The response of the upper ocean to surface buoyancy forcing: a characteristic solution to wave propagation

Abstract The overturning circulation of the global oceans is examined from an energetics viewpoint. A general framework for stratified turbulence is used for this purpose; first, it highlights the importance of available potential energy in facilitating the transfer of kinetic energy to the background potential energy (defined as the adiabatically rearranged state with no motion). Next, it is shown that it is the rate of transfer between different energy reservoirs that is important for the maintenance of the ocean overturning, rather than the total amount of potential or kinetic energy. A series of numerical experiments is used to assess which energy transfers are significant in the overturning circulation. In the steady state, the rate of irreversible diapycnal mixing is necessarily balanced by the production of available potential energy sourced from surface buoyancy fluxes. Thus, the external inputs of available potential energy from surface buoyancy forcing and of kinetic energy from other sources (such as surface winds and tides, and leading to turbulent mixing) are both necessary to maintain the overturning circulation.

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On the Importance of Surface Forcing in Conceptual Models of the Deep Ocean

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0206.1 ◽

2014 ◽

Vol 44 (3) ◽

pp. 891-899 ◽

Cited By ~ 15

Author(s):

Andrew L. Stewart ◽

Raffaele Ferrari ◽

Andrew F. Thompson

Keyword(s):

Conceptual Models ◽

Deep Ocean ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Surface Boundary ◽

Buoyancy Forcing ◽

Surface Boundary Condition ◽

Meridional Overturning ◽

Surface Buoyancy ◽

The Antarctic

Abstract In the major ocean basins, diapycnal mixing upwells dense Antarctic Bottom Water, which returns southward and closes the deepest cell of the meridional overturning circulation (MOC). This cell ventilates the deep ocean and regulates the partitioning of CO2 between the atmosphere and the ocean. The oceanographic community's conceptual understanding of the deep stratification and MOC has evolved from classic “abyssal recipes” arguments to a more recent appreciation of along-isopycnal upwelling in the Southern Ocean, consistent with a weakly mixed ocean interior. Both the deep stratification and the deep MOC are shown here to be sensitive to the form of the surface buoyancy forcing in a two-dimensional model that includes a circumpolar channel and northern basin. For a fixed surface buoyancy condition, the deep stratification is essentially prescribed, whereas for a fixed surface buoyancy flux, the deep stratification varies by orders of magnitude over the range of diapycnal diffusivity κ observed in the ocean. These cases also produce different scalings for the deep MOC with κ, in both weak and strong κ regimes. In addition, these scalings are shown to be sensitive not only to the type of surface boundary condition, but also to the latitudinal structure of the surface fluxes. This latter point is crucial as buoyancy budgets and dynamical features of the circulation are poorly constrained along the Antarctic margins. This study emphasizes the need for caution in the interpretation of simple conceptual models that, while useful, may not include all mechanisms that contribute to the MOC’s strength and structure.

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The dependence of upper ocean gyres on wind and buoyancy forcing

Geoscience Letters ◽

10.1186/s40562-022-00213-2 ◽

2022 ◽

Vol 9 (1) ◽

Author(s):

Tongya Liu ◽

Hsien-Wang Ou ◽

Xiaohui Liu ◽

Yu-Kun Qian ◽

Dake Chen

Keyword(s):

Wind Forcing ◽

Upper Ocean ◽

Subpolar Gyre ◽

Meridional Temperature Gradient ◽

Buoyancy Forcing ◽

Vertical Diffusivity ◽

Ocean Gyres ◽

Eddy Heat Transport ◽

Diapycnal Diffusivity

AbstractA series of numerical simulations with different forcing conditions are carried out, to investigate the roles played by buoyancy and wind forcing on the upper ocean gyres, and to contrast the laminar and eddying regimes. Model experiments show that the buoyancy-driven eastward geostrophic flow tends to suppress the formation of the wind-driven subpolar gyre, but the northward eddy heat transport can homogenize the subpolar water and reduce the meridional temperature gradient by about two-third, thus counteracting the buoyancy effect and saving the subpolar gyre. For the subtropical gyre, its transport is enhanced by eddy mixing, and the role of buoyancy forcing is very sensitive to the choice of diapycnal diffusivity. Our results suggest that eddy effects must be considered in the dynamics of the subpolar gyre, and vertical diffusivity should be selected carefully in simulating the basin-wide circulations.

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Ocean gyres driven by surface buoyancy forcing

10.1002/essoar.10503133.1 ◽

2020 ◽

Author(s):

Andrew McC. Hogg ◽

Bishakhdatta Gayen

Keyword(s):

Buoyancy Forcing ◽

Ocean Gyres ◽

Surface Buoyancy

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Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current

Ocean Science ◽

10.5194/os-3-491-2007 ◽

2007 ◽

Vol 3 (4) ◽

pp. 491-507 ◽

Cited By ~ 45

Author(s):

A. M. Treguier ◽

M. H. England ◽

S. R. Rintoul ◽

G. Madec ◽

J. Le Sommer ◽

...

