their trajectories was 5.1 and 6.2 cm/s. Most of the mesoscale eddies were associated with currents of the Bering Sea. This shows that eddies are mainly generated due to baroclinic/barotropic instability of the Bering Sea slope and Kamchatka currents. The eddy generation in the North Aleutian slope current should be influenced by water exchange through the straits of the Aleutian ridge. The number of "new" anticyclonic eddies is maximum in April–July. Most of the cyclonic eddies are generated in the cold half of the year (January–March). Seasonal variability of the Bering Sea circulation affects the intensity of eddy generation. [ABSTRACT FROM AUTHOR]

The paper presents an analysis of stable and unstable perturbations of ocean currents of a finite transverse scale with a parabolic vertical profiles of velocity (Poiseuille–Couette-type flow), based on the potential vorticity equation in the quasi-geostrophic approximation and taking into account both linear and constant flow velocity shear. The model takes into account the effect of vertical diffusion of buoyancy and vertical friction and assumes that the maximum mean current velocity takes place at the boundary of the layer. The analysis is based on the small perturbation method. The problem depends on several physical parameters and reduces to solving a spectral non-self-adjoint problem for a fourth-order equation with a small parameter at the highest derivative. Asymptotic expansions of the eigenfunctions and eigenvalues are constructed for small values of the wavenumber k. Using the method of continuation in the parameter k, the trajectories of the eigenvalues are calculated for different values of the problem's physical parameters. A detailed analysis of how the features of the vertical flow structure influence the characteristics of stable and unstable perturbations is presented. It is shown that the phase velocities of unstable perturbations can vary significantly depending on the linear vertical shear of the flow velocity. [ABSTRACT FROM AUTHOR]

A submesoscale‐permitting global ocean simulation is used to study the upper ocean turbulence in high kinetic energy (KE) regions. Submesoscale processes peak in winter so that the geostrophic KE spectra tend to be relatively shallow in winter (∼k−2) with steeper spectra in summer (∼k−3). This transition in KE spectral scaling has two phases. In the first phase (late autumn), KE spectra show the presence of two spectral regimes: ∼k−3 power‐law in mesoscales and ∼k−2 power‐law in submesoscales. The first phase appears with the onset of mixed‐layer instabilities, which convert available potential energy into KE, and this process results in a flattening of KE spectra at submesoscales. However, KE spectra at longer wavelengths follow ∼k−3 scaling associated with a forward enstrophy transfer. In the second phase (late winter), KE produced through mixed‐layer instabilities is transferred to larger scales, and k−2 power‐law also develops in mesoscales. Plain Language Summary: Mesoscale and submesoscale motions are important for heat and material transport in the oceans. In the upper ocean, submesoscale turbulence shows seasonal variability and is pronounced in winter. The same distinction is reflected in horizontal wavenumber spectra of kinetic energy (KE) and spectral energy fluxes. In this study, geostrophic KE spectra are analyzed in a submesoscale‐permitting global ocean model to study the seasonal variability in the upper ocean turbulence. We interpret the results in terms of different physical mechanisms and their effects on the evolution of KE spectra over an annual cycle. We find that both mesoscale and submesoscale processes contribute to their characterization. Key Points: Seasonality in the upper ocean turbulence is interpreted in terms of interior baroclinic instabilities and mixed‐layer instabilitiesTemporal variability in the geostrophic kinetic energy spectrum can be subdivided into distinct seasonal phasesBoth a forward enstrophy transfer and an inverse kinetic energy transfer control the spectral slope in the kinetic energy spectrum [ABSTRACT FROM AUTHOR]

Here, we explore the kinematics and dynamics of coastal density fronts (within 10 km from shore and <30-m depth), identified using an edge detection algorithm, in a realistic high-resolution model of the San Diego Bight with relatively weak winds and small freshwater input. The density fronts have lengths spanning 4–10 km and surface density gradients spanning 2–20 × 10−4 kg m−4. Cross-shore-oriented fronts are more likely with northward subtidal flow and are 1/3 as numerous as alongshore-oriented fronts, which are more likely with onshore surface baroclinic diurnal flow. Using a subset of the cross-shore fronts, decomposed into cross-front mean and perturbation components, an ensemble front is created. The ensemble cross-front mean flow is largely geostrophic in the cross- and alongfront directions. The ensemble cross-shore front extends several kilometers from shore, with a distinct linear front axis and downwelling (upwelling) on the dense (light) side of the front, convergent perturbation cross-front flow within the upper 5 m, strengthening the ensemble front. Vertical mixing of momentum is weak, counter to the turbulent thermal wind mechanism. The ensemble cross-shore front resembles a gravity current and is generated by a convergent strain field acting on the large-scale density field. The ensemble front is bounded by the shoreline and is alongfront geostrophic and cross-front ageostrophic. This contrasts with the cross-front geostrophic and alongfront ageostrophic balances of classic deformation frontogenesis, but is consistent with semigeostrophic coastal circulation. [ABSTRACT FROM AUTHOR]

A spectral problem of the Orr–Sommerfeld type is considered to describe unstable disturbances of oceanic geostrophic currents with a linear vertical velocity profile by taking into account the vertical diffusion of buoyancy and friction. Numerical solutions are obtained for different values of the dimensionless parameters of the problem. Calculations of the spectra of eigenvalues and growth rates are compared with those of a similar problem for an ideal fluid. It is shown that (a) dissipation expands the range of wave numbers of unstable disturbances, (b) dissipation can increase the growth rates of baroclinic disturbances, (c) disturbances driven by the instability of the critical layer can grow faster than baroclinic disturbances, and (d) currents with a width equal to or less than the Rossby radius can be unstable; nearly circular (axisymmetric) unstable disturbances (submesoscale eddies) can develop in narrow currents or frontal zones. [ABSTRACT FROM AUTHOR]

