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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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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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Laurent M. Chérubin, Nicolas Le Paih, and Xavier Carton

Abstract

The Florida Current (FC) flows in the Straits of Florida (SoF) and connects the Loop Current in the Gulf of Mexico to the Gulf Stream (GS) in the western Atlantic Ocean. Its journey through the SoF is at time characterized by the formation and presence of mesoscale but mostly submesoscale frontal eddies on the cyclonic side of the current. The formation of those frontal eddies was investigated in a very high-resolution two-way nested simulation using the Regional Oceanic Modeling System (ROMS). Frontal eddies were either locally formed or originated from outside the SoF. The northern front of the incoming eddies was susceptible to superinertial shear instability over the shelf slope when the eddies were pushed up against the slope by the FC. Otherwise, incoming eddies could be advected, relatively unaffected by the current, when in the southern part of the straits. In the absence of incoming eddies, submesoscale eddies were locally formed by the roll-up of superinertial barotropically unstable vorticity filaments when the FC was pushed up against the shelf slope. The vorticity filaments were intensified by the friction-induced bottom-layer vorticity flux as previously demonstrated by Gula et al. in the GS. When the FC retreated farther south, negative-vorticity west Florida shelf waters overflowed into the SOF and led to the formation of submesoscale eddies by baroclinic instability. The instability regimes, that is, the submesoscale frontal eddies formation, appear to be controlled by the lateral “sloshing” of the FC in the SoF.

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Abhishek Savita, Jan D. Zika, Catia M. Domingues, Simon J. Marsland, Gwyn Dafydd Evans, Fabio Boeira Dias, Ryan M. Holmes, and Andrew McC. Hogg

Abstract

Ocean circulation and mixing regulate Earth’s climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities, to diagnose circulation. Using quasi-steady-state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature–depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of “Super Residual Transport,” which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies but excludes small-scale three-dimensional turbulent mixing effect, to construct a new overturning circulation—the “Super Residual Circulation” (SRC). We find that in the coarse-resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11-Sv (1 Sv ≡ 106 m3 s−1) circulation that transports heat upward. The SRC’s upward heat transport is ~2 times as large in a finer-horizontal-resolution (0.1°) version of ACCESS, suggesting that a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into superresidual processes, because the SRC elucidates the pathways in temperature and depth space along which water mass transformation occurs.

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Mohammad Hadi Bordbar, Volker Mohrholz, and Martin Schmidt

Abstract

Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind-stress-curl. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine ecosystem.

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William G. Large, Edward G. Patton, and Peter P. Sullivan

Abstract

Empirical rules for both entrainment and detrainment are developed from LES of the Southern Ocean boundary layer when the turbulence, stratification and shear cannot be assumed to be in equilibrium with diurnal variability in surface flux and wave (Stokes drift) forcing. A major consequence is the failure of down-gradient eddy viscosity, which becomes more serious with Stokes drift and is overcome by relating the angle between the stress and shear vectors to the orientations of Lagrangian shear to the surface and of local Eulerian shear over five meters. Thus, the momentum flux can be parameterized as a stress magnitude and this empirical direction. In addition, the response of a deep boundary layer to sufficiently strong diurnal heating includes boundary layer collapse and the subsequent growth of a morning boundary layer, whose depth is empirically related to the time history of the forcing, as are both morning detrainment and afternoon entrainment into weak diurnal stratification. Below the boundary layer, detrainment rules give the maximum buoyancy flux and its depth, as well a specific stress direction. Another rule relates both afternoon and night-time entrainment depth and buoyancy flux to surface layer turbulent kinetic energy production integrals. These empirical relationships are combined with rules for boundary layer transport to formulate two parameterizations; one based on eddy diffusivity and viscosity profiles and another on flux profiles of buoyancy and of stress magnitude. Evaluations against LES fluxes show the flux profiles to be more representative of the diurnal cycle, especially with Stokes drift.

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Sultan Hameed, Christopher L. P. Wolfe, and Lequan Chi

Abstract

Previous work to find an association between variations of annually averaged Florida Current transport and the North Atlantic Oscillation (NAO) have yielded negative results (Meinen et al. 2010). Here we show that Florida current in winter is impacted by displacements in the positions of the Azores High and the Icelandic Low, the constituent pressure centers of the NAO. As a one-dimensional representation of North Atlantic atmospheric circulation, the NAO index does not distinguish displacements of the pressure centers from fluctuations in their intensity. Florida Current transport is significantly correlated with Icelandic Low longitude with a lag of less than one season. We carried out perturbation experiments in the ECCOv4 model to investigate these correlations. These experiments reveal that east-west shifts of the Icelandic Low perturb the wind stress in mid-latitudes adjacent to the American coast, driving downwelling (through longshore winds) and offshore sea level anomalies (through wind stress curl) which travel to the Florida Straits within the same season. Florida Current transport is also correlated with the latitude variations of both the Icelandic Low and the Azores High with a lag of four years. Regression analysis shows that latitude variations of the Icelandic Low and the Azores High are associated with positive wind stress curl anomalies over extended regions in the ocean east of Florida. Rossby wave propagation from this region to the Florida Straits has been suggested as a mechanism for perturbing FCT transport in several previous studies (DiNezio et al. 2009; Czeschel et al. 2012; Frajka-Williams et al. 2013; Domingues et al. 2016, 2019).

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Shuo Li, Alexander V. Babanin, Fangli Qiao, Dejun Dai, Shumin Jiang, and Changlong Guan

Abstract

The CO 2 gas transfer velocity (KCO2) at air-sea interface is usually parameterized with the wind speed, but to a great extent is defined by waves and wave breaking. To investigate the direct relationship between KCO2 and waves, laboratory experiments are conducted in a wind-wave flume. Three types of waves are forced in the flume: modulational wave trains generated by a wave maker, wind waves with 10-meter wind speed ranging from 4.5 m/s to 15.5 m/s, and (mechanically-generated) modulational wave trains coupled with superimposed wind force. The wave height and wave orbital velocity are found to be well correlated with KCO2 while wind speed alone can not adequately describe KCO2. To reconcile the measurements, non-dimensional equations are established in which gas transfer velocity is expressed as a main function of wave parameters and an additional secondary factor to account for influence of the wind.

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Tobias Kukulka and Todd Thoman

Abstract

Dispersion processes in the ocean surface boundary layer (OSBL) determine marine material distributions such as those of plankton and pollutants. Sheared velocities drive shear dispersion, which is traditionally assumed to be due to mean horizontal currents that decrease from the surface. However, OSBL turbulence supports along-wind jets; located in near-surface convergence and downwelling regions, such turbulent jets contain strong local shear. Through wind-driven idealized and large eddy simulation (LES) models of the OSBL, this study examines the role of turbulent along-wind jets in dispersing material. In the idealized model, turbulent jets are generated by prescribed cellular flow with surface convergence and associated downwelling regions. Numeric and analytic model solutions reveal that horizontal jets substantially contribute to along-wind dispersion for sufficiently strong cellular flows and exceed contributions due to vertical mean shear for buoyant surface-trapped material. However, surface convergence regions also accumulate surface-trapped material, reducing shear dispersion by jets. Turbulence resolving LES results of a coastal depth-limited ocean agree qualitatively with the idealized model and reveal long-lived coherent jet structures that are necessary for effective jet dispersion. These coastal results indicate substantial jet contributions to along-wind dispersion. However, jet dispersion is likely less effective in the open ocean because jets are shorter lived, less organized, and distorted due to spiraling Ekman currents.

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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 days, 50 days 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-day 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 along-shore 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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