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Tomas Chor
,
Jacob O. Wenegrat
, and
John Taylor

Abstract

Submesoscale processes provide a pathway for energy to transfer from the balanced circulation to turbulent dissipation. One class of submesoscale phenomena that has been shown to be particularly effective at removing energy from the balanced flow is centrifugal–symmetric instabilities (CSIs), which grow via geostrophic shear production. CSIs have been observed to generate significant mixing in both the surface boundary layer and bottom boundary layer flows along bathymetry, where they have been implicated in the mixing and water mass transformation of Antarctic Bottom Water. However, the mixing efficiency (i.e., the fraction of the energy extracted from the flow used to irreversibly mix the fluid) of these instabilities remains uncertain, making estimates of mixing and energy dissipation due to CSI difficult. In this work we use large-eddy simulations to investigate the mixing efficiency of CSIs in the submesoscale range. We find that centrifugally dominated CSIs (i.e., CSI mostly driven by horizontal shear production) tend to have a higher mixing efficiency than symmetrically dominated ones (i.e., driven by vertical shear production). The mixing efficiency associated with CSIs can therefore alternately be significantly higher or significantly lower than the canonical value used by most studies. These results can be understood in light of recent work on stratified turbulence, whereby CSIs control the background state of the flow in which smaller-scale secondary overturning instabilities develop, thus actively modifying the characteristics of mixing by Kelvin–Helmholtz instabilities. Our results also suggest that it may be possible to predict the mixing efficiency with more readily measurable parameters (viz., the Richardson and Rossby numbers), which would allow for parameterization of this effect.

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Junbiao Tu
,
Daidu Fan
,
Feixiang Sun
,
Alexis Kaminski
, and
William Smyth

Abstract

This study presents field observations of fluid mud and the flow instabilities that result from the interaction between mud-induced density stratification and current shear. Data collected by shipborne and bottom-mounted instruments in a hyperturbid estuarine tidal channel reveal the details of turbulent sheared layers in the fluid mud that persist throughout the tidal cycle. Shear instabilities form during periods of intense shear and strong mud-induced stratification, particularly with gradient Richardson number smaller than or fluctuating around the critical value of 0.25. Turbulent mixing plays a significant role in the vertical entrainment of fine sediment over the tidal cycle. The vertical extent of the billows identified seen in the acoustic images is the basis for two useful parameterizations. First, the aspect ratio (billow height/wavelength) is indicative of the initial Richardson number that characterizes the shear flow from which the billows grew. Second, we describe a scaling for the turbulent dissipation rate ε that holds for both observed and simulated Kelvin–Helmholtz billows. Estimates for the present observations imply, however, that billows growing on a lutocline obey an altered scaling whose origin remains to be explained.

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Kristin L. Zeiden
,
Daniel L. Rudnick
,
Jennifer A. MacKinnon
,
Verena Hormann
, and
Luca Centurioni

Abstract

Wake eddies are important to physical oceanographers because they tend to dominate current variability in the lee of islands. However, their generation and evolution has been difficult to study due to their intermittency. In this study, 2 years of observations from Surface Velocity Program (SVP) drifters are used to calculate relative vorticity (ζ) and diffusivity (κ) in the wake generated by westward flow past the archipelago of Palau. Over 2 years, 19 clusters of five SVP drifters ∼5 km in scale were released from the north end of the archipelago. Out of these, 15 were entrained in the wake. We compare estimates of ζ from both velocity spatial gradients (least squares fitting) and velocity time series (wavelet analysis). Drifters in the wake were entrained in either energetic submesoscale eddies with initial ζ up to 6f, or island-scale recirculation and large-scale lateral shear with ζ ∼ 0.1f. Here f is the local Coriolis frequency. Mean wake vorticity is initially 1.5f but decreases inversely with time (t), while mean cluster scale (L) increases as Lt. Kinetic energy measured by the drifters is comparatively constant. This suggests ζ is predominantly a function of scale, confirmed by binning enstrophy (ζ 2) by inverse scale. We find κL 4/3 and upper and lower bounds for L(t) are given by t 3/2 and t 1/2, respectively. These trends are predicted by a model of dispersion due to lateral shear. We argue the observed time dependence of cluster scale and vorticity suggest island-scale shear controls eddy growth in the wake of Palau.