The spatial scale of submesoscales is an important parameter for studies of submesoscale dynamics and multiscale interactions. The horizontal spatial scales of baroclinic, geostrophic-branch mixed layer instabilities (MLI) are investigated globally (without the equatorial or Arctic oceans) based on observations and simulations in the surface and bottom mixed layers away from significant topography. Three high-verticalresolution boundary layer schemes driven with profiles from a MITgcm global submesoscale-permitting model improve robustness. The fastest-growing MLI wavelength decreases toward the poles. The zonal median surface MLI wavelength is 51-2.9 km when estimated from the observations and from 32, 25, and 27 km to 2.5, 1.2, and 1.1 km under the K-profile parameterization (KPP), Mellor-Yamada (MY), and κ-ε schemes, respectively. The surface MLI wavelength has a strong seasonality with a median value 1.6 times smaller in summer (10 km) than winter (16 km) globally from the observations. The median bottom MLI wavelengths estimated from simulations are 2.1, 1.4, and 0.41 km globally under the KPP, MY, and κ-ε schemes, respectively, with little seasonality. The estimated required ocean model grid spacings to resolve wintertime surface mixed layer eddies are 1.9 km (50% of regions resolved) and 0.92 km (90%) globally. To resolve summertime eddies or MLI seasonality requires grids finer than 1.3 km (50%) and 0.55 km (90%). To resolve bottom mixed layer eddies, grids finer than 257, 178, and 51m (50%) and 107, 87, and 17m (90%) are estimated under the KPP, MY, and κ-ε schemes. [ABSTRACT FROM AUTHOR]

The North Equatorial Subsurface Current (NESC) is a subthermocline ocean current uncovered recently in the tropical Pacific Ocean, flowing westward below the North Equatorial Countercurrent. In this study, the dynamics of the seasonal cycle of this current are studied using historical shipboard acoustic Doppler current profiler measurements and Argo absolute geostrophic currents. Both data show a westward current at the depths of 200-1000m between 4° and 6°N, with a typical core speed of about 5 and 2 cms-1, respectively. The subsurface current originates in the eastern Pacific, with its core descending to deeper isopycnal surfaces and moving to the equator as it flows westward. The zonal velocity of the NESC shows pronounced seasonal variability, with the annual-cycle harmonics of vertical isothermal displacement and zonal velocity presenting characters of vertically propagating baroclinic Rossby waves. A simple analytical Rossby wave model is employed to simulate the propagation of the seasonal variations of the westward zonal currents successfully, which is the basis for exploring the wind forcing dynamics. The results suggest that the wind curl forcing in the central-eastern basin between 170° and 140°W associated with the meridional movement of the intertropical convergence zone dominates the NESC seasonal variability in the western Pacific, with the winds west of 170°W and east of 140°W playing a minor role in the forcing. [ABSTRACT FROM AUTHOR]

The North Equatorial Subsurface Current (NESC) has recently been found to flow westward below the North Equatorial Countercurrent in the subsurface layer across the Pacific Ocean. The structure, water mass properties, and the dynamics of the NESC are studied using Argo profiles and geostrophic currents, combined with moored current meter observations. The mean westward geostrophic currents of the NESC has been validated with moored current meter measurements at 4.7° N, 142° E in the far western tropical Pacific Ocean. Sizable seasonal-to-interannual variability of the NESC is indicated by the observations, with strong transports in boreal summer and during La Niña events, whereas weak transports in boreal winter and during El Niño events. The water masses of the NESC appear to be the mixture of the North and South Pacific intermediate waters, with the waters immediately below the thermocline closer to the North than to the South Pacific waters. A simulation using a linear continuously stratified model of ocean circulation suggests that the mean NESC is forced by wind curl through low baroclinic mode responses of the ocean. [ABSTRACT FROM AUTHOR]

Near-inertial waves (NIWs) are often an energetic component of the internal wave field on windy continental shelves. The effect of baroclinic geostrophic currents, which introduce both relative vorticity and baroclinicity, on NIWs is not well understood. Relative vorticity affects the resonant frequency feff, while both relative vorticity and baroclinicity modify the minimum wave frequency of freely propagating waves ωmin. On a windy and narrow shelf, we observed wind-forced oscillations that generated NIWs where feff was less than the Coriolis frequency f. If everywhere feff > f then NIWs were generated where ωmin < f and feff was smallest. The background current not only affected the location of generation, but also the NIWs' propagation direction. The estimated NIW energy fluxes show that NIWs propagated predominantly toward the equator because ωmin > f on the continental slope for the entire sample period. In addition to being laterally trapped on the shelf, we observed vertically trapped and intensified NIWs that had a frequency ω within the anomalously low-frequency band (i.e., ωmin < ω < feff), which only exists if the baroclinicity is nonzero. We observed two periods when ωmin < f on the shelf, but the relative vorticity was positive (i.e., feff > f) for one of these periods. The process of NIW propagation remained consistent with the local ωmin, and not feff, emphasizing the importance of baroclinicity on the NIW dynamics. We conclude that windy shelves with baroclinic background currents are likely to have energetic NIWs, but the current and seabed will adjust the spatial distribution and energetics of these NIWs. [ABSTRACT FROM AUTHOR]

