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F. Sévellec
,
A. Colin de Verdière
, and
N. Kolodziejczyk

Abstract

Observations of deep Argo displacements (located between 950 and 1150 dbar) and their associated integrated Lagrangian velocities allow for the first time to compute worldwide deep horizontal transfers of Kinetic Energy (KE) between the 3°×3°-Mean and the Eddy reservoirs (MKE and EKE, respectively). This diagnostic reveals that the transfers are mainly localized along western boundaries and in the Southern Ocean. Overall the MKE-to-EKE transfers appear dominant globally and in all specifically tested regions (i.e., Gulf Stream, Kuroshio, Agulhas Current, and Antarctic Circumpolar Current). However an important exception is the Zapiola gyre where the EKE-to-MKE transfers dominate. Beyond that, we find that horizontal KE transfers are better described by the horizontal properties of the mean flow deformation (divergence and strain) than by the horizontal properties of the turbulent velocities. Our theoretical analysis also demonstrates that the mean flow vorticity does not contribute to KE transfers. We show the existence of two consistent transfer modes: one from MKE to EKE and one from EKE to MKE, which are based on the eigendirections of the mean flow deformation tensor. The alignment of the turbulence along these directions selects the transfer modes and it is the competition between these two transfer modes that leads to the actual transfers. We compute these transfer modes globally, regionally, and locally. We explain the distinctive situation of the Zapiola gyre by the favoured alignment of the turbulence with the EKE-to-MKE transfer mode. Overall, the dominance of the large-scale flow properties on the structure of the MKE-to-EKE transfers suggests the potential for a large-scale parameterization.

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Theresa J. Morrison
,
Julie L. McClean
,
Sarah T. Gille
,
Mathew E. Maltrud
,
Detelina P. Ivanova
, and
Anthony P. Craig

Abstract

Meltwater from the Greenland Ice Sheet can alter the continental shelf/slope circulation, cross-shelf freshwater fluxes, and limit deep convection in adjacent basins through surface freshening. We explore the impacts on the West Greenland Current and Eastern Labrador Sea with different vertical distributions of the meltwater forcing. In this study, we present results from global coupled ocean/sea-ice simulations, forced with atmospheric reanalysis, that are mesoscale eddy-active (~2–3 km horizontal spacing) and eddy-permitting (~6–7 km horizontal spacing) in the study region. We compare the West Greenland Current in mesoscale eddy-active and eddy-permitting without meltwater to highlight the role of small scale features. The mesoscale eddy-active configuration is then used to assess the change in the Eastern Labrador Sea when meltwater is added to the surface or vertically distributed to account for mixing within fjords. In both simulations with meltwater, the West Greenland and West Greenland Coastal Currents are faster than in the simulation with no meltwater; their mean surface speeds are highest in the vertical distribution case. In the latter case, there is enhanced baroclinic conversion at the shelf break compared to the simulation with no meltwater. When meltwater is vertically distributed, there is an increase in baroclinic conversion at the shelf break associated with increased eddy kinetic energy. In addition, in the Eastern Labrador Sea the salinity is lower and the meltwater volume greater when meltwater is vertically distributed. Therefore, the West Greenland Current is sensitive to how meltwater is added to the ocean with implications for the freshening of the Labrador Sea.

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R C Musgrave
,
D Winters
,
V E Zemskova
, and
J A Lerczak

Abstract

A series of idealized numerical simulations is used to examine the generation of mode-one superinertial coastally trapped waves (CTW). In the first set of simulations, CTW are resonantly generated when freely propagating mode-one internal tides are incident on the coast such that the angle of incidence of the internal wave causes the projected wavenumber of the tide on the coast to satisfy a triad relationship with the wavenumbers of the bathymetry and the CTW. In the second set of simulations, CTW are generated by the interaction of the barotropic tide with topography that has the same scales as the CTW. Under resonant conditions superinertial coastally trapped waves are a leading order coastal process, with along-shore current magnitudes that can be larger than the barotropic or internal tides from which they are generated.

