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Pranav Puthan, Geno Pawlak, and Sutanu Sarkar

Abstract

Large-eddy simulations (LES) are employed to investigate the role of time-varying currents on the form drag and vortex dynamics of submerged 3D topography in a stratified rotating environment. The current is of the form Uc + Utsin(2πftt), where Uc is the mean, Ut is the tidal component, and ft is its frequency. A conical obstacle is considered in the regime of low Froude number. When tides are absent, eddies are shed at the natural shedding frequency fs , c. The relative frequency f*=fs,c/ft is varied in a parametric study, which reveals states of high time-averaged form drag coefficient. There is a twofold amplification of the form drag coefficient relative to the no-tide (Ut = 0) case when f* lies between 0.5 and 1. The spatial organization of the near-wake vortices in the high drag states is different from a Kármán vortex street. For instance, the vortex shedding from the obstacle is symmetric when f*=5/12 and strongly asymmetric when f*=5/6. The increase in form drag with increasing f* stems from bottom intensification of the pressure in the obstacle lee which we link to changes in flow separation and near-wake vortices.

Open access
Qiong Xia, Changming Dong, Yijun He, Gaocong Li, and Jihai Dong

Abstract

By using a Lagrangian-averaged vorticity deviation (LAVD)-based vortex detection scheme, rotationally coherent Lagrangian vortices in the South Atlantic Ocean are detected. These vortices act as agents for water transport and can stay coherent in a limited time scale. Our study starts from the life cycle of several long-lived Agulhas rings detected with the LAVD-based vortex detection method. The life cycle of those long-lived Agulhas rings can be separated into two distinct stages: the growing stage and the decaying stage. It is found that at the growing stage, the ambient water spins in and provides effective shielding for the coherent core. The rate of change of material belt width with respect to the detection time scale at the end of the growing stage can represent the decay rate of coherence. We further find a linear relationship between the mean strain rate and the mean square root of kinetic energy (KE1/2). Mean finite-time Lyapunov exponents (FTLE) increase monotonically with the mean strain rate or mean KE1/2. The long existence of the Agulhas rings can be partly attributed to the energetic boundaries around the rings. The ratio of the boundary kinetic energy to the spatial mean kinetic energy (KE/MKE) is also found to be a contributing factor that can influence the lifetime of Agulhas rings. In the retroflection area, the short-lived Agulhas rings might be attributed to the low KE/MKE in this area.

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Lisa Maillard, Julien Boucharel, and Lionel Renault

Abstract

Tropical instability waves (TIWs) are oceanic features propagating westward along the northern front of the pacific cold tongue. Observational and modeling studies suggest that TIWs may have a large impact on the eastern tropical Pacific background state from seasonal to interannual time-scales, through heat advection and mixing. However, observations are coarse or limited to surface data, and modeling studies are often based on the comparison of low- vs. high-resolution simulations. In this study, we perform a set of regional high-resolution ocean simulations (CROCO 1/12°) in which we strongly damp (NOTIWs-RUN) or not (TIWs-RUN) TIW propagation, by nudging meridional current velocities in the TIW region toward their monthly climatological values. This approach, while effectively removing TIW mesoscale activity, does not alter the model internal physics in particular related to the equatorial Kelvin wave dynamics. The impact of TIWs on the oceanic mean state is then assessed by comparing the two simulations. While the well-known direct effect of TIW heat advection is to weaken the meridional temperature gradient by warming up the cold tongue (0.34°C/month), the rectified effect of TIWs onto the mean state attenuates this direct effect by cooling down the cold tongue (-0.10°C/month). This rectified effect occurs through the TIW induced deepening and weakening of the Equatorial Undercurrent, that subsequently modulates the mean zonal advection and counterbalances TIWs direct effect. This approach allows quantifying the rectified effect of TIWs without degrading the model horizontal resolution, and may lead to a better characterization of the eastern tropical Pacific mean state and to the development of TIW parameterizations in Earth system models.

