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Eric Kunze, John B. Mickett, and James B. Girton

Abstract

Destratification and restratification of a ~50-m thick surface boundary layer in the North Pacific Subtropical Front are examined during 24-31 March 2017 in the wake of a storm using a ~ 5-km array of 23 chi-augmented EM profiling floats (u, v, T, S, χT), as well as towyo and ADCP ship surveys, shipboard air-sea surface fluxes and parameterized shortwave penetrative radiation. During the first four days, nocturnal destabilizing buoyancy-fluxes mixed the surface layer over almost its full depth every night followed by restratification to N ~ 2 × 10–3 rad s–1 during daylight. Starting on 28 March, nocturnal destabilizing buoyancy-fluxes weakened because weakening winds reduced the latent heat-flux. Shallow mixing and stratified transition layers formed above ~20-m depth. The remnant layer in the lower part of the surface layer was insulated from destabilizing surface forcing. Penetrative radiation, turbulent buoyancy-fluxes and horizontal buoyancy advection all contribute to restratification of this remnant layer, closing the budget to within measurement uncertainties. Buoyancy advective restratification (slumping) plays a minor role. Before 28 March, measured advective restratification ∫(uzbx + vzby)dt is confined to daytime, is often destratifying and is much stronger than predictions of geostrophic adjustment, mixed-layer eddy instability and Ekman buoyancy-flux predictions because of storm-forced inertial shear. Starting on 28 March, the subinertial envelope of measured buoyancy advective restratification in the remnant layer resembles MLE parameterization predictions.

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Xiaolin Bai, Kevin G. Lamb, Jianyu Hu, and Zhiyu Liu

Abstract

Internal solitary-like waves (ISWs) evolve considerably when passing through a critical point separating the deep water where ISWs are waves of depression and shallower water where they are waves of elevation. The location of the critical point is determined by the background current and stratification. In this study, we investigate the influence of tidal currents on the cross-shelf movement of the critical point and elucidate the underlying processes via fully nonlinear numerical simulations. Our simulations reveal phase-locked tidal variations of the critical point, which are mainly attributed to stratification fluctuations that are modulated by the combined effects of cross-shelf barotropic tidal currents and locally generated baroclinic tides. The barotropic tidal currents drive isopycnal displacements as they flow over the slope, and as this occurs baroclinic tides are generated, modulating the stratification and inducing sheared currents. This results in a cross-shelf movement of the critical point, which moves onshore (offshore) when the pycnocline is elevated (depressed) by the flood (ebb) tide. Our idealized numerical simulations for the study region in the South China Sea suggest that the cross-shelf movement of the critical point reaches to O(10) km within a tidal cycle. This distance depends on the strength of tidal currents, stratification, and bathymetry. Because of tidal currents, ISWs of depression may undergo a complex evolution even in a stratification with a shallow pycnocline. For the stratification with a deep pycnocline, the critical point may be at a location deep enough so that its tidal movement becomes insignificant.

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Kenneth G. Hughes, James N. Moum, Emily L. Shroyer, and William D. Smyth

Abstract

In low winds (≲2 m s−1), diurnal warm layers form but shear in the near-surface jet is too weak to generate shear instability and mixing. In high winds (≳8ms−1), surface heat is rapidly mixed downward and diurnal warm layers do not form. Under moderate winds of 3–5 m s−1, the jet persists for several hours in a state that is susceptible to shear instability. We observe low Richardson numbers of Ri ≈ 0.1 in the top 2 m between 10:00 and 16:00 local time (from 4 h after sunrise to 2 h before sunset). Despite Ri being well below the Ri = 1/4 threshold, instabilities do not grow quickly, nor do they overturn. The stabilizing influence of the sea surface limits growth, a result demonstrated by both linear stability analysis and two-dimensional simulations initialized from observed profiles. In some cases, growth rates are sufficiently small (≪1 h−1) that mixing is not expected even though Ri < 1/4. This changes around 16:00–17:00. Thereafter, convective cooling causes the region of unstable flow to move downward, away from the surface. This allows shear instabilities to grow an order of magnitude faster and mix effectively. We corroborate the overall observed diurnal cycle of instability with a freely evolving, two-dimensional simulation that is initialized from rest before sunrise.

