Water-Mass Transformations in a Neutral Density Framework and the Key Role of Light Penetration

Iudicone, Daniele; Madec, Gurvan; McDougall, Trevor J.

doi:10.1175/2007jpo3464.1

Cited by 104 publications

(171 citation statements)

References 43 publications

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“…It requires for example a complex water mass framework (Iudicone et al 2008;Downes et al 2011). In comparison, biogeochemical tracers that are integrated for thousand years or more, in order to reach near-steady state conditions, contain strong signatures from the circulation.…”

Section: Spinup Strategy Versus Skill Assessmentmentioning

confidence: 99%

Skill assessment of three earth system models with common marine biogeochemistry

et al. 2012

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We have assessed the ability of a common ocean biogeochemical model, PISCES, to match relevant modern data fields across a range of ocean circulation fields from three distinct Earth system models: IPSL-CM4-LOOP, IPSL-CM5A-LR and CNRM-CM5.1. The first of these Earth system models has contributed to the IPCC 4th assessment report, while the latter two are contributing to the ongoing IPCC 5th assessment report. These models differ with respect to their atmospheric component, ocean subgrid-scale physics and resolution. The simulated vertical distribution of biogeochemical tracers suffer from biases in ocean circulation and a poor representation of the sinking fluxes of matter. Nevertheless, differences between upper and deep ocean model skills significantly point to changes in the underlying model representations of ocean circulation. IPSL-CM5A-LR and CNRM-CM5.1 poorly represent deep-ocean circulation compared to IPSL-CM4-LOOP degrading the vertical distribution of biogeochemical tracers. However, their representations of surface wind, wind stress, mixed-layer depth and geostrophic circulations (e.g., Antarctic Circumpolar Current) have been improved compared to IPSL-CM4-LOOP. These improvements result in a better representation of large-scale structure of biogeochemical fields in the upper ocean. In particular, a deepening of 20-40 m of the summer mixed-layer depth allows to capture the 0-0.5 lgChl L -1 concentrations class of surface chlorophyll in the Southern Ocean. Further improvements in the representation of the ocean mixedlayer and deep-ocean ventilation are needed for the next generations of models development to better simulate marine biogeochemistry. In order to better constrain ocean dynamics, we suggest that biogeochemical or passive tracer modules should be used routinely for both model development and model intercomparisons.

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Section: Spinup Strategy Versus Skill Assessmentmentioning

confidence: 99%

Skill assessment of three earth system models with common marine biogeochemistry

et al. 2012

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“…Moreover, its action is precisely limited to bottom water (an endpoint of the T-S diagram), which means that geothermal heating tends to transform the densest water masses into warmer, lighter ones, much like air-sea fluxes transform surface waters. Indeed, a parcel of Antarctic Bottom Water (AABW) experiences a steady warming while heading North in the vicinity of the seafloor, in the same fashion as air-sea fluxes determine the thermal history of North Atlantic surface water while it ascends to high Northern latitudes (Walin, 1982;Speer and Tziperman, 1992;Large and Nurser, 2001;Marshall et al, 1998;Nurser et al, 1999;Iudicone et al, 2008b). To observe this analogy, one just needs to conceptually flip the ocean upside down, as done in Fig.…”

Section: The Formation/consumption Cycle Of Bottom Watermentioning

confidence: 99%

“…One can show (Walin, 1982;Speer et al, 2000;Iudicone et al, 2008b) that mass and density conservation then lead to the transformation equation: and the water mass formation equation:…”

Section: The Transformation Frameworkmentioning

confidence: 99%

Geothermal heating, diapycnal mixing and the abyssal circulation

Emile‐Geay

Madec

2009

Ocean Sci.

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Abstract. The dynamical role of geothermal heating in abyssal circulation is reconsidered using three independent arguments. First, we show that a uniform geothermal heat flux close to the observed average (86.4 mW m −2 ) supplies as much heat to near-bottom water as a diapycnal mixing rate of ∼10 −4 m 2 s −1 -the canonical value thought to be responsible for the magnitude of the present-day abyssal circulation. This parity raises the possibility that geothermal heating could have a dynamical impact of the same order. Second, we estimate the magnitude of geothermally-induced circulation with the density-binning method (Walin, 1982), applied to the observed thermohaline structure of Levitus (1998). The method also allows to investigate the effect of realistic spatial variations of the flux obtained from heatflow measurements and classical theories of lithospheric cooling. It is found that a uniform heatflow forces a transformation of ∼6 Sv at σ 4 =45.90, which is of the same order as current best estimates of AABW circulation. This transformation can be thought of as the geothermal circulation in the absence of mixing and is very similar for a realistic heatflow, albeit shifted towards slightly lighter density classes. Third, we use a general ocean circulation model in global configuration to perform three sets of experiments: (1) a thermally homogenous abyssal ocean with and without uniform geothermal heating; (2) a more stratified abyssal ocean subject to (i) no geothermal heating, (ii) a constant heat flux of 86.4 mW m −2 , (iii) a realistic, spatially varying heat flux of identical global average; (3) experiments (i) and (iii) with enhanced vertical mixing at depth. Geothermal heating and diapycnal mixing are found to interact non-linearly through the density field, with geothermal heating eroding the deep stratification supporting a downward diffusive flux, while diapycnal mixing acts to map near-surface temperature gradientsCorrespondence to: J. Emile-Geay (julieneg@usc.edu) onto the bottom, thereby altering the density structure that supports a geothermal circulation. For strong vertical mixing rates, geothermal heating enhances the AABW cell by about 15% (2.5 Sv) and heats up the last 2000 m by ∼0.15 • C, reaching a maximum of by 0.3 • C in the deep North Pacific. Prescribing a realistic spatial distribution of the heat flux acts to enhance this temperature rise at mid-depth and reduce it at great depth, producing a more modest increase in overturning than in the uniform case. In all cases, however, poleward heat transport increases by ∼10% in the Southern Ocean. The three approaches converge to the conclusion that geothermal heating is an important actor of abyssal dynamics, and should no longer be neglected in oceanographic studies.

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“…In order to do this accurately in a numerical model it is necessary to store all the tendencies of the temperature and salinity equation terms at every grid point, which is not yet feasible in a global eddy-resolving model (such diagnostics are used by Iudicone et al, 2007 in a low resolution version of the same model). The largest difficulty comes from the vertical mixing terms, which are highly nonlinear due to the turbulent closure scheme and cannot be estimated from our archived 5-days output.…”

Section: Depth-integrated Balancesmentioning

confidence: 99%

“…Moreover, to define potential density a reference depth must be chosen, and none is adequate in the whole Southern Ocean where isopycnals undergo large vertical excursions. It is better to use neutral density as Iudicone et al, 2007, but at the present time there is no algorithm efficient enough to "label" neutral density in a large model output dataset like ORCA025. In this paper, the emphasis being on the upper ocean circulation, we mainly consider the potential density referenced at the surface (σ 0 ).…”

Section: Depth-integrated Balancesmentioning

confidence: 99%

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

et al. 2007

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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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Water-Mass Transformations in a Neutral Density Framework and the Key Role of Light Penetration

Cited by 104 publications

References 43 publications

Skill assessment of three earth system models with common marine biogeochemistry

Skill assessment of three earth system models with common marine biogeochemistry

Geothermal heating, diapycnal mixing and the abyssal circulation

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

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