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Andrew W. Smith
,
Brian K. Haus
, and
Rachel H. R. Stanley

Abstract

Bubbles directly link sea surface structure to the dissipation rate of turbulence in the ocean surface layer through wave breaking, and they are an important vehicle for air–sea transfer of heat and gases and important for understanding both hurricanes and global climate. Adequate parameterization of bubble dynamics, especially in high winds, requires simultaneous measurements of surface waves and breaking-induced turbulence; collection of such data would be hazardous in the field, and they are largely absent from laboratory studies to date. We therefore present data from a series of laboratory wind-wave tank experiments designed to observe bubble size distributions in natural seawater beneath hurricane conditions and connect them to surface wave statistics and subsurface turbulence. A shadowgraph imager was used to observe bubbles in three different water temperature conditions. We used these controlled conditions to examine the role of stability, surface tension, and water temperature on bubble distributions. Turbulent kinetic energy dissipation rates were determined from subsurface ADCP data using a robust inertial-subrange identification algorithm and related to wind input via wave-dependent scaling. Bubble distributions shift from narrow to broadbanded and toward smaller radius with increased wind input and wave steepness. TKE dissipation rate and shear were shown to increase with wave steepness; this behavior is associated with a larger number of small bubbles in the distributions, suggesting shear is dominant in forcing bubbles in hurricane wind-wave conditions. These results have important implications for bubble-facilitated air–sea exchanges, near-surface ocean mixing, and the distribution of turbulence beneath the air–sea interface in hurricanes.

Significance Statement

Bubbles are a vehicle for the flux of heat, momentum, and gases between the atmosphere and ocean. These fluxes contribute to the energy budgets of hurricanes, climate, and upper-ocean biology. Few to no simultaneous measurements of surface waves, bubbles, and turbulence have been made in hurricane conditions. To improve numerical model representation of bubbles, we performed laboratory experiments to parameterize bubble size distributions using physical variables including wind and waves. Bubble distributions were found to become broadbanded and shift toward smaller radius with increased wind stress and wave steepness. Turbulence dissipation rate and shear were shown to increase with wave steepness. Our results give the first physically based bubble distribution parameterization from naturally breaking waves in hurricane-force conditions.

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Ashwita Chouksey
,
Alexa Griesel
,
Manita Chouksey
, and
Carsten Eden

Abstract

We investigate changes in the ocean circulation due to the variation of isopycnal diffusivity (κ iso) in a global non-eddy-resolving model. Although isopycnal diffusion is thought to have minor effects on interior density gradients, the model circulation shows a surprisingly large sensitivity to the changes: with increasing κ iso, the strength of the Atlantic residual overturning circulation (AMOC) and the Antarctic Circumpolar Current (ACC) transport weaken. At high latitudes, the isopycnal diffusion diffuses temperature and salinity upward and poleward, and at low latitudes downward close to the surface. Increasing isopycnal diffusivity increases the meridional isopycnal fluxes whose meridional gradient is equatorward, hence leading to a negative contribution to the flux divergence in the tracer equations and predominant cooling and freshening equatorward of 40°. The effect on temperature overcompensates the countering effect of salinity diffusion, such that the meridional density differences decrease, along with which ACC and AMOC decrease. We diagnose the adjustment process to the new equilibrium with increased isopycnal diffusion to assess how the other terms in the tracer equations react to the increased κ iso. It reveals that around ±40° latitude, the cooling induced by the increased isopycnal flux is only partly compensated by warming by advection, explaining the net cooling. Overall, the results emphasize the importance of isopycnal diffusion on ocean circulation and dynamics, and hence the necessity of its careful representation in models.