An analysis of water level time series from 20 tide gauges in Southeast Asia finds that diurnal and semidiurnal astronomical tides exhibit strong seasonal variability of both amplitude and phase, which is not caused by known modulations of the astronomical tide-generating forces. Instead, it is found that the tidal properties are coherent with the western North Pacific monsoon index (WNPMI), indicating that monsoonal mechanisms are the likely cause. The study domain includes the Malacca Strait, Gulf of Thailand, the southern SouthChina Sea, and Java Sea. The character of the geography and the tidal variability is different in each of these subregions. A new barotropic regional tide model is developed that incorporates the coupling between geostrophic currents, wind-driven (Ekman) currents, and tidal currents in the bottom boundary layer in order to examine the influence of these factors on tides. The dynamics thus preserve the frictional nonlinearities while neglecting advective nonlinearities and baroclinic tides, approximations that should be valid on the wide and shallow continental shelves in the study region. Themodel perturbation approach uses the climatological seasonal variability of wind stress and geostrophic currents, which are prescribed singly and in combination in the model, to explain the observed tidal variability. Results are most successful in the southern Gulf of Thailand and near Singapore, where it is found that the combined effect of geostrophic and Ekman currents shows increased skill in reproducing the tidal variability than individual models. Ambiguous results at other locations suggest more localized processes such as river runoff. [ABSTRACT FROM AUTHOR]

Isolated anticyclones are frequently observed below the mixed layer in the Arctic Ocean. Some of these subsurface anticyclones are thought to originate at surface fronts. However, previous idealized simulations with no surface stress show that only cyclone-anticyclone dipoles can propagate away from baroclinically unstable surface fronts. Numerical simulations of fronts subject to a surface stress presented here show that a surface stress in the same direction as the geostrophic flow inhibits dipole propagation away from the front. On the other hand, a surface stress in the opposite direction to the geostrophic flow helps dipoles to propagate away from the front. Regardless of the surface stress at the point of dipole formation, these dipoles can be broken up on a time scale of days when a surface stress is applied in the right direction. The dipole breakup leads to the deeper anticyclonic component becoming an isolated subsurface eddy. The breakup of the dipole occurs because the cyclonic component of the dipole in the mixed layer is subject to an additional advection because of the Ekman flow. When the Ekman transport has a component oriented from the anticyclonic part of the dipole toward the cyclonic part then the cyclone is advected away from the anticyclone and the dipole is broken up. When the Ekman transport is in other directions relative to the dipole axis, it also leads to deviations in the trajectory of the dipole. A scaling is presented for the rate at which the surface cyclone is advected that holds across a range of mixed layer depths and surface stress magnitudes in these simulations. The results may be relevant to other regions of the ocean with similar near-surface stratification profiles. [ABSTRACT FROM AUTHOR]

The Kuroshio south of Japan shows bimodal path fluctuations between the large meander (LM) path and the nonlarge meander (NLM) path. The transition from the NLM path to the LM path is triggered by a small meander generated off southwestern Japan. The small meander first propagates eastward (downstream) along the Kuroshio and then rapidly amplifies over Koshu Seamount, located about 200 km south of Japan, leading to the formation of the LM path of the Kuroshio. Although Koshu Seamount is essential for the rapid amplification of the small meander, the underlying physical mechanism is not fully understood. In this study, the role of Koshu Seamount is revisited using a two-layer quasi-geostrophic model that takes into account the effects of bottom topography. Numerical experiments show that the transition from the NLM path to the LM path can be successfully reproduced only when bottom topography mimicking Koshu Seamount is incorporated. In this case, the upper-layer meander trough is rapidly amplified together with a lower-layer anticyclone by baroclinic instability during their passage over the northern slope of Koshu Seamount. A linear stability analysis shows that baroclinic instability over a seamount is caused by resonant coupling between the upper-layer Rossby wave in the eastward background flow and the lower-layer seamount-trapped wave during their eastward propagation over the northern slope of the seamount. The spatial scale and structure of this baroclinically unstable mode are close to those of the numerically reproduced small meander in its early amplification stage over the seamount. [ABSTRACT FROM AUTHOR]

Accurate uncertainty quantification for the mean and variance about forced responses to general external perturbations in the climate system is an important subject in understanding Earth's atmosphere and ocean in climate change science. A low-dimensional reduced-order method is developed for uncertainty quantification and capturing the statistical sensitivity in the principal model directions with largest variability and in various regimes in two-layer quasigeostrophic turbulence. Typical dynamical regimes tested here include the homogeneous flow in the high latitudes and the anisotropic meandering jets in the low latitudes and/or midlatitudes. The idea in the reduced-order method is from a self-consistent mathematical framework for general systems with quadratic nonlinearity, where crucial high-order statistics are approximated by a systematic model calibration procedure. Model efficiency is improved through additional damping and noise corrections to replace the expensive energy-conserving nonlinear interactions. Model errors due to the imperfect nonlinear approximation are corrected by tuning the model parameters using linear response theory with an information metric in a training phase before prediction. Here a statistical energy principle is adopted to introduce a global scaling factor in characterizing the higher-order moments in a consistent way to improve model sensitivity. The reduced-order model displays uniformly high prediction skill for the mean and variance response to general forcing for both homogeneous flow and anisotropic zonal jets in the first 102 dominant low-wavenumber modes, where only about 0.15% of the total spectral modes are resolved, compared with the full model resolution of 2562 horizontal modes. [ABSTRACT FROM AUTHOR]

