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Tobias Kukulka and Todd X. 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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Kaylie Cohanim, Ken X. Zhao, and Andrew L. Stewart

Abstract

Interaction between the atmosphere and ocean in sea ice–covered regions is largely concentrated in leads, which are long, narrow openings between sea ice floes. Refreezing and brine rejection in these leads inject salt that plays a key role in maintaining the polar halocline. The injected salt forms dense plumes that subsequently become baroclinically unstable, producing submesoscale eddies that facilitate horizontal spreading of the salt anomalies. However, it remains unclear which properties of the stratification and leads most strongly influence the vertical and horizontal spreading of lead-input salt anomalies. In this study, the spread of lead-injected buoyancy anomalies by mixed layer and eddy processes are investigated using a suite of idealized numerical simulations. The simulations are complemented by dynamical theories that predict the plume convection depth, horizontal eddy transfer coefficient, and eddy kinetic energy as functions of the ambient stratification and lead properties. It is shown that vertical penetration of buoyancy anomalies is accurately predicted by a mixed layer temperature and salinity budget until the onset of baroclinic instability (~3 days). Subsequently, these buoyancy anomalies are spread horizontally by eddies. The horizontal eddy diffusivity is accurately predicted by a mixing-length scaling, with a velocity scale set by the potential energy released by the sinking salt plume and a length scale set by the deformation radius of the ambient stratification. These findings indicate that the intermittent opening of leads can efficiently populate the polar halocline with submesoscale coherent vortices with diameters of ~10 km, and they provide a step toward parameterizing their effect on the horizontal redistribution of salinity anomalies.

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

Abstract

The CO2 gas transfer velocity (KCO2) at air–sea interface is usually parameterized with the wind speed, but to a great extent it 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-m wind speed ranging from 4.5 to 15.5 m s−1, 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 whereas wind speed alone cannot adequately describe KCO2. To reconcile the measurements, nondimensional 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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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 and satellite-derived chlorophyll-a concentration, along with a state-of-the-art ocean model simulation, spanning 2008–18 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 is fairly consistent across the entire BUS and is 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 submesoscale 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 are 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 that this relationship is far more complex than is currently thought and seems to be sensitive to climate changes, with short- and far-reaching consequences for this vulnerable marine ecosystem.

Open access
Sydney Sroka and Kerry Emanuel

Abstract

The intensity of tropical cyclones is sensitive to the air–sea fluxes of enthalpy and momentum. Sea spray plays a critical role in mediating enthalpy and momentum fluxes over the ocean’s surface at high wind speeds, and parameterizing the influence of sea spray is a crucial component of any air–sea interaction scheme used for the high wind regime where sea spray is ubiquitous. Many studies have proposed parameterizations of air–sea flux that incorporate the microphysics of sea spray evaporation and the mechanics of sea spray stress. Unfortunately, there is not yet a consensus on which parameterization best represents air–sea exchange in tropical cyclones, and the different proposed parameterizations can yield substantially different tropical cyclone intensities. This paper seeks to review the developments in parameterizations of the sea spray–mediated enthalpy and momentum fluxes for the high wind speed regime and to synthesize key findings that are common across many investigations.

Open access
Øyvind Saetra, Trygve Halsne, Ana Carrasco, Øyvind Breivik, Torstein Pedersen, and Kai Håkon Christensen

Abstract

The Lofoten Maelstrom has been known for centuries as one of the strongest open-ocean tidal currents in the world, estimated to reach 3 m s−1, and by some estimates as much as 5 m s−1. The strong current gives rise to choppy seas when waves enter the Moskenes Sound, making the area extremely difficult to navigate. Despite its reputation, few studies of its strength exist and no stationary in situ measurements for longer time periods have been made due to the challenging conditions. By deploying for the first time in situ wave and current instruments, we confirm some previous estimates of the strength of the current. We also show that its strength is strongly connected with wave breaking. From a consideration of specific forcing terms in the dynamical energy balance equation for waves on a variable current, we assess the impact of the underlying current using a convenient metric formulated as a function of the horizontal current gradients. We find that the horizontal gradients are a likely explanation for the observed enhanced wave breaking during strong currents at a rising tide.

