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Varvara E. Zemskova and Nicolas Grisouard

Abstract

Linear theory for steady stratified flow over topography sets the range for topographic wavenumbers over which freely propagating internal waves are generated, and the radiation and breaking of these waves contribute to energy dissipation away from the ocean bottom. However, previous numerical work demonstrated that dissipation rates can be enhanced by flow over large-scale topographies with wavenumbers outside of the lee wave radiative range. We conduct idealized 3D numerical simulations of steady stratified flow over 1D topography in a rotating domain and quantify vertical distribution of kinetic energy dissipation. We vary two parameters: the first determines whether the topographic obstacle is within the lee wave radiative range and the second, proportional to the topographic height, measures the degree of flow nonlinearity. For certain combinations of topographic width and height, breaking occurs in pulses every inertial period, such that kinetic energy dissipation develops inertial periodicity. In these simulations, kinetic energy dissipation rates are also enhanced in the interior of the domain. In the radiative regime the inertial motions arise due to resonant wave–wave interactions. In the small wavenumber nonradiative regime, instabilities downstream of the obstacle can facilitate the generation and propagation of nonlinearly forced inertial motions, especially as topographic height increase. In our simulations, dissipation rates for tall and wide nonradiative topography are comparable to those of radiative topography, even away from the bottom, which is relevant to the ocean where the topographic spectrum is such that wider abyssal hills also tend to be taller.

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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, and 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−1), increases with depth, indicates predominant upwelling with isolated downwelling events, and sometimes changes sign between 1 and 50 m. Vortex stretching is one of the significant terms, but not the only one, in the vorticity balance. Two-dimensional FTLEs are 2 × 10−5 s−1 after 1 day, 2 times as large as in a 400-m-resolution numerical model. Three-dimensional 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 are partially due to only measuring at four 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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Jordi Isern-Fontanet and Antonio Turiel

Abstract

The multifractal theory of turbulence is used to investigate the energy cascade in the northwestern Atlantic Ocean. The statistics of singularity exponents of horizontal velocity gradients computed from in situ measurements at 2-km resolution are used to characterize the anomalous scaling of the velocity structure functions at depths between 50 and 500 m. Here, we show that the degree of anomalous scaling can be quantified using singularity exponents. Observations reveal, on one side, that the anomalous scaling has a linear dependence on the exponent characterizing the strongest velocity gradient and, on the other side, that the slope of this linear dependence decreases with depth. Since the observed distribution of exponents is asymmetric about the mode at all depths, we use an infinitely divisible asymmetric model of the energy cascade, the log–Poisson model, to derive the functional dependence of the anomalous scaling with the exponent of the strongest velocity gradient, as well as the dependence with dissipation. Using this model we can interpret the vertical change of the linear slope between the anomalous scaling and the exponents of the strongest velocity gradients as a change in the energy cascade. This interpretation assumes the validity of the multifractal theory of turbulence, which has been assessed in previous studies.

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Olivier Marchal and Ning Zhao

Abstract

Measurements of radiocarbon concentration (Δ14C) in fossil biogenic carbonates have been interpreted as reflecting a reduced ventilation of the deep Atlantic during the last ice age. Here we evaluate the (in)consistency of an updated compilation of fossil Δ14C data for the last deglaciation with the abyssal circulation in the modern Atlantic. A Δ14C transport equation, in which the mean velocity field is a modern field estimate and turbulent flux divergence is treated as a random fluctuation, is fitted to deglacial Δ14C records by using recursive weighted least squares. This approach allows us to interpret the records in terms of deviations from the modern flow with due regard for uncertainties in the fossil data, the Δ14C transport equation, and its boundary conditions. We find that the majority of fit residuals could be explained by uncertainties in fossil Δ14C data, for two distinct estimates of the modern flow and of the error variance in the boundary conditions. Thus, most, not all, deglacial data appear consistent with present-day ventilation rates. From 20% to 32% of the residuals exceed in magnitude the published errors in the fossil data by a factor of 2. Residuals below 4000 m in the western North Atlantic are all negative, suggesting that deglacial Δ14C values from this region are too low to be explained by modern ventilation. While deep water ventilation appeared different from today at some locations, a larger database and a better understanding of error (co)variances are needed to make reliable paleoceanographic inferences from fossil Δ14C records.

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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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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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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

Here, we develop a framework for understanding the observations presented in Part I. In this framework, the internal tide saturates as it shoals as a result of amplitude limitation with decreasing water depth H. From this framework evolves estimates of averaged energetics of the internal tide; specifically, energy ⟨APE⟩, energy flux ⟨F E⟩, and energy flux divergence ∂xF E⟩. Since we observe that dissipation ⟨D⟩ ≈ ∂xF E⟩, we also interpret our estimate of ∂xF E⟩ as ⟨D⟩. These estimates represent a parameterization of the energy in the internal tide as it saturates over the inner continental shelf. The parameterization depends solely on depth-mean stratification and bathymetry. A summary result is that the cross-shelf depth dependencies of ⟨APE⟩, ⟨F E⟩, and ∂xF E⟩ are analogous to those for shoaling surface gravity waves in the surf zone, suggesting that the inner shelf is the surf zone for the internal tide. A test of our simple parameterization against a range of datasets suggests that it is broadly applicable.

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Yisen Zhong, Meng Zhou, Joanna J. Waniek, Lei Zhou, and Zhaoru Zhang

Abstract

The long-term satellite altimeter and reanalysis data show that large seasonal variations of the Kuroshio intrusion into the South China Sea are associated with geostrophic transport through the Luzon Strait, but not with the current intensity, width, and axis position east of Luzon island. To address this issue, we examine the seasonal variability of surface intrusion patterns by a new streamline-based method. The along-streamline analysis reveals that the seasonality of geostrophic intrusion is only attributed to the cyclonic shear part of the flow, while the anticyclonic shear part always leaps across the Luzon Strait. A possible physical mechanism is proposed to accommodate these seasonal characteristics based globally on the vorticity (torque work) balance between the basinwide negative wind stress curl and the positive vorticity fluxes induced by the lateral wall, as well as locally on loss of balance between the torques of frictional stresses and normal stresses owing to the boundary gap. Through modifying the nearshore sea surface level, the northeasterly (southeasterly) monsoon increases (decreases) the positive vorticity fluxes in response to global vorticity balance, and simultaneously amplifies (alleviates) the local imbalance by enhancing (reducing) the positive frictional stress torque within the cyclonic shear layer. Therefore, in winter when the positive torque is large enough, the Kuroshio splits and the intrusion occurs, while in summer the stress torque is so weak that the entire current keeps flowing north.

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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 (8 m s−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 1000 and 1600 local time (LT) (from 4 h after sunrise to 2 h before sunset). Despite Ri being well below the Ri = ¼ 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 < ¼. This changes around 1600–1700 LT. 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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