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Yoeri M. Dijkstra and Henk M. Schuttelaars

Abstract

The classification diagram developed by Hansen and Rattray is one of the classic papers on classification of estuarine salinity dynamics. However, we found several inconsistencies in both their stratification–circulation and estuarine classification diagrams. These findings considerably change the interpretation of their work. Furthermore, while their classification includes salt wedge estuaries, the model used to derive this is only applicable to well-mixed and partially mixed estuaries. Here, we identify and solve these inconsistencies, and we propose new adjusted and extended stratification–circulation and classification diagrams. To this end, we summarize the model of Hansen and Rattray and extend their work to find analytical model solutions and an adjusted stratification–circulation diagram. Using this new diagram, it is shown that Hansen and Rattray incorrectly discussed the behavior of dispersion-dominated estuaries and that several parts of the diagram correspond to physically unrealistic model solutions. This is then used to demonstrate that several estuarine classes identified by Hansen and Rattray correspond to physically unrealistic model solutions and can therefore not be interpreted. A new and extended classification is proposed by using a recently developed model that extends the work of Hansen and Rattray to salt wedge estuaries. This results in an extended estuarine classification including examples of the location of 12 estuaries in this new diagram.

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Zhiling Liao, Shaowu Li, Ye Liu, and Qingping Zou

Abstract

The theoretical model for group-forced infragravity (IG) waves in shallow water is not well established for nonbreaking conditions. In the present study, analytical solutions of the group-forced IG waves at O(β 1) (β 1 = h x/(Δkh), h x = bottom slope, Δk = group wavenumber, h = depth) in intermediate water and at O(β11) in shallow water are derived separately. In case of off-resonance [β 1 μ −1 = O(β 1), where μ=1cg2/(gh) is the resonant departure parameter, c g = group speed] in intermediate water, additional IG waves in quadrature with the wave group forcing (hereinafter, the nonequilibrium response or component) are induced at O(β 1) relative to the equilibrium bound IG wave solution of in phase with the wave group. The present theory indicates that the nonequilibrium response is mainly attributed to the spatial variation of the equilibrium bound IG wave amplitude instead of group-forcing. In case of near-resonance [β 1 μ −1 = O(1)] in shallow water; however, both the equilibrium and nonequilibrium components are ~O(β11) at the leading order. Based on the nearly-resonant solution, the shallow water limit of the local shoaling rate of bound IG waves over a plane sloping beach is derived to be ~h −1 for the first time. The theoretical predictions compare favorably with the laboratory experiment by and the present numerical model results generated using SWASH. Based on the proposed solution, the group-forced IG waves over a symmetric shoal are investigated. In case of off-resonance, the solution predicts a roughly symmetric reversible spatial evolution of the IG wave amplitude, while in cases of near to full resonance the IG wave is significantly amplified over the shoal with asymmetric irreversible spatial evolution.

Open access
P. Vélez-Belchí, V. Caínzos, E. Romero, M. Casanova-Masjoan, C. Arumí-Planas, D. Santana-Toscano, A. González-Santana, M. D. Pérez-Hernández, and A. Hernández-Guerra

Abstract

Poleward undercurrents are well-known features in eastern boundary upwelling systems. In the California Current upwelling system, the California poleward undercurrent has been widely studied, and it has been demonstrated that it transports nutrients from the equatorial waters to the northern limit of the subtropical gyre. However, in the Canary Current upwelling system, the Canary intermediate poleward undercurrent (CiPU) has not been properly characterized, despite recent studies arguing that the dynamics of the eastern Atlantic Ocean play an important role in the Atlantic meridional overturning circulation, specifically on its seasonal cycle. Here, we use trajectories of Argo floats and model simulations to characterize the CiPU, including its seasonal variability and its driving mechanism. The Argo observations show that the CiPU flows from 26°N, near Cape Bojador, to approximately 45°N, near Cape Finisterre and flows deeper than any poleward undercurrent in other eastern boundaries, with a core at a mean depth of around 1000 dbar. Model simulations manifest that the CiPU is driven by the meridional alongshore pressure gradient due to general ocean circulation and, contrary to what is observed in the other eastern boundaries, is still present at 1000 dbar as a result of the pressure gradient between the Antarctic Intermediate Waters in the south and Mediterranean Outflow waters in the north. The high seasonal variability of the CiPU, with its maximum strength in autumn and minimum in spring, is due to the poleward extension of AAIW, forced by Ekman pumping in the tropics.

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Ewa Jarosz, Hemantha W. Wijesekera, and David W. Wang

Abstract

Velocity, hydrographic, and microstructure observations collected under moderate to high winds, large surface waves, and significant ocean-surface heat losses were utilized to examine coherent velocity structures (CVS) and turbulent kinetic energy (TKE) budget in the mixed layer on the outer shelf in the northern Gulf of Mexico in February 2017. The CVS exhibited larger downward velocities in downwelling regions and weaker upward velocities in broader upwelling regions, elevated vertical velocity variance, vertical velocity maxima in the upper part of the mixed layer, and phasing of crosswind velocities relative to vertical velocities near the base of the mixed layer. Temporal scales ranged from 10 to 40 min, and estimated lateral scales ranged from 90 to 430 m, which were 1.5–6 times as large as the mixed layer depth. Nondimensional parameters, Langmuir and Hoenikker numbers, indicated that plausible forcing mechanisms were surface-wave-driven Langmuir vortex and destabilizing surface buoyancy flux. The rate of change of TKE, shear production, Stokes production, buoyancy production, vertical transport of TKE, and dissipation in the TKE budget were evaluated. The shear and Stokes productions, dissipation, and vertical transport of TKE were the dominant terms. The buoyancy production term was important at the sea surface, but it decreased rapidly in the interior. A large imbalance term was found under the unstable, high-wind, and high–sea state conditions. The cause of this imbalance cannot be determined with certainty through analyses of the available observations; however, our speculation is that the pressure transport is significant under these conditions.