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Christoph S. Funke, Marc P. Buckley, Larissa K.P. Schultze, Fabrice Veron, Mary-Louise E. Timmermans, and Jeffrey R. Carpenter

Abstract

The quantification of pressure fields in the airflow over water waves is fundamental for understanding the coupling of the atmosphere and the ocean. The relationship between the pressure field, and the water surface slope and velocity, are crucial in setting the fluxes of momentum and energy. However, quantifying these fluxes is hampered by difficulties in measuring pressure fields at the wavy air-water interface. Here we utilise results from laboratory experiments of wind-driven surface waves. The data consist of particle image velocimetry of the airflow combined with laser-induced fluorescence of the water surface. These data were then used to develop a pressure field reconstruction technique based on solving a pressure Poisson equation in the airflow above water waves. The results allow for independent quantification of both the viscous stress and pressure-induced form drag components of the momentum flux. Comparison of these with an independent bulk estimate of the total momentum flux (based on law-of-the-wall theory) shows that the momentum budget is closed to within approximately 5%. In the partitioning of the momentum flux between viscous and pressure drag components, we find a greater influence of form drag at high wind speeds and wave slopes. An analysis of the various approximations and assumptions made in the pressure reconstruction, along with the corresponding sources of error, is also presented.

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Ratnaksha Lele, Sarah G. Purkey, Jonathan D. Nash, Jennifer A. MacKinnon, Andreas M. Thurnherr, Caitlin B. Whalen, Sabine Mecking, Gunnar Voet, and Lynne D. Talley

Abstract

The abyssal Southwest Pacific Basin has warmed significantly between 1992-2017, consistent with warming along the bottom limb of the meridional overturning circulation seen throughout the global oceans. Here we present a framework for assessing the abyssal heat budget that includes the time-dependent unsteady effects of decadal warming and direct and indirect estimates of diapycnal mixing from microscale temperature measurements and finescale parameterizations. The unsteady terms estimated from the decadalwarming rate are shown to be within a factor of 3 of the steady state terms in the abyssal heat budget for the coldest portion of the water column and therefore, cannot be ignored. We show that a reduction in the lateral heat flux for the coldest temperature classes compensated by an increase in warmer waters advected into the basin has important implications for the heat balance and diffusive heat fluxes in the basin. Finally, vertical diffusive heat fluxes are estimated in different ways: using the newly available CTD-mounted microscale temperature measurements, a finescale strain parameterization, and a vertical kinetic energy parameterization from data along the P06 transect along 32.5°S. The unsteady-state abyssal heat budget for the basin shows closure within error estimates, demonstrating that (i) unsteady terms have become consequential for the heat balance in the isotherms closest to the ocean bottom and (ii) direct and indirect estimates from full depth GO-SHIP hydrographic transects averaged over similarly large spatial and temporal scales can capture the basin-averaged abyssal mixing needed to close the deep overturning circulation.

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He Wang, Julie L. McClean, and Lynne D. Talley

Abstract

The Arabian Sea, influenced by the Indian monsoon, has many unique features including its basin scale seasonally reversing surface circulation and the Great Whirl, a seasonal anti-cyclonic system appearing during the southwest monsoon close to the western boundary. To establish a comprehensive dynamical picture of the Arabian Sea, we utilize numerical model output and design a full vorticity budget that includes a fully-decomposed nonlinear term. The ocean general circulation model has 0.1° resolution and is mesoscale eddy-resolving in the region. In the western boundary current system, we highlight the role of nonlinear eddies in the life cycle of the Great Whirl. The nonlinear eddy term is of leading order importance in this feature’s vorticity balance. Specifically, it contributes to the Great Whirl’s persistence in boreal fall after the weakening of the southwesterly winds. In the open ocean, Sverdrup dynamics and annual Rossby waves are found to dominate the vorticity balance; the latter is considered as a key factor in the formation of the Great Whirl and the sea-sonal reversal of the western boundary current. In addition, we discuss different forms of vertically-integrated vorticity equations in the model and argue that the bottom pressure torque term can be interpreted analogously as friction in the western boundary and vortex stretching in the open ocean.

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Gerardo Hernández-Dueñas, M.-Pascale Lelong, and Leslie M. Smith

Abstract

Submesoscale lateral transport of Lagrangian particles in pycnocline conditions is investigated by means of idealized numerical simulations with reduced-interaction models. Using a projection technique, the models are formulated in terms of wave-mode and vortical-mode nonlinear interactions, and they range in complexity from full Boussinesq to waves-only and vortical-modes-only (QG) models. We find that, on these scales, most of the dispersion is done by vortical motions, but waves cannot be discounted because they play an important, albeit indirect, role. In particular, we show that waves are instrumental in filling out the spectra of vortical-mode energy at smaller scales through non-resonant vortex-wave-wave triad interactions. We demonstrate that a richer spectrum of vortical modes in the presence of waves enhances the effective lateral diffusivity, compared to QG. Waves also transfer energy upscale to vertically sheared horizontal flows which are a key ingredient for internal-wave shear dispersion. In the waves-only model, the dispersion rate is an order of magnitude smaller and is attributed entirely to internal-wave shear dispersion.

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