Open access
Qi Li, Zhaohui Chen, Shoude Guan, Haiyuan Yang, Zhao Jing, Yongzheng Liu, Bingrong Sun, and Lixin Wu

Abstract

Shipboard observations of upper-ocean current, temperature/salinity, and turbulent dissipation rate were used to study near-inertial waves (NIWs) and turbulent diapycnal mixing in cold-core eddy (CE) and warm-core eddy (WE) in the Kuroshio Extension (KE) region. The two eddies shed from the KE were energetic, with the maximum velocity exceeding 1 m s−1 and relative vorticity magnitude as high as 0.6 f. The Mode Regression Method was proposed to extract NIWs from the shipboard-ADCP velocities. The NIW amplitudes were 0.15 and 0.3 m s−1 in the CE and WE, respectively, and their constant phase lines were nearly slanted along the heaving isopycnals. In the WE, the NIWs were trapped in the negative vorticity core and amplified at the eddy base (at 350–650 m), which was consistent with the “inertial chimney” effect documented in existing literature. Outstanding NIWs in the background wavefield were also observed inside the positive vorticity core of the CE, despite their lower strength and shallower residence (above 350 m) compared to the counterparts in the WE. Particularly, the near-inertial kinetic energy efficiently propagated downward and amplified below the surface layer in both eddies, leading to an elevated turbulent dissipation rate of up to 10−7 W kg−1. In addition, bidirectional energy exchanges between the NIWs and mesoscale balanced flow occurred during NIWs’ downward propagation. The present study provides observational evidence for the enhanced downward NIW propagation by mesoscale eddies, which has significant implications for parameterizing the wind-driven diapycnal mixing in the eddying ocean.

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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 data set. In situ data collected by a glider along the Bari-Dubrovnik transect captured the mixing and the spreading/re-stratification phase of the water column in winter 2018. After a period of about two 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 of the order of internal Rossby radius of deformation (∼ 6 km) populate the ∼ 110-km wide convective site, develop beneath it and have a short life time 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 post-convection 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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Catherine A. Vreugdenhil, John R. Taylor, Peter E. D. Davis, Keith W. Nicholls, Paul R. Holland, and Adrian Jenkins

Abstract

The melt rate of Antarctic ice shelves is of key importance for rising sea levels and future climate scenarios. Recent observations beneath Larsen C Ice Shelf revealed an ocean boundary layer that was highly turbulent and raised questions on the effect of these rich flow dynamics on the ocean heat transfer and the ice shelf melt rate (Davis and Nicholls 2019). Directly motivated by the field observations, we have conducted large-eddy simulations (LES) to further examine the ocean boundary layer beneath Larsen C Ice Shelf. The LES was initialised with uniform temperature and salinity (T/S) and included a realistic tidal cycle and a small basal slope. A new parameterization based on Vreugdenhil and Taylor (2019) was applied at the top boundary to model near-wall turbulence and basal melting. The resulting vertical T/S profiles, melt rate and friction velocity matched well with the Larsen C Ice Shelf observations. The instantaneous melt rate varied strongly with the tidal cycle, with faster flow increasing the turbulence and mixing of heat towards the ice base. An Ekman layer formed beneath the ice base and, due to the strong vertical shear of the current, Ekman rolls appeared in the mixed layer and stratified region (depth ≈ 20–60m). In an additional high-resolution simulation (conducted with a smaller domain) the Ekman rolls were associated with increased turbulent kinetic energy, but a relatively small vertical heat flux. Our results will help with interpreting field observations and parameterizing the ocean-driven basal melting of ice shelves.

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Samuel M. Kelly and Sebastine Ogbuka

Abstract

Coastal trapped waves (CTWs) transport energy along coastlines and drive coastal currents and upwelling. CTW modes are non-orthogonal when frequency is treated as the eigen-value, preventing the separation of modal energy fluxes and quantification of longshore topographic scattering. Here, CTW modes are shown to be orthogonal with respect to energy flux (but not energy) when the longshore wavenumber is the eigenvalue. The modal evolution equation is a simple harmonic oscillator forced by longshore bathymetric variability, where downstream distance is treated like time. The energy equation includes an expression for modal topographic scattering. The eigenvalue problem is carefully discretized to produce numerically orthogonal modes, allowing CTW amplitudes, energy fluxes, and generation to be precisely quantified in numerical simulations. First, a spatially-uniform K1 longshore velocity is applied to a continental slope with a Gaussian bump in the coastline. Mode-1 CTW generation increases quadratically with the amplitude of the bump and is maximum when the bump’s length of coastline matches the natural wavelength of the CTW mode, as predicted by theory. Next, a realistic K1 barotropic tide is applied to the Oregon coast. The forcing generates mode-1 and 2 CTWs with energy fluxes of 6 and 2 MW, respectively, which are much smaller the 80 MW of M2 internal-tide generation in this region. CTWs also produce 1 cm sea surface displacements along the coast, potentially complicating the interpretation of future satellite altimetry. Prospects and challenges for quantifying the global geography of CTWs are discussed.