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Callum J. Shakespeare, Brian K. Arbic, and Andrew McC. Hogg

Abstract

Internal waves generated at the seafloor propagate through the interior of the ocean, driving mixing where they break and dissipate. However, existing theories only describe these waves in two limiting cases. In one limit, the presence of an upper boundary permits bottom-generated waves to reflect from the ocean surface back to the seafloor, and all the energy flux is at discrete wavenumbers corresponding to resonant modes. In the other limit, waves are strongly dissipated such that they do not interact with the upper boundary and the energy flux is continuous over wavenumber. Here, a novel linear theory is developed for internal tides and lee waves that spans the parameter space in between these two limits. The linear theory is compared with a set of numerical simulations of internal tide and lee wave generation at realistic abyssal hill topography. The linear theory is able to replicate the spatially-averaged kinetic energy and dissipation of even highly non-linear wave fields in the numerical simulations via an appropriate choice of the linear dissipation operator, which represents turbulent wave breaking processes.

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Elizabeth Brasseale and Parker MacCready

Abstract

The inflow to an estuary originates on the shelf. It flushes the estuary and can bring in nutrients, heat, salt, and hypoxic water, having consequences for estuarine ecosystems and fjordic glacial melt. However, the source of estuarine inflow has only been explored in simple models that do not resolve interactions between inflow and outflow outside of the estuarine channel. This study addressed the estuary inflow problem using variations on a three-dimensional primitive equation model of an idealized estuarine channel next to a sloping, unstratified shelf with mixing provided by a single frequency, 12-hour tide. Inflow was identified using particle tracking, momentum budgets, and Total Exchange Flow. Inflow sources were found in shelf water downstream of the estuary, river plume water, and shelf water upstream of the estuary. Downstream is defined here with respect to the direction of coastal trapped wave propagation, which is to the right for an observer looking seaward from the estuary mouth in the northern hemisphere. Downstream of the estuary and offshore of the plume, the dynamics were quasi-geostrophic, consistent with previous simple models. The effect of this inflowing current on the geometry of the river plume front was found to be small. Novel sources of inflow were identified which originated from within the plume and upstream of the estuary.

Open access
Abhishek Savita, Jan D. Zika, Catia M. Domingues, Simon J. Marsland, Gwyn Dafydd Evans, Fabio Boeira Dias, Ryan M. Holmes, and Andrew McC. Hogg

Abstract

Ocean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities to diagnose circulation. Using quasi-steady state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature-depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of Super-Residual Transport (SRT), which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies, but excludes small-scale three dimensional turbulent mixing effect, to construct a new overturning circulation – the ‘Super Residual Circulation’ (SRC).

We find that in the coarse resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11 Sv circulation which transports heat upward. The SRC’s upward heat transport is ~2 times larger in a finer horizontal resolution (0.1°) version of ACCESS, suggesting a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into super-residual processes, as the SRC elucidates the pathways in temperature and depth space along which watermass transformation occurs.

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Michael A. Spall

Abstract

The frequency and latitudinal dependence of the mid-latitude wind-driven meridional overturning circulation (MOC) is studied using theory and linear and nonlinear applications of a quasi-geostrophic numerical model. Wind-forcing is varied by either changing the strength of the wind or by shifting the meridional location of the wind stress curl pattern. At forcing periods less than the first mode baroclinic Rossby wave basin crossing time scale the linear response in the mid-depth and deep ocean is in phase and opposite to the Ekman transport. For forcing periods close to the Rossby wave basin crossing time scale, the upper and deep MOC are enhanced, and the mid-depth MOC becomes phase shifted, relative to the Ekman transport. At longer forcing periods the deep MOC weakens and the mid-depth MOC increases, but eventually for long enough forcing periods (decadal) the entire wind-driven MOC spins down. Nonlinearities and mesoscale eddies are found to be important in two ways. First, baroclinic instability causes the mid-depth MOC to weaken, lose correlation with the Ekman transport, and lose correlation with the MOC in the opposite gyre. Second, eddy thickness fluxes extend the MOC beyond the latitudes of direct wind forcing. These results are consistent with several recent studies describing the four-dimensional structure of the MOC in the North Atlantic.