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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-APEX profiling floats (u, υ, T, S, χ T), 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 latent heat flux. Shallow mixing and stratified transition layers formed above ~20-m depth. A 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 its restratification, closing the budget to within measurement uncertainties. Buoyancy advective restratification (slumping) plays a minor role. Before 28 March, measured advective restratification (uzbx+υzby)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 because of storm-forced inertial shear. Starting on 28 March, while small, the subinertial envelope of measured buoyancy advective restratification in the remnant layer proceeds as predicted by mixed-layer eddy parameterizations.

Open access
Jen-Ping Peng, Julia Dräger-Dietel, Ryan P. North, and Lars Umlauf

Abstract

Recent high-resolution numerical simulations have shown that the diurnal variability in the atmospheric forcing strongly affects the dynamics, stability, and turbulence of submesoscale structures in the surface boundary layer (SBL). Field observations supporting the real-ocean relevance of these studies are, however, largely lacking at the moment. Here, the impact of large diurnal variations in the surface heat flux on a dense submesoscale upwelling filament in the Benguela upwelling system is investigated, based on a combination of densely spaced turbulence microstructure observations and surface drifter data. Our data show that during nighttime and early morning conditions, when solar radiation is still weak, frontal turbulence is generated by a mix of symmetric and shear instability. In this situation, turbulent diapycnal mixing is approximately balanced by frontal restratification associated with the cross-front secondary circulation. During daytime, when solar radiation is close to its peak value, the SBL quickly restratifies, the conditions for frontal instability are no longer fulfilled, and SBL turbulence collapses except for a thin wind-driven layer near the surface. The drifter data suggest that inertial oscillations periodically modulate the stability characteristics and energetics of the submesoscale fronts bounding the filament.

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Qingyang Song and Hidenori Aiki

Abstract

Intraseasonal waves in the tropical Atlantic Ocean have been found to carry prominent energy that affects interannual variability of zonal currents. This study investigates energy transfer and interaction of wind-driven intraseasonal waves using single-layer model experiments. Three sets of wind stress forcing at intraseasonal periods of around 30, 50, and 80 days with a realistic horizontal distribution are employed separately to excite the second baroclinic mode in the tropical Atlantic. A unified scheme for calculating the energy flux, previously approximated and used for the diagnosis of annual Kelvin and Rossby waves, is utilized in the present study in its original form for intraseasonal waves. Zonal velocity anomalies by Kelvin waves dominate the 80-day scenario. Meridional velocity anomalies by Yanai waves dominate the 30-day scenario. In the 50-day scenario, the two waves have comparable magnitudes. The horizontal distribution of wave energy flux is revealed. In the 30- and 50-day scenarios, a zonally alternating distribution of cross-equatorial wave energy flux is found. By checking an analytical solution excluding Kelvin waves, we confirm that the cross-equatorial flux is caused by the meridional transport of geopotential at the equator. This is attributed to the combination of Kelvin and Yanai waves and leads to the asymmetric distribution of wave energy in the central basin. Coastally trapped Kelvin waves along the African coast are identified by alongshore energy flux. In the north, the bend of the Guinea coast leads the flux back to the equatorial basin. In the south, the Kelvin waves strengthened by local wind transfer the energy from the equatorial to Angolan regions.

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Luwei Yang, Maxim Nikurashin, Andrew McC. Hogg, and Bernadette M. Sloyan

Abstract

Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee-wave parameterization that includes both lee-wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee-wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation.

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Luc Lenain and Nick Pizzo

Abstract

Internal waves are a regular feature of the open-ocean and coastal waters. As a train of internal waves propagates, their surface induced currents modulate the surface waves, generating a characteristic rough- and smooth-banded structure. While the surface expression of these internal waves is well known and has been observed from a variety of remote sensing instruments, direct quantitative observations of the directional properties of the surface gravity wave field modulated by an internal wave remain sparse. In this work, we report on a comprehensive field campaign conducted off the coast of Point Sal, California, in September 2017. Using a unique combination of airborne remote sensing observations, along with in situ surface and subsurface measurements, we investigate and quantify the interaction between surface gravity and internal wave processes. We find that surface waves are significantly modulated by the currents induced by the internal waves. Through novel observations of ocean topography, we characterize the rapid modification of the directional and spectral properties of surface waves over very short spatial scales [O(100) m or less]. Over a range of wavelengths (3–9-m waves), geometrical optics and wave action conservation predictions show good agreement with the observed wavenumber spectra in smooth and rough regions of the modulated surface waves. If a parameterization of wave action source terms is used, good agreement is found over a larger range of wavenumbers, down to 4 rad m−1. These results elucidate properties of surface waves interacting with a submesoscale ocean current and should provide insight into more general interactions between surface waves and the fine-scale structure of the upper ocean.

Open access