Keyword(s):

Ocean Circulation ◽

Antarctic Circumpolar Current ◽

Meridional Circulation ◽

Dimensional Structure ◽

Residual Circulation ◽

Mean Flow ◽

Buoyancy Forcing ◽

Circumpolar Current ◽

Surface Buoyancy ◽

The Antarctic

Abstract. An eddying global model is used to study the characteristics of the Antarctic Circumpolar Current (ACC) in a streamline-following framework. Previous model-based estimates of the meridional circulation were calculated using zonal averages: this method leads to a counter-intuitive poleward circulation of the less dense waters, and underestimates the eddy effects. We show that on the contrary, the upper ocean circulation across streamlines agrees with the theoretical view: an equatorward mean flow partially cancelled by a poleward eddy mass flux. Two model simulations, in which the buoyancy forcing above the ACC changes from positive to negative, suggest that the relationship between the residual meridional circulation and the surface buoyancy flux is not as straightforward as assumed by the simplest theoretical models: the sign of the residual circulation cannot be inferred from the surface buoyancy forcing only. Among the other processes that likely play a part in setting the meridional circulation, our model results emphasize the complex three-dimensional structure of the ACC (probably not well accounted for in streamline-averaged, two-dimensional models) and the distinct role of temperature and salinity in the definition of the density field. Heat and salt transports by the time-mean flow are important even across time-mean streamlines. Heat and salt are balanced in the ACC, the model drift being small, but the nonlinearity of the equation of state cannot be ignored in the density balance.

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Generation mechanisms of mesoscale eddies in the Mauritanian Upwelling Region

Journal of Physical Oceanography ◽

10.1175/jpo-d-21-0092.1 ◽

2021 ◽

Author(s):

Ahmad Fehmi Dilmahamod ◽

Johannes Karstensen ◽

Heiner Dietze ◽

Ulrike Löptien ◽

Katja Fennel

Keyword(s):

Current System ◽

Ekman Transport ◽

Boreal Summer ◽

Unstable State ◽

Coastal Current ◽

Boundary Current ◽

Mode Water ◽

Buoyancy Forcing ◽

Surface Buoyancy ◽

Anticyclonic Eddies

AbstractThe physical processes driving the genesis of surface- and subsurface-intensified cyclonic and anticyclonic eddies originating from the coastal current system of the Mauritanian Upwelling Region are investigated using a high-resolution (~1.5 km) configuration of GFDL’s Modular Ocean Model. Estimating an energy budget for the boundary current reveals a baroclinically unstable state during its intensification phase in boreal summer and which is driving eddy generation within the near-coastal region. The mean poleward coastal flow’s interaction with the sloping topography induces enhanced anticyclonic vorticity, with potential vorticity close to zero generated in the bottom boundary layer. Flow separation at sharp topographic bends intensifies the anticyclonic vorticity, and submesoscale structures of low PV coalesce to form anticyclonic vortices. A combination of offshore Ekman transport and horizontal advection determined the amount of SACW in an anticyclonic eddy. A vortex with a relatively dense and low PV core will form an anticyclonic mode-water eddy, which will subduct along isopycnals while propagating offshore and hence be shielded from surface buoyancy forcing. Less contribution of dense SACW promotes the generation of surface anticyclonic eddies as the core is composed of a lighter water mass, which causes the eddy to stay closer to the surface and hence be exposed to surface buoyancy forcing. Simulated cyclonic eddies are formed between the rotational flow of an offshore anticyclonic vortex and a poleward flowing boundary current, with eddy potential energy being the dominant source of eddy kinetic energy. All three types of eddies play a key role in the exchange between the Mauritanian Coastal currents system and the adjacent eastern boundary shadow zone region.

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Moored Turbulence Measurements using Pulse-Coherent Doppler Sonar

Journal of Atmospheric and Oceanic Technology ◽

10.1175/jtech-d-21-0005.1 ◽

2021 ◽

Author(s):

Seth F. Zippel ◽

J. Thomas Farrar ◽

Christopher J. Zappa ◽

Una Miller ◽

Louis St. Laurent ◽

...

Keyword(s):

Mechanical Design ◽

Surface Fluxes ◽

Upper Ocean ◽

Data Sets ◽

Regional Study ◽

Ocean Turbulence ◽

Dissipation Rates ◽

Optimal Programming ◽

Long Time ◽

Surface Buoyancy

AbstractUpper-ocean turbulence is central to the exchanges of heat, momentum, and gasses across the air/sea interface, and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed-layer depths and sea surface temperature. In part, progress has been limited due to the difficulty of measuring turbulence from fixed moorings which can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring Turbulent Kinetic Energy (TKE) dissipation rates, ϵ, from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-ocean Regional Study (SPURS) to collect two year-long data sets. We find the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate two-week missions for (10−8) ≤ ϵ ≤ (10−5) m2 s−3. Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1-10 km. We also find that dissipation estimates from two different moorings at 12.5 m, and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory.

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