This paper explores stability of and transport by baroclinic vortices on the β plane using a two-layer, quasigeostrophic model. The study adapts a wave-mean flow formalism and examines interactions between the axisymmetric flow ('the vortex') and residuals ('the waves'). Unlike baroclinically unstable vortices on the f plane, such vortices on the β plane can be also vulnerable to barotropic instability as revealed by the globally integrated energy balance analysis. The spatial structure of energy fluxes shows the energy leakage inside the vortex core when its breakdown occurs. Mixing by stable and unstable vortical flows is quantified through the computation of finite-time Lyapunov exponent (FTLE) maps. Depending on the strength of wave radiation, the upper-layer FTLE maps of stable vortices show either an annulus or volute ring of vigorous mixing inside the vortex interior. This ring region is disrupted when the vortex becomes unstable. Both stable and unstable vortices show the wavy patterns of FTLE in the near and far fields. Despite the fact that the initial vortex resides in the top layer only, significant FTLE patterns are observed in the deep layer at later times. Lagrangian analysis of the vortex-induced change of large-scale tracer gradient demonstrates significant effects of vortex instability in the top layer and the importance of the wavelike anomalies in the bottom layer. [ABSTRACT FROM AUTHOR]

A set of absolute geostrophic current (AGC) data for the period January 2004 to December 2012 are calculated using the P-vector method based on monthly gridded Argo profiles in the world tropical oceans. The AGCs agree well with altimeter geostrophic currents, Ocean Surface Current Analysis-Real time currents, and moored current-meter measurements at 10-m depth, based on which the classical Sverdrup circulation theory is evaluated. Calculations have shown that errors of wind stress calculation, AGC transport, and depth ranges of vertical integration cannot explain non-Sverdrup transport, which is mainly in the subtropical western ocean basins and equatorial currents near the Equator in each ocean basin (except the North Indian Ocean, where the circulation is dominated by monsoons). The identified non-Sverdrup transport is thereby robust and attributed to the joint effect of baroclinicity and relief of the bottom (JEBAR) and mesoscale eddy nonlinearity. [ABSTRACT FROM AUTHOR]

Large-scale baroclinic instability is investigated as a potential source of Rossby waves and large-scale variability in the ocean. This baroclinic instability is first reviewed in a 2.5-layer model. As already noticed by several authors, the instability arises in westward surface mean flow when the phase velocities of the two vertical modes are made equal by mean flow influence. This large-scale instability is stronger at low latitudes and thus is likely to happen in the westward return flow of the subtropical gyres. Further investigations with a continuous stratification quasigeostrophic model show that the instability is stronger where the mean flow projects negatively on the second baroclinic mode (imposing positive vertical modes at the surface). The linear stability calculation is then performed on Argo-derived mean flow along with mean stratification data. The results show that the unstable regions are situated at low latitudes in every oceanic basin, in western boundary currents, and in some part of the Antarctic Circumpolar Current. The location of these unstable regions is well correlated with the region of negative projection of the mean flow on the second baroclinic mode. Given that the unstable mode growth times are generally smaller than 6 months at low latitudes, these unstable modes are likely to be observable in satellite altimetry. Therefore, results of the present article suggest that the large-scale instability is indeed a source of large-scale variability at low latitudes. [ABSTRACT FROM AUTHOR]

This article reports an analysis of the intrinsic dynamics of the life cycle of fronts in a three-dimensional (3D) inviscid adiabatic semi-geostrophic model performed in physical coordinates instead of geostrophic coordinates. The Sawyer-Eliassen equation is generalized to two coupled equations that govern the ageostrophic circulation in a 3D balanced flow. We present a formal solution of the two ageostrophic potentials in the form of two separate functionals of the geostrophic stream function. The integration reveals simultaneous development of realistic cold and warm fronts from a weak unstable wave disturbance in the presence of an initial baroclinic jet. They reach full intensity in a few days. The model fronts evolve towards a singularity that signals their impending demise. Frontal collapse occurs shortly afterwards. It is diagnosed with the concept of semi-geostrophic potential vorticity. The dynamical difference between cold and warm fronts stems from the differences in the thermal advection by the geostrophic and ageostrophic flow components in the two frontal regions. [ABSTRACT FROM AUTHOR]

Absolute geostrophic currents in the North Pacific Ocean were calculated using P-vector method from newly gridded Argo profiling float data collected during 2004-2009. The meridional volume transport of geostrophic currents differed significantly from the classical Sverdrup balance, with differences of 10×10-20×10m/s in the interior tropical Northwest Pacific Ocean. Analyses showed that errors of wind stress estimation could not explain all of the differences. The largest differences were found in the areas immediately north and south of the bifurcation latitude of the North Equatorial Current west of the dateline, and in the recirculation area of the Kuroshio and its extension, where nonlinear eddy activities were robust. Comparison of the geostrophic meridional transport and the wind-driven Sverdrup meridional transport in a high-resolution OFES simulation showed that nonlinear effects of the ocean circulation were the most likely reason for the differences. It is therefore suggested that the linear, steady wind-driven dynamics of the Sverdrup theory cannot completely explain the meridional transport of the interior circulation of the tropical Northwest Pacific Ocean. [ABSTRACT FROM AUTHOR]

An analysis and physical interpretation of near-inertial waves (NIWs) propagating perpendicular to a steady, two-dimensional, strongly baroclinic, geostrophic current are presented. The analysis is appropriate for geostrophic currents with order-one Richardson numbers such as those associated with fronts experiencing strong, wintertime atmospheric forcing. This work highlights the underlying physics behind the properties of the NIWs using parcel arguments and the principles of conservation of density and absolute momentum. Baroclinicity introduces lateral gradients in density and vertical gradients in absolute momentum that significantly modify the dispersion and polarization relations and propagation of NIWs relative to classical internal wave theory. In particular, oscillations at the minimum frequency are not horizontal but, instead, are slanted along isopycnals. Furthermore, the polarization of the horizontal velocity is not necessarily circular at the minimum frequency and the spiraling of the wave's velocity vector with time and depth can be in the opposite direction from that predicted by classical theory. Ray tracing and numerical solutions illustrate the trapping and amplification of NIWs in regions of strong baroclinicity where the wave frequency is lower than the effective Coriolis frequency. The largest amplification is found at slantwise critical layers that align with the tilted isopycnals of the current. Such slantwise critical layers are seen in wintertime observations of the Gulf Stream and, consistent with the theory, coincide with regions of intensified ageostrophic shear characterized by a banded structure that is spatially coherent along isopycnals. [ABSTRACT FROM AUTHOR]

