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Xingchi Wang and Tobias Kukulka

Abstract

Turbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Using a large-eddy simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik–Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments in which winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases as a result of reduced TKE production. Stage 2 is characterized by TKE increasing, with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values because of inertial resonance and nonequilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts as a result of slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian shear TKE production exceeding Stokes drift shear TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier–Stokes equation models indicate that Stokes drift shear production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.

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Olivier Marchal and Ning Zhao

Abstract

Radiocarbon dates of fossil carbonates sampled from sediment cores and the seafloor have been used to infer that deep ocean ventilation during the last ice age was different from today. In this first of two companion papers, the time-averaged abyssal circulation in the modern Atlantic is estimated by combining a hydrographic climatology, observational estimates of volume transports, Argo float velocities at 1000 m, radiocarbon data, and geostrophic dynamics. Different estimates of modern circulation, obtained from different prior assumptions about the abyssal flow and different errors in the geostrophic balance, are produced for use in a robust interpretation of fossil records in terms of deviations from the present-day flow, which is undertaken in Part II. We find that, for all estimates, the meridional transport integrated zonally and averaged over a hemisphere, ⟨V k⟩, is southward between 1000 and 4000 m in both hemispheres, northward between 4000 and 5000 m in the South Atlantic, and insignificant between 4000 and 5000 m in the North Atlantic. Estimates of ⟨V k⟩ obtained from two distinct prior circulations—one based on a level of no motion at 4000 m and one based on Argo float velocities at 1000 m—become statistically indistinguishable when Δ14C data are considered. The transport time scale, defined as τk=Vk/Vk, where Vk is the volume of the kth layer, is estimated to about a century between 1000 and 3000 m in both the South and North Atlantic, 124 ± 9 yr (203 ± 23 yr) between 3000 and 4000 m in the South (North) Atlantic, and 269 ± 115 yr between 4000 and 5000 m in the South Atlantic.

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Wilton Sturges

Abstract

A previous study by the author concluded that either there were errors in the satellite results or that some long-term means were not in geostrophic balance. Ship-drift results are in good agreement with surface drifters, but these two do not agree with satellite sea surface heights (SSH). The agreement between the first two suggested the possibility that there could be errors in the SSH or that the mean surface flow is not in geostrophic balance. The present results, using the addition of a fourth long-term mean from hydrographic data, which agrees with the SSH, resolves the issue. The lack of agreement between different long-term means is from inadequate coverage in space and time in data from ship drifts and drifters.

Open access
Lu Han, Harvey Seim, John Bane, Robert E. Todd, and Mike Muglia

Abstract

Carbon-rich Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) shelf waters typically converge on the continental shelf near Cape Hatteras. Both are often exported to the adjacent open ocean in this region. During a survey of the region in mid-January 2018, there was no sign of shelf water export at the surface. Instead, a subsurface layer of shelf water with high chlorophyll and dissolved oxygen was observed at the edge of the Gulf Stream east of Cape Hatteras. Strong cooling over the MAB and SAB shelves in early January led to shelf waters being denser than offshore surface waters. Driven by the density gradient, the denser shelf waters cascaded beneath the Gulf Stream and were subsequently entrained into the Gulf Stream, as they were advected northeastward. Underwater glider observations 80 km downstream of the export location captured 0.44 Sv (1 Sv ≡ 106 m3 s−1) of shelf waters transported along the edge of the Gulf Stream in January 2018. In total, as much as 7 × 106 kg of carbon was exported from the continental shelf to a greater depth in the open ocean during this 5-day-long cascading event. Earlier observations of near-bottom temperature and salinity at a depth of 230 m captured several multiday episodes of shelf water at a location that was otherwise dominated by Gulf Stream water, indicating that the January 2018 cascading event was not unique. Cascading is an important, yet little-studied pathway of carbon export and sequestration at Cape Hatteras.