Significance Statement

The effect of mixing by mesoscale eddies, represented as diffusion along surfaces of constant density in models, on the ocean circulation is not well understood. Here, we show that an increase in the eddy diffusivity in different setups of a global ocean model leads to a surprisingly large change of the ocean circulation. The strength of the Atlantic overturning circulation and the Antarctic Circumpolar Current decrease. We find that the interior ocean becomes cooler and fresher and that the temperature effect on density dominates over salinity, resulting in a decrease in the density gradients. Our results point out the importance of eddy diffusion on ocean circulation, and hence the necessity of its correct representation in ocean and climate models.

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Gabin H. Urbancic
,
Kevin G. Lamb
,
Ilker Fer
, and
Laurie Padman

Abstract

The propagation of internal waves (IWs) of tidal frequency is inhibited poleward of the critical latitude, where the tidal frequency is equal to the Coriolis frequency (f). These subinertial IWs may propagate in the presence of background vorticity, which can reduce rotational effects. Additionally, for strong tidal currents, the isopycnal displacements may evolve into internal solitary waves (ISWs). In this study, wave generation by the subinertial K1 and M2 tides over the Yermak Plateau (YP) is modeled to understand the linear response and the conditions necessary for the generation of ISWs. The YP stretches out into Fram Strait, a gateway into the Arctic Ocean for warm Atlantic-origin waters. We consider the K1 tide for a wide range of tidal amplitudes to understand the IW generation for different forcing. For weak tidal currents, the baroclinic response is predominantly at the second harmonic due to critical slopes. For sufficiently strong diurnal currents, ISWs are generated and their generation is not sensitive to the range of f and stratifications considered. The M2 tide is subinertial yet the response shows propagating IW beams with frequency just over f. We discuss the propagation of these waves and the influence of variations of f, as a proxy for variations in the background vorticity, on the energy conversion to IWs. An improved understanding of tidal dynamics and IW generation at high latitudes is needed to quantify the magnitude and distribution of turbulent mixing, and its consequences for the changes in ocean circulation, heat content, and sea ice cover in the Arctic Ocean.

Open access
A. Pirro
,
E. Mauri
,
R. Gerin
,
R. Martellucci
,
P. Zuppelli
, and
P. M. Poulain

Abstract

The deepwater formation in the northern part of the South Adriatic Pit (Mediterranean Sea) is investigated using a unique oceanographic dataset. In situ data collected by a glider along the Bari–Dubrovnik transect captured the mixing and the spreading/restratification phase of the water column in winter 2018. After a period of about 2 weeks from the beginning of the mixing phase, a homogeneous convective area of ∼300-m depth breaks up due to the baroclinic instability process in cyclonic cones made of geostrophically adjusted fluid. The base of these cones is located at the bottom of the mixed layer, and they extend up to the theoretical critical depth Zc . These cones, with a diameter on the order of internal Rossby radius of deformation (∼6 km), populate the ∼110-km-wide convective site, develop beneath it, and have a short lifetime of weeks. Later on, the cones extend deeper and intrusion from deep layers makes their inner core denser and colder. These observed features differ from the long-lived cyclonic eddies sampled in other ocean sites and formed at the periphery of the convective area in a postconvection period. So far, to the best of our knowledge, only theoretical studies, laboratory experiments, and model simulations have been able to predict and describe our observations, and no other in situ information has yet been provided.

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Maciej Janecki
,
Dawid Dybowski
,
Daniel Rak
, and
Lidia Dzierzbicka-Glowacka

Abstract

This paper introduces a new method for finding the top of thermocline (TTD) and halocline (THD) depths that may become a powerful tool for applications in shallow marine basins around the world. The method calculates the moving average of the ocean vertical profile’s short-scale spatial variability (standard deviation) and then processes it to determine the potential depth at which temperature or salinity rapidly changes. The method has been calibrated using an extensive set of data from the ecohydrodynamic model EcoFish. As a result of the calibration, the values of the input parameters that allowed the correct determination of TTD and THD were established. It was confirmed by the validation carried out on the in situ profiles collected by the research vessel S/Y Oceania during statutory cruises in the southern Baltic Sea. The “MovSTD” algorithm was then used to analyze the seasonal variability of the vertical structure of the waters in Gdańsk Deep for temperature and salinity. The thermocline deepening speed was also estimated in the region analyzed.