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Thomas Wilder, Xiaoming Zhai, David Munday, and Manoj Joshi

Abstract

Including the ocean surface current in the calculation of wind stress is known to damp mesoscale eddies through a negative wind power input, and have potential ramifications for eddy longevity. Here, we study the spin-down of a baroclinic anticyclonic eddy subject to absolute (no ocean surface current) and relative (including ocean surface current) wind stress forcing by employing an idealised high-resolution numerical model. Results from this study demonstrate that relative wind stress dissipates surface mean kinetic energy (MKE) and also generates additional vertical motions throughout the whole water column via Ekman pumping. Wind stress curl-induced Ekman pumping generates additional baroclinic conversion (mean potential to mean kinetic energy) that is found to offset the damping of surface MKE by increasing deep MKE. A scaling analysis of relative wind stress-induced baroclinic conversion and relative wind stress damping confirms these numerical findings, showing that additional energy conversion counteracts relative wind stress damping. What is more, wind stress curl-induced Ekman pumping is found to modify surface potential vorticity gradients that lead to an earlier destabilisation of the eddy. Therefore, the onset of eddy instabilities and eventual eddy decay takes place on a shorter timescale in the simulation with relative wind stress.

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Anda Vladoiu, Ren-Chieh Lien, and Eric Kunze

Abstract

Horizontal and vertical wavenumbers (kx, kz) immediately below the Ozmidov wavenumber (N3)1/2 are spectrally distinct from both isotropic turbulence (kx, kz > 1 cpm) and internal waves as described by the Garrett-and-Munk (GM) model spectrum (kz < 0.1 cpm). Towed CTD chain, augmented with concurrent EM-APEX profiling float microstructure measurements and shipboard ADCP surveys, are used to characterize 2D wavenumber (kx, kz) spectra of isopycnal slope, vertical strain and isopycnal salinity-gradient on horizontal wavelengths of 50 m - 250 km and vertical wavelengths of 2 - 48 m. For kz < 0.1 cpm, 2D spectra of isopycnal slope and vertical strain resemble GM. Integrated over the other wavenumber, the isopycnal slope 1D kx spectrum exhibits a roughly + 1/3 slope for kx > 3 × 10−3 cpm, and the vertical strain 1D kz spectrum a −1 slope for kz > 0.1 cpm, consistent with previous 1D measurements, numerical simulations and anisotropic stratified turbulence theory. Isopycnal salinity-gradient 1D kx spectra have a + 1 slope for kx > 2 × 10−3 cpm, consistent with nonlocal stirring. Turbulent diapycnal diffusivities inferred in the (i) internal-wave subrange using a vertical strain-based finescale parameterization are consistent with those inferred from finescale horizonal wavenumber spectra of (ii) isopycnal slope and (iii) isopycnal salinity-gradients using Batchelor model spectra. This suggests that horizontal submesoscale and vertical finescale subranges participate in bridging the forward cascade between weakly nonlinear internal waves and isotropic turbulence, as hypothesized by anisotropic turbulence theory.

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Xiaohui Zhou, Tetsu Hara, Isaac Ginis, Eric D’Asaro, Je-Yuan Hsu, and Brandon G. Reichl

Abstract

The drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper ocean current observations (using EM-APEX floats deployed under five tropical cyclones) and a coupled ocean-wave (Modular Ocean Model 6 - WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2−3×10−3 for wind speeds 25–55 m/s. While the drag coefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is significantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 45°, a feature which is underestimated by current models of sea state dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state dependent parameterizations of the drag coefficient in tropical cyclone conditions.

Open access