The importance of bottom topography in the linear baroclinic instability of zonal flows on the β plane is examined by using analytical calculations and a quasigeostrophic eddy-resolving numerical model. The particular focus is on the effects of a zonal topographic slope, compared with the effects of a meridional slope. A zonal slope always destabilizes background zonal flows that are otherwise stable in the absence of topography regardless of the slope magnitude, whereas the meridional slopes stabilize/destabilize zonal flows only through changing the lower-level background potential vorticity gradient beyond a known critical value. Growth rates, phase speeds, and vertical structure of the growing solutions strongly depend on the slope magnitude. In the numerical simulations configured with an isolated meridional ridge, unstable modes develop on both sides of the ridge and propagate eastward of the ridge, in agreement with analytical results. [ABSTRACT FROM AUTHOR]

Variability of the mesoscale eddy field in the Caribbean Sea is analyzed over the period 1993-2009 using geostrophic anomalies derived from altimeter data and a high-resolution (1/12°) regional model. The Colombia Basin presents the largest values of eddy kinetic energy (EKE) and its semiannual cycle, with a main peak in August-October and a secondary peak in February-March, is the dominant feature in the whole Caribbean EKE cycle. The analysis of energy conversion terms between low-frequency currents and eddies explains these peaks by enhanced baroclinic and barotropic instabilities, in response to seasonally varying currents in the region of the Guajira Peninsula. The semiannual acceleration of the atmospheric Caribbean low-level jet intensifies the southern Caribbean Current (sCC) twice a year in this region, together with its vertical and horizontal velocity shears. The asymmetry of the EKE seasonal cycle in the Colombia Basin is explained by a summer peak in the annual cycle of the whole sCC. Numerical results suggest that the arrival of more energetic North Brazil Current rings during part of the year have almost no impact on the seasonal cycle of EKE in the Colombia Basin. Instead, they are shown to contribute, together with the annual cycle of the Caribbean inflow through the southern passages of the Lesser Antilles, to an annual peak of EKE in the Venezuela Basin between May and August. At the interannual scale the mechanism is similar: interannual variability of the alongshore wind stress controls the speed of the southern Caribbean Current and the energy of the eddies in the Colombia Basin through instability. [ABSTRACT FROM AUTHOR]

The nonlinear collision of two western boundary currents (WBCs) of equal transport at a gap of the western boundary is studied using a 1.5-layer reduced-gravity quasigeostrophic ocean model. It is found that, when the gap (of width 2a) is narrow, a ≤ 7.3LM (LM the Munk thickness), neither of the WBCs can penetrate into the western basin because of the restriction of the viscous force. When 7.3LM < a < 9.0LM, both WBCs penetrate into the western basin for small transport and choke for large transport. When 9.0LM ≤ a ≤ 9.6LM, the two WBCs penetrate for small transport, choke for intermediate transport, and shed eddies periodically for large transport. When a > 9.6LM, no steady choking state is found. Instead, the WBCs have only two equilibrium states: the penetrating and the periodic eddy shedding states. A Hopf bifurcation is found for a > 9.0LM. The Reynolds number (Re) of the Hopf bifurcation is sensitive to the magnitude of y(a/LM) and the baroclinic deformation radius, being small for larger y or deformation radius. In addition, a reverse Hopf bifurcations is identified in the decreased Re experiments, occurring at a smaller Re than that of the Hopf bifurcation. The Re of the reverse Hopf bifurcation is not sensitive to the magnitude of the baroclinic deformation radius. Hysteresis behavior of the WBCs is found for a > 9.0LM, because of the existence of the Hopf and reverse Hopf bifurcations. In between them, steady penetrating or choking states coexist with eddy-shedding states. The steady states are found to be sensitive to perturbations of relative vorticity and can transit to periodic eddy-shedding states at the forcing of a mesoscale eddy. [ABSTRACT FROM AUTHOR]

High-resolution simulations of β-channel, zonal-jet, baroclinic turbulence with a three-dimensional quasigeostrophic (QG) model including surface potential vorticity (PV) are analyzed with emphasis on the competing role of interior and surface PV (associated with isopycnal outcropping). Two distinct regimes are considered: a Phillips case, where the PV gradient changes sign twice in the interior, and a Charney case, where the PV gradient changes sign in the interior and at the surface. The Phillips case is typical of the simplified turbulence test beds that have been widely used to investigate the effect of ocean eddies on ocean tracer distribution and fluxes. The Charney case shares many similarities with recent high-resolution primitive equation simulations. The main difference between the two regimes is indeed an energization of submesoscale turbulence near the surface. The energy cycle is analyzed in the ( k, z) plane, where k is the horizontal wavenumber. In the two regimes, the large-scale buoyancy forcing is the primary source of mechanical energy. It sustains an energy cycle in which baroclinic instability converts more available potential energy (APE) to kinetic energy (KE) than the APE directly injected by the forcing. This is due to a conversion of KE to APE at the scale of arrest. All the KE is dissipated at the bottom at large scales, in the limit of infinite resolution and despite the submesoscales energizing in the Charney case. The eddy PV flux is largest at the scale of arrest in both cases. The eddy diffusivity is very smooth but highly nonuniform. The eddy-induced circulation acts to flatten the mean isopycnals in both cases. [ABSTRACT FROM AUTHOR]