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Irina I. Rypina, Timothy R. Getscher, Lawrence J. Pratt, and Baptiste Mourre

Abstract

This paper presents analyses of drifters with drogues at different depths – 1, 10, 30, 50 m – that were deployed in the Mediterranean Sea to investigate frontal subduction and upwelling. Drifter trajectories were used to estimate divergence, vorticity, vertical velocity, and finite-size Lyapunov exponents (FTLEs), and to investigate the balance of terms in the vorticity equation. The divergence and vorticity are O(f) and change sign along trajectories. Vertical velocity is O(1 mm/s), increases with depth, indicates predominant upwelling with isolated downwelling events, and sometimes changes sign between 1 and 50 m. Vortex stretching is one of, but not the only, significant term in the vorticity balance. 2D FTLEs are 2 × 10−51/s after 1 day, twice larger than in a 400-m-resolution numerical model. 3D FTLEs are 50% larger than 2D FTLEs and are dominated by the vertical shear of horizontal velocity. Bootstrapping suggests uncertainty levels of ~10% of the time-mean absolute values for divergence and vorticity. Analysis of simulated drifters in a model suggests that drifter-based estimates of divergence and vorticity are close to the Eulerian model estimates, except when drifters get aligned into long filaments. Drifter-based vertical velocity is close to the Eulerian model estimates at 1 m but differs at deeper depths. The errors in the vertical velocity are largely due to the lateral separation between drifters at different depths, and partially due to only measuring at 4 depths. Overall, this paper demonstrates how drifters, heretofore restricted to 2D near-surface observations, can be used to learn about 3D flow properties throughout the upper layer of the water column.

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Johannes Becherer, James N. Moum, Joseph Calantoni, John A. Colosi, John A. Barth, James A. Lerczak, Jacqueline M. McSweeney, Jennifer A. MacKinnon, and Amy F. Waterhouse

Abstract

Broadly-distributed measurements of velocity, density and turbulence spanning the inner shelf off central California indicate that (i) the average shoreward-directed internal tide energy flux (〈FE〉) decreases to near 0 at the 25 m isobath; (ii) the vertically-integrated turbulence dissipation rate (〈D〉) is approximately equal to the flux divergence of internal tide energy (xFE〉); (iii) the ratio of turbulence energy dissipation in the interior relative to the bottom boundary layer (BBL) decreases toward shallow waters; (iv) going inshore, 〈FE〉 becomes decorrelated with the incoming internal wave energy flux; and (v) 〈FE〉 becomes increasingly correlated with stratification toward shallower water.

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Xiaodong Wu, Falk Feddersen, and Sarah N. Giddings

Abstract

Rip currents are generated by surfzone wave breaking and are ejected offshore inducing inner-shelf flow spatial variability (eddies). However, surfzone effects on the inner-shelf flow spatial variability have not been studied in realistic models that include both shelf and surfzone processes. Here, these effects are diagnosed with two nearly identical twin realistic simulations of the San Diego Bight over summer to fall where one simulation includes surface gravity waves (WW) and the other that does not (NW). The simulations include tides, weak to moderate winds, internal waves, submesoscale processes, and have surfzone width L sz of 96(±41) m (≈ 1 m significant wave height). Flow spatial variability metrics, alongshore root mean square vorticity, divergence, and eddy cross-shore velocity, are analyzed in a L sz normalized cross-shore coordinate. At the surface, the metrics are consistently (> 70%) elevated in the WW run relative to NW out to 5L sz offshore. At 4L sz offshore, WW metrics are enhanced over the entire water column. In a fixed coordinate appropriate for eddy transport, the eddy cross-shore velocity squared correlation betweenWWand NW runs is < 0.5 out to 1.2 km offshore or 12 time-averaged L sz. The results indicate that the eddy tracer (e.g., larvae) transport and dispersion across the inner-shelf will be significantly different in the WW and NW runs. The WW model neglects specific surfzone vorticity generation mechanisms. Thus, these inner-shelf impacts are likely underestimated. In other regions with larger waves, impacts will extend farther offshore.

Open access