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Alan Gaul
,
Weifeng (Gordon) Zhang
, and
Claudia Cenedese

Abstract

This study examines the link between near-bottom outflows of dense water formed in Antarctic coastal polynyas and onshore intrusions of Circumpolar Deep Water (CDW) through prograde troughs cutting across the continental shelf. Numerical simulations show that the dense water outflow is primarily in the form of cyclonic eddies. The trough serves as a topographic guide that organizes the offshore-moving dense water eddies into a chain pattern. The offshore migration speed of the dense water eddies is similar to the velocity of the dense water offshore flow in the trough, which scaling analysis finds to be proportional to the reduced gravity of the dense water and the slope of the trough side walls and to be inversely proportional to the Coriolis parameter. Our model simulations indicate that, as these cyclonic dense water eddies move across the trough mouth into the deep ocean, they entrain CDW from offshore and carry CDW clockwise along their periphery into the trough. Subsequent cyclonic dense water eddies then entrain the intruding CDW further toward the coast along the trough. This process of recurring onshore entrainment of CDW by a topographically constrained chain of offshore-flowing dense water eddies is consistent with topographic hotspots of onshore intrusion of CDW around Antarctica identified by other studies. It can bring CDW from offshore to close to the coast and thus impact the heat flux into Antarctic coastal regions, affecting interactions among ocean, sea ice, and ice shelves.

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Anna Lo Piccolo
,
Christopher Horvat
, and
Baylor Fox-Kemper

Abstract

During polar winter, refreezing of exposed ocean areas results in the rejection of brine, i.e., salt-enriched plumes of water, a source of available potential energy that can drive ocean instabilities. As this process is highly localized, and driven by sea ice physics, not gradients in oceanic or atmospheric buoyancy, it is not currently captured in modern climate models. This study aims to understand the energetics and lateral transfer of density at a semi-infinite, instantaneously-opened and continuously re-freezing sea ice edge through a series of high resolution model experiments. We show that kilometer-scale submesoscale eddies grow from baroclinic instabilities via an inverse energy cascade. These eddies meander along the ice edge and propagate laterally. The lateral transfer of buoyancy by eddies is not explained by existing theories. We isolate the fundamental forcing-independent quantities driving lateral mixing, and discuss the implications for the overall strength of submesoscale activity in the Arctic Ocean.

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Varvara E. Zemskova
,
Ruth C. Musgrave
, and
James A. Lerczak

Abstract

The generation of internal tides at coastal margins is an important mechanism for the loss of energy from the barotropic tide. Although some previous studies attempted to quantify energy loss from the barotropic tides into the deep ocean, global estimates are complicated by the coastal geometry and spatially and temporally variable stratification. Here, we explore the effects of supercritical, finite amplitude bottom topography, which is difficult to solve analytically. We conduct a suite of 2D linear numerical simulations of the barotropic tide interacting with a uniform along-shore coastal shelf, representing the tidal forcing by a body force derived from the vertical displacement of the isopycnals by the gravest coastal trapped wave (of which a Kelvin wave is a close approximation). We explore the effects of latitude, topographic parameters, and non-uniform stratification on the baroclinic tidal energy flux propagating into the deep ocean away from the shelf. By varying the pycnocline depth and thickness, we extend previous studies of shallow and infinitesimally thin pycnoclines to include deep permanent pycnoclines. We find that scaling laws previously derived in terms of continental shelf width and depth for shallow and thin pycnoclines generally hold for the deeper and thicker pycnoclines considered in this study. We also find baroclinic tidal energy flux is more sensitive to topographic than stratification parameters. Interestingly, we find that the slope of the shelf itself to be an important parameter, but not the width of the continental slope in the case of these steep topographies.

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Malcolm E. Scully
and
Seth F. Zippel

Abstract

Data from an air-sea interaction tower are used to close the turbulent kinetic energy (TKE) budget in the wave-affected surface layer of the upper ocean. Under energetic wind forcing with active wave breaking, the dominant balance is between the dissipation rate of TKE and the downward convergence in vertical energy flux. The downward energy flux is driven by pressure work, and the TKE transport is upward, opposite to the downgradient assumption in most turbulence closure models. The sign and the relative magnitude of these energy fluxes are hypothesized to be driven by an interaction between the vertical velocity of Langmuir circulation (LC) and the kinetic energy and pressure of wave groups, which is the result of small-scale wave-current interaction. Consistent with previous modeling studies, the data suggest that the horizontal velocity anomaly associated with LC refracts wave energy away from downwelling regions and into upwelling regions, resulting in negative covariance between the vertical velocity of LC and the pressure anomaly associated with the wave groups. The asymmetry between downward pressure work and upward TKE flux is explained by the Bernoulli response of the sea surface, which results in groups of waves having a larger pressure anomaly than the corresponding kinetic energy anomaly, consistent with group-bound long wave theory.