The linear baroclinic and parametric instabilities of boundary currents with piecewise-constant potential vorticity are studied in a two-layer quasi-geostrophic model. The growth rates of both the exponential modes and of the optimal perturbations are calculated for the baroclinic instability of steady coastal currents. We show that the growth rates of the exponential modes are maximal for a vertically symmetric flow. Furthermore, the vertical asymmetries induced by different layer thicknesses, the presence of a barotropic potential vorticity or bottom topography, all act to dampen the growth rates and favor growth at shorter wavelengths. It is shown that this behavior can be predicted from the conditions for vertical resonance of Rossby waves on the two potential vorticity fronts. Also, the baroclinic instability of the optimal perturbations has larger growth rates at shorter wavelengths and shorter time scales. As well, the presence of a sloping bottom of moderate amplitude favors the growth of these optimal perturbations. Finally, we compute the growth rates of parametric instability of oscillatory coastal flows. We show that subharmonic resonance is the most unstable mode of growth. In addition, a second region of parametric instability is found (for the first time) away from marginality of exponential-mode baroclinic instability. It is shown that the functional dependency of the growth rates of parametric instability, for optimal excitation, are similar to that of the optimal perturbations of baroclinic instability. To explain this a mechanism for parametric instability, involving the rapid growth of short-wave optimal perturbations, is proposed. [ABSTRACT FROM AUTHOR]

Using 1-year simulated data from extended Prince William Sound (PWS) nowcast/forecast system, both barotropic and baroclinic transports through two-strait, semi-enclosed PWS are examined. With major tidal constituents removed, hourly time series of volume transports through two straits are significantly correlated with net transport well balanced by the time rate of change of the PWS spatial-mean sea level. A transition frequency band occurs within the coherence function of hourly volume transports, which is characterized by a nearly 180° phase shift between low-frequency (>30 h) and high-frequency (<6 h) bands. The transition band is implicitly related to the horizontally divergent and horizontally non-divergent flows inside the Sound. Further investigation of monthly and annual mean volume transports indicates strong seasonal variability of flows through two straits. On the other hand, baroclinic transport through PWS demonstrates the transition between a two-layered flow structure during the wintertime and a well-defined three-layered structure, i.e., inflow in both the surface and bottom layer with outflow in the intermediate layer, in the remainder of the year. This three-layer exchange flow is determined to be mainly buoyancy-driven, geostrophic flow, and thus largely affected by seasonal variability of buoyancy over the shelf and PWS. [ABSTRACT FROM AUTHOR]

Mesoscale eddy activity in the southeast Indian Ocean (15°°--30°°S, 60°°--110°°E) is investigated based on available satellite altimetry observations. The observed sea level anomaly data show that this region is the only eastern basin among the global oceans where strong eddy activity exists. Furthermore, the eddy kinetic energy (EKE) level in this region displays a distinct seasonal cycle with the maximum in austral summer and minimum in austral winter. It is found that this seasonal modulation of EKE is mediated by baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equatorial Current (SEC) system. In austral spring and summer the enhanced flux forcing of combined meridional Ekman and geostrophic convergence strengthens the upper-ocean meridional temperature gradient, intensifying the SICC front and its vertical velocity shear. Modulation of the vertical velocity shear results in the seasonal changes in the strength of baroclinic instability, leading to the seasonal EKE variations in the southeast Indian Ocean. [ABSTRACT FROM AUTHOR]

In a large-scale deformation flow, lateral and vertical buoyancy gradients sharpen through baroclinic frontogenesis near the surface boundary. A ''thermally direct'' ageostrophic secondary circulation cell arises during frontogenesis to maintain geostrophic, hydrostatic (thermal wind) momentum balance for the alongfront flow. Unstable three-dimensional fluctuations can grow during frontogenesis by baroclinic instability of the alongfront shear flow that converts frontal potential energy to fluctuation energy. At finite amplitude, the fluctuations provide alongfront-averaged eddy momentum and buoyancy fluxes that arrest the frontal sharpening even while the deformation flow persists. The frontal ageostrophic secondary circulation reverses to become a ''thermally indirect'' cell in the center of the front. This allows an approximate opposition between ageostrophic advection and eddy-flux divergence in the frontal buoyancy gradient variance (i.e., frontal strength) balance equation, implying frontal equilibration. During the approximately equilibrated phase, the energy exchange rates among the deformation flow, front, and fluctuations are all reduced in comparison with a solution without eddy-flux feedback on the frontal evolution. The mean stratification is enhanced by both frontogenesis and eddy vertical buoyancy flux. The thermally indirect secondary circulation arises from eddy fluxes acting to force a departure in thermal-wind balance for the alongfront flow, overwhelming the single-cell thermally direct circulation induced by the deformation flow. The equilibrated thermal-wind imbalance in the frontal flow is appreciable, and its magnitude is set by the cross-front eddy flux of alongfront vorticity. This demonstrates an essentially inviscid, baroclinic, dynamical process for frontogenetic arrest through frontal instability. [ABSTRACT FROM AUTHOR]