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Huan Mei
,
Yiquan Qi
,
Xuhua Cheng
,
Xiangbai Wu
, and
Qiang Wang

Abstract

We study a hysteresis western boundary current (WBC) flowing across a gap impinged by a mesoscale eddy, with an island of variable meridional size in the gap, using a 1.5-layer ocean model. The hysteresis curves suggest the island with a larger size facilitates the WBC intrusion by shedding the eddy more easily. Both anticyclonic and cyclonic eddies are able to induce the critical WBC transition from penetration regime to leap regime, and vice versa. The vorticity balance analysis indicates increased (decreased) meridional advection that induces the critical WBC shifting from the eddy shedding (leaping) regime to the leaping (eddy shedding) regime. The meridional size of the island significantly affects the critical WBC transition in terms of the critical strength of the mesoscale eddy. The regime shift from penetration to leap is most sensitive to the eddy upstream of the WBC for small islands and most sensitive to the southern anticyclonic eddy and northern cyclonic eddy for moderate and large islands. It is least sensitive to the central cyclonic eddy for small islands and to the cyclonic eddy upstream of the WBC for moderate and large islands and to the northern anticyclonic eddy regardless of island size. The regime shift from leap to penetration is most sensitive to the cyclonic eddy upstream of the WBC and to the northern anticyclonic eddy. It is least sensitive to the anticyclonic eddy from the south, and the least sensitive location of the cyclonic eddy shifts northward from the gap center as the island size increases.

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Hua Zheng
,
Xiao-Hua Zhu
,
Chuanzheng Zhang
,
Ruixiang Zhao
,
Ze-Nan Zhu
,
Qiang Ren
,
Yansong Liu
,
Feng Nan
, and
Fei Yu

Abstract

South China Sea (SCS) abyssal circulation largely contributes to water renewal, energy budget, and sedimentary processes in the deep ocean. The three-dimensional abyssal circulation west of the Luzon Strait (LS) in the northern SCS was investigated using an array comprising 27 current- and pressure-recording inverted echo sounders. Over 400 days of measurements from June 2018 to July 2019 showed a narrow and strong (∼70 km, ∼2.3 cm s−1 at 2500 dbar) northward current near the steep eastern boundary, while a wide and weak (∼180 km, ∼1.5 cm s−1 at 2500 dbar) southwestward current lies along the subdued western boundary. The circulation showed conspicuous cyclonic patterns with a volume transport of ∼1.21 ± 0.93 Sv (1 Sv ≡ 106 m3 s−1) and ∼1.59 ± 0.95 Sv below 2500 dbar along the eastern and western boundaries, respectively. The current near the LS was strong in late autumn and early winter but weak in late winter and spring, following the seasonal variation of LS deep-water overflow. However, the southwestward current in the interior SCS was stronger in summer and early autumn but weaker in late winter and early spring. The different seasonal patterns identified near the LS and the interior SCS are attributed to the propagation of seasonal variation. The weak current along the western boundary in August 2018 and February 2019 was dominated by LS deep-water overflow with a time lag of ∼7.5 months. Although eddies in the upper ocean may also contribute to such variation through pressure work, the effect is minor.

Significance Statement

Cyclonic circulation in the deep South China Sea (SCS) largely contributes to water renewal, energy budget, and sedimentary processes and influences the transport of dissolved elements, minerals, and pollutants. As an important part of the SCS throughflow, an in-depth analysis of the SCS abyssal circulation may also contribute to understanding Indonesian Throughflow and global climate change. The three-dimensional abyssal circulation west of the Luzon Strait was investigated using large-scale data from June 2018 to July 2019, which provided unprecedented coverage of abyssal circulation in the northeast SCS. The study provides important observational evidence for the existence of SCS abyssal cyclonic circulation. Detailed spatiotemporal structure of abyssal circulation and its variations are presented, and related dynamic processes are discussed.

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