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Zhumin Lu
and
Xiaodong Shang

Abstract

Despite the large radius (R 17) of gale-force wind of a tropical cyclone (TC), the observed TC-induced effects on mesoscale and large-scale ocean via the baroclinic geostrophic response are found to have a limited cross-track width; this strange but important phenomenon is interpreted here. Driven by the wind stress curl (WSC), the TC-induced geostrophic response is in fact regulated by along-track integration of the WSC (AIWSC). Constrained by atmospheric TC dynamics, the violent winds outside the radius (R max) of maximum wind of any TC must have nearly zero WSC. Consequently, the AIWSC function can be fit as a boxcar function with an extraordinarily large positive value between ±R max about the track. Based on this boxcar function, the theoretical estimate of the cross-track length scale of the baroclinic geostrophic response, Ld + R max, is presented, where Ld is the first-mode baroclinic Rossby deformation radius. Further, this scale is validated by numerical experiments to well explain the width of the altimetry-observed geostrophic response induced by any TC. Evidently, Ld + R max is far smaller than R 17 and thus the baroclinic geostrophic response generally has a limited width. This study implies that, although for a TC the violent winds outside R max are generally ∼90% of all winds, in an open ocean these winds may be useless to perturb the ocean interior due to the nearly zero WSC.

Significance Statement

Despite the large radius of gale-force wind of a tropical cyclone, the effects of a tropical cyclone on mesoscale and large-scale ocean are confined in a limited cross-track width; this strange but important phenomenon is interpreted here. In essence, the effects are exerted by the wind stress curl rather than by the wind stress. However, constrained by atmospheric dynamics, a tropical cyclone has most of the positive wind stress curl in the inner core and nearly zero wind stress curl far away from the inner core. Consequently, albeit violent, the winds outside the inner core cannot make an appreciable contribution to the physical processes below the mixed layer.

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T. Sohail
and
J.D. Zika

Abstract

The ocean surrounding Antarctica, also known as the Antarctic margins, is characterised by complex and heterogeneous process interactions which have major impacts on the global climate. A common way to understand changes in the Antarctic margins is to categorise regions into similar ‘regimes’, thereby guiding process-based studies and observational analyses. However, this categorisation is traditionally largely subjective and based on temperature, density and bathymetric criteria that are bespoke to the dataset being analysed. In this work, we introduce a method to classify Antarctic shelf regimes using unsupervised learning. We apply Gaussian Mixture Modelling to the across-shelf temperature and salinity properties along the Antarctic margins from a high-resolution ocean model, ACCESS-OM2-01. Three clusters are found to be optimum based on the Bayesian Information Criterion and an assessment of regime properties. The three clusters correspond to the fresh, dense and warm regimes identified canonically via subjective approaches. Our analysis allows us to track changes to these regimes in a future projection of the ACCESS-OM2-01 model. We identify the future collapse of dense water formation, and the merging of dense and fresh shelf regions into a single fresh regime that covers the entirety of the Antarctic shelf except for the West Antarctic. Our assessment of these clusters indicates that the Antarctic margins may shift into a two-regime system in the future, consisting only of a strengthening warm shelf in the West Antarctic and a fresh shelf regime everywhere else.

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M. Schmitt
,
H. T. Pham
,
S. Sarkar
,
K. Klingbeil
, and
L. Umlauf

Abstract

Diurnal warm layers (DWLs) form near the surface of the ocean on days with strong solar radiation, weak to moderate winds, and small surface-wave effects. Here, we use idealized second-moment turbulence modeling, validated with large-eddy simulations (LES), to study the properties, dynamics, and energetics of DWLs across the entire physically relevant parameter space. Both types of models include representations of Langmuir turbulence (LT). We find that LT only slightly modifies DWL thicknesses and other bulk parameters under equilibrium wave conditions, but leads to a strong reduction in surface temperature and velocity with possible implications for air–sea coupling. Comparing tropical and the less frequently studied high-latitude DWLs, we find that LT has a strong impact on the energy budget and that rotation at high latitudes strongly modifies the DWL energetics, suppressing net energy turnover and entrainment. We identify the key nondimensional parameters for DWL evolution and find that the scaling relations of Price et al. provide a reliable representation of the DWL bulk properties across a wide parameter space, including high-latitude DWLs. We present different sets of revised model coefficients that include the deepening of the DWL due to LT and other aspects of our more advanced turbulence model to describe DWL properties at midday and during the DWL temperature peak in the afternoon, which we find to occur around 1500–1630 local time for a broad range of parameters.

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