Multiple alternating zonal jets observed in the ocean are studied with an idealized quasigeostrophic zonal-channel model, with the supercritical, zonal background flow imposed. Both eastward and westward background flows with vertical shear are considered. The underlying nonlinear dynamics is illuminated with analysis of the vertical-mode interactions and time-mean eddy fluxes. Interactions between the vertical modes are systematically studied. The barotropic component of the jets is maintained by both barotropic–barotropic and baroclinic–baroclinic time-mean interactions; thus, the barotropic component of the jets cannot be accurately simulated with a randomly forced barotropic model. The roles of the vertical-mode interactions in driving the baroclinic component of the jets are also characterized. Not only the first but also the second baroclinic mode is found to be important for maintaining the baroclinic component of the jets, whereas the barotropic component of the jets is maintained mostly by the barotropic and first baroclinic modes. The properties of the eddy forcing were systematically studied. It is shown that the baroclinic component of the jets is maintained by Reynolds stress forcing and resisted by form stress forcing only in the eastward background flow. In the westward background flow, the jets are maintained by form stress forcing and resisted by Reynolds stress forcing. The meridional scaling and kinematical properties of the jets are studied as well as the roles of meridional boundaries. The Rhines scaling for meridional spacing of the jets is not generally confirmed, and it is also shown that there are multiple stable equilibria with different numbers of the time-mean jets. It is also found that the jets are associated with alternating weak barriers to the meridional material transport, but the locations of these barriers are not unique and depend on the direction of the background flow and depth. Finally, if the channel is closed with meridional walls, then the jets become more latent but the eddy forcing properties do not change qualitatively. [ABSTRACT FROM AUTHOR]

Temperature–salinity profiles from the region studied in the North Atlantic Tracer Release Experiment (NATRE) show large isopycnal excursions at depths just below the thermocline. It is proposed here that these thermohaline filaments result from the mesoscale stirring of large-scale temperature and salinity gradients by geostrophic turbulence, resulting in a direct cascade of thermohaline variance to small scales. This hypothesis is investigated as follows: Measurements from NATRE are used to generate mean temperature, salinity, and shear profiles. The mean stratification and shear are used as the background state in a high-resolution horizontally homogeneous quasigeostrophic model. The mean state is baroclinically unstable, and the model produces a vigorous eddy field. Temperature and salinity are stirred laterally in each density layer by the geostrophic velocity and vertical advection is by the ageostrophic velocity. The simulated temperature–salinity diagram exhibits fluctuations at depths just below the thermocline of similar magnitude to those found in the NATRE data. It is shown that vertical diffusion is sufficient to absorb the laterally driven cascade of tracer variance through an amplification of filamentary slopes by small-scale shear. These results suggest that there is a strong coupling between vertical mixing and horizontal stirring in the ocean at scales below the deformation radius. [ABSTRACT FROM AUTHOR]

The parameterization of geostrophic eddies represents a large sink of energy in most ocean models, yet the ultimate fate of this eddy energy in the ocean remains unclear. The authors conjecture that a significant fraction of the eddy energy may be transferred to internal lee waves and oscillations over rough bottom topography, leading to bottom-enhanced diapycnal mixing. A range of circumstantial evidence in support of this conjecture is presented and discussed. The authors further propose a modification to the Gent and McWilliams eddy parameterization to account for the bottom-enhanced diapycnal mixing. [ABSTRACT FROM AUTHOR]

Ageostrophic baroclinic instabilities develop within the surface mixed layer of the ocean at horizontal fronts and efficiently restratify the upper ocean. In this paper a parameterization for the restratification driven by finite-amplitude baroclinic instabilities of the mixed layer is proposed in terms of an overturning streamfunction that tilts isopycnals from the vertical to the horizontal. The streamfunction is proportional to the product of the horizontal density gradient, the mixed layer depth squared, and the inertial period. Hence restratification proceeds faster at strong fronts in deep mixed layers with a weak latitude dependence. In this paper the parameterization is theoretically motivated, confirmed to perform well for a wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. It is shown to be superior to alternative extant parameterizations of baroclinic instability for the problem of mixed layer restratification. Two companion papers discuss the numerical implementation and the climate impacts of this parameterization. [ABSTRACT FROM AUTHOR]

The authors propose a parameterization for restratification by mixed layer eddies that develop from baroclinic instabilities of ocean fronts. The parameterization is cast as an overturning streamfunction that is proportional to the product of horizontal buoyancy gradient, mixed layer depth, and inertial period. The parameterization has remarkable skill for an extremely wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. In this paper a coarse resolution prognostic model of the parameterization is compared with submesoscale mixed layer eddy resolving simulations. The parameterization proves accurate in predicting changes to the buoyancy. The climate implications of the proposed parameterization are estimated by applying the restratification scaling to observations: the mixed layer depth is estimated from climatology, and the buoyancy gradients are from satellite altimetry. The vertical fluxes are comparable to monthly mean air–sea fluxes in large areas of the ocean and suggest that restratification by mixed layer eddies is a leading order process in the upper ocean. Critical regions for ocean–atmosphere interaction, such as deep, intermediate, and mode water formation sites, are particularly affected. [ABSTRACT FROM AUTHOR]

Abstract: A historical data set is used to describe the coastal transition zone off Northwest Africa during spring 1973 and fall 1975, from 17° to 26°N, with special emphasis on the interaction between subtropical (North Atlantic Central Waters) and tropical (South Atlantic Central Waters) gyres. The near-surface geostrophic circulation, relative to 300m, is quite complex. Major features are a large cyclonic pattern north of Cape Blanc (21°N) and offshore flow at the Cape Verde front. The large cyclone occurs in the region of most intense winds, and resembles a large meander of the baroclinic southward upwelling jet. The Cape Verde frontal system displays substantial interleaving that may partly originate as mesoscale features at the coastal upwelling front. Property–property diagrams show that the front is an effective barrier to all properties except temperature. The analysis of the Turner angle suggests that the frontal system is characterized by large heat horizontal diffusion as a result of intense double diffusion, which results in the smoothing of the temperature horizontal gradients. Nine cross-shore sections are used to calculate along-shore geostrophic water-mass and nutrient transports and to infer exchanges between the coastal transition zone and the deep ocean (import: deep ocean to transition zone; export: transition zone to deep ocean). These exchanges compare well with mean wind-induced transports and actual geostrophic cross-shore transport estimates. The region is divided into three areas: southern (18–21°N), central (21–23.5°N), and northern (23.5–26°N). In the northern area geostrophic import is roughly compensated with wind-induced export during both seasons. In the central area geostrophic import is greater than wind-induced export during spring, resulting in net import of both water (0.8Sv) and nitrate (14kmols−1), but during fall both factors again roughly cancel. In the southern area geostrophy and wind join to export water and nutrients during both seasons, they increase from 0.6Sv and 3kmols−1 during fall to 2.9Sv and 53kmols−1 during spring. [Copyright &y& Elsevier]