Open access
S. W. Stevens, R. Pawlowicz, and S. E. Allen

Abstract

The intermediate circulation of the Strait of Georgia, British Columbia, Canada, plays a key role in dispersing contaminants throughout the Salish Sea, yet little is known about its dynamics. Here, we use hydrographic observations and hindcast fields from a regional 3D model to approach the intermediate circulation from three perspectives. First, we derive and model a “seasonality” tracer from temperature observations to age the water, estimate mixing, and infer circulation. Second, we analyze modeled velocity fields to create mean current maps and examine the advective and diffusive components of the mean flow field. Last, we calculate Lagrangian trajectories to derive transit time distributions and Lagrangian statistics. In combination, these analyses provide an overview of the mean intermediate circulation that can be summarized as follows: subducting water in Haro Strait ventilates the intermediate water primarily via an up-strait boundary current that flows along the eastern shores of the southernmost basin in 1–2 months. This inflowing water is either incorporated into the interior of the basin, recirculated southward, or transported into the northernmost basin, mixing steadily with adjacent water masses during its transit. A second, shallower ventilating jet emanates southward from Discovery Passage, locally modifying the Haro Strait inflow signal. Outside of these well-defined advective features, diffusive transport dominates in the majority of the region. The intermediate renewal signal fully ventilates the region in 100–140 days, which serves as a benchmark for contaminant dispersal time scale estimates.

Open access
Fraser W. Goldsworth, David P. Marshall, and Helen L. Johnson

Abstract

The upper limb of the Atlantic meridional overturning circulation draws waters with negative potential vorticity from the Southern Hemisphere into the Northern Hemisphere. The North Brazil Current is one of the cross-equatorial pathways in which this occurs: upon crossing the equator, fluid parcels must modify their potential vorticity to render them stable to symmetric instability and to merge smoothly with the ocean interior. In this work a linear stability analysis is performed on an idealized western boundary current, dynamically similar to the North Brazil Current, to identify features that are indicative of symmetric instability. Simple two-dimensional numerical models are used to verify the results of the stability analysis. The two-dimensional models and linear stability theory show that symmetric instability in meridional flows does not change when the nontraditional component of the Coriolis force is included, unlike in zonal flows. Idealized three-dimensional numerical models show anticyclonic barotropic eddies being spun off as the western boundary current crosses the equator. These eddies become symmetrically unstable a few degrees north of the equator, and their PV is set to zero through the action of the instability. The instability is found to have a clear fingerprint in the spatial Fourier transform of the vertical kinetic energy. An analysis of the water mass formation rates suggest that symmetric instability has a minimal effect on water mass transformation in the model calculations; however, this may be the result of unresolved dynamics, such as secondary Kelvin–Helmholtz instabilities, which are important in diabatic transformation.

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Qi Quan, Zhongya Cai, Guangzhen Jin, and Zhiqiang Liu

Abstract

Topographic Rossby waves (TRWs) in the abyssal South China Sea (SCS) are investigated using observations and high-resolution numerical simulations. These energetic waves can account for over 40% of the kinetic energy (KE) variability in the deep western boundary current and seamount region in the central SCS. This proportion can even reach 70% over slopes in the northern and southern SCS. The TRW-induced currents exhibit columnar (i.e., in phase) structure in which the speed increases downward. Wave properties such as the period (5–60 days), wavelength (100–500 km), and vertical trapping scale (102–103 m) vary significantly depending on environmental parameters of the SCS. The TRW energy propagates along steep topography with phase propagation offshore. TRWs with high frequencies exhibit a stronger climbing effect than low-frequency ones and hence can move further upslope. For TRWs with a certain frequency, the wavelength and trapping scale are dominated by the topographic beta, whereas the group velocity is more sensitive to the internal Rossby deformation radius. Background circulation with horizontal shear can change the wavelength and direction of TRWs if the flow velocity is comparable to the group velocity, particularly in the central, southern, and eastern SCS. A case study suggests two possible energy sources for TRWs: mesoscale perturbation in the upper layer and large-scale background circulation in the deep layer. The former provides KE by pressure work, whereas the latter transfers the available potential energy (APE) through baroclinic instability.

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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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W. D. Smyth, S. J. Warner, J. N. Moum, H. Pham, and S. Sarkar

Abstract

Factors thought to influence deep cycle turbulence in the equatorial Pacific are examined statistically for their predictive capacity using a 13-year moored record that includes microstructure measurements of the turbulent kinetic energy dissipation rate. Wind stress and mean current shear are found to be most predictive of the dissipation rate. Those variables, together with the solar buoyancy flux and the diurnal mixed layer thickness, are combined to make a pair of useful parameterizations. The uncertainty in these predictions is typically 50% greater than the uncertainty in present-day in situ measurements. To illustrate the use of these parameterizations, the record of deep cycle turbulence, measured directly since 2005, is extended back to 1990 based on historical mooring data. The extended record is used to refine our understanding of the seasonal variation of deep cycle turbulence.

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