• Fronts over the South China Sea shelf are submesoscale. • Fronts can generate submesoscale meanders and eddies by baroclinic instability. • Fronts are generated by the flow straining of geostrophic and Ekman currents. Fronts are ubiquitous dynamic processes in the ocean, which play a significant role in the ocean dynamical and ecological environments. In this paper strong temperature fronts are investigated on the shelf of the Northern South China Sea using high resolution satellite data. These fronts have large horizontal gradients exceeding 1 °C km−1 with spatial scales around several kilometers. The fronts generate meanders and eddies due to baroclinic instability, since these instabilities have spatial scales around the local first baroclinic mode deformation radius. The estimated Rossby number of the fronts is O (0.4), suggesting that the fronts tend to be ageostrophic and show submesoscale features. The Finite Size Lyapunov Exponent analysis of the generation mechanism indicates that the fronts are tightly related to the combined flow straining of geostrophic and Ekman currents. [ABSTRACT FROM AUTHOR]

Argo float trajectory data has been used to estimate the velocities of mid-depth (1000 db) currents in the North Indian Ocean (NIO). The structure of the upper ocean absolute geostrophic currents can be derived more accurately from the Argo temperature and salinity profiles by referring to the estimated velocities rather than by assuming a level of "no motion". The derived flow field reveals that the eastward zonal velocities generate striation-like structures in the Arabian Sea, which is most prominent above the layer of 500 db layer, with a meridional scale of ~300 km; however, such structures are barely observed in the Bay of Bengal. This meridional scale as well as the distribution of the mid-depth striations are unique to the NIO, as compared with the other ocean basins. The nonlinear 1 1 2 -layer reduced-gravity model combined with the triad baroclinic Rossby wave interaction theory have been used to determine the essential factors controlling the characteristics of the quasi-zonal striation structure. Compared with the North Pacific Ocean, the meridional scale in the NIO is narrower, and is caused by the smaller basin size in the equatorial zone, rather than by the semiannual wind stress forcing period or the eastern boundary angle. Furthermore, the coastal trapped Kelvin waves significantly contribute to the generation of the zonal striations in the Arabian Sea. • Mid-depth striations in the North Indian Ocean have been observed by the Argo trajectory data and T/S profiles. • The striations are more prominent in the Arabian Sea while barely observed in the Bay of Bengal. • The meridional scale of those striations is 300 km, which is because of the smaller basin scale in the equatorial zone. • Coastal trapped Kelvin waves contribute significantly to the generation of the zonal striation in the Arabian Sea. [ABSTRACT FROM AUTHOR]

A comparative analysis of estimates of the formation time for intrusions observed under absolutely stable thermohaline stratification in the upper layer of the Deep Polar Water (DPW) is presented based on two different approaches: 1) 2D instability model of the baroclinic front accounting for differential mixing [1], 2) 3D instability model of the geostrophic flow with a linear vertical velocity shear accounting for diffusion of buoyancy and momentum [2]. The instability of the baroclinic front described by model 1 is a monotonic instability. The unstable modes are presented by periodic trigonometric functions; they have no along-front slope. According to model 2, the phase velocity of unstable disturbances depends on the along-front wavenumber k, and the instability is observed in a wide range of horizontal scales: there are unstable long-wave modes with a phase velocity exceeding the maximum value of flow velocity (see [3, 4] for detail), unstable modes on the scales of the Rossby radius, and short-wave perturbations with scales significantly smaller than the Rossby radius (submesoscale structures).For weak geostrophic currents with velocity of about 1 cm/s, which is typical for the upper layer of the DPW, and for typical hydrological parameters of the observed fronts, the formation time of intrusions was found to be about several years for model 1 and about 50-100 days on the scales of the Rossby radius for model 2. A discussion on the description of DPW intrusions based on these two approaches is presented.This research was funded by the state assignment of IO RAS, theme No 0149-2019-0003.Literature1. Kuzmina N.P., Zhurbas N.V., Emelianov M.V., Pyzhevich M.L. Application of interleaving models for description of intrusive layering at the fronts of Deep polar water in the Eurasian Basin (Arctic) // Oceanology. 2014. Vol. 54. No. 5. PP. 557–566. (Engl).2. Kuzmina N.P., Skorokhodov S.L., Zhurbas N.V., Lyzhkov D.A. Description of the disturbances of oceanic geostrophic currents with linear vertical velocity shear taking into account friction and diffusion of density // 2019. Fizika Atmosfery i okeana. In press.3. Kuzmina N.P. Generation of large-scale intrusions at baroclinic fronts: an analytical consideration with a reference to the Arctic Ocean // Ocean Science. 2016. 12(6). P. 1269–1277. doi: 10.5194/os-12-1269-2016.4. Kuzmina N.P., Skorokhodov S.L., Zhurbas N.V., Lyzhkov D.A. Instability of a Geostrophic Current with Linear Vertical velocity shear at scales of interleaving // Fizika Atmosfery i okeana. 2018. 54(1). P. 54–63. [ABSTRACT FROM AUTHOR]