Diagnosing Water Mass Formation from Air–Sea Fluxes and Surface Mixing

Nurser, A. J. George; Marsh, Robert; Williams, Richard G.

doi:10.1175/1520-0485(1999)029<1468:dwmffa>2.0.co;2

Cited by 110 publications

(151 citation statements)

References 34 publications

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“…Here we extend the classical water mass diagnostics of the impact of physical terms on the diapycnal fluxes (initiated by Walin, 1982; see also Nurser et al, 1999, andMarshall et al, 1999) to derive a tool for the quantitative analysis of factors (advection and sources/sinks) that determine tracer transport across a particular oceanic boundary (e.g., the boundary of a particular basin) (see Fig. 4).…”

Section: The Water Mass Frameworkmentioning

confidence: 99%

Water masses as a unifying framework for understanding the Southern Ocean Carbon Cycle

et al. 2011

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Abstract. The scientific motivation for this study is to understand the processes in the ocean interior controlling carbon transfer across 30 • S. To address this, we have developed a unified framework for understanding the interplay between physical drivers such as buoyancy fluxes and ocean mixing, and carbon-specific processes such as biology, gas exchange and carbon mixing. Given the importance of density in determining the ocean interior structure and circulation, the framework is one that is organized by density and water masses, and it makes combined use of Eulerian and Lagrangian diagnostics. This is achieved through application to a global ice-ocean circulation model and an ocean biogeochemistry model, with both components being part of the widely-used IPSL coupled ocean/atmosphere/carbon cycle model.Our main new result is the dominance of the overturning circulation (identified by water masses) in setting the vertical distribution of carbon transport from the Southern Ocean towards the global ocean. A net contrast emerges between the role of Subantarctic Mode Water (SAMW), associated with large northward transport and ingassing, and Antarctic Intermediate Water (AAIW), associated with a much smaller export and outgassing. The differences in their export rate reflects differences in their water mass formation processes. For SAMW, two-thirds of the surface waters are provided as Correspondence to: D. Iudicone (iudicone@szn.it) a result of the densification of thermocline water (TW), and upon densification this water carries with it a substantial diapycnal flux of dissolved inorganic carbon (DIC). For AAIW, principal formatin processes include buoyancy forcing and mixing, with these serving to lighten CDW. An additional important formation pathway of AAIW is through the effect of interior processing (mixing, including cabelling) that serve to densify SAMW.A quantitative evaluation of the contribution of mixing, biology and gas exchange to the DIC evolution per water mass reveals that mixing and, secondarily, gas exchange, effectively nearly balance biology on annual scales (while the latter process can be dominant at seasonal scale). The distribution of DIC in the northward flowing water at 30 • S is thus primarily set by the DIC values of the water masses that are involved in the formation processes.

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Section: The Water Mass Frameworkmentioning

confidence: 99%

Water masses as a unifying framework for understanding the Southern Ocean Carbon Cycle

et al. 2011

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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%

“…The set of possible values for σ is [45.6, 46.2]. F (σ ) is called the "transformation" by geothermal heat flux, and its derivative with respect to σ the "formation", denoted by M (Speer and Tziperman, 1992;Nurser et al, 1999). Define the net advective diapycnal flux A(σ ) and the diffusive diapycnal flux D(σ ) by their integral along the isopycnal surface intersecting the bottom (but not the surface; this requires the outcropping zone of AABW to be excluded from the domain):…”

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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“…This approach makes a direct connection between surface buoyancy forcing and water mass formation in the upper ocean; thus, it can provide useful information regarding the role of climate change and water mass formation/erosion. This approach has been widely used by many investigators in analyzing hydrographic data to diagnose the corresponding water mass formation/erosion rate, for example, Speer and Tziperman (1992), Speer et al (1995), Marshall et al (1999), Nurser et al (1999), Brambilla and Talley (2008), and Brambilla et al (2008).…”

Section: Introductionmentioning

confidence: 99%

The Global Subduction/Obduction Rates: Their Interannual and Decadal Variability

Liu

Huang

2012

Journal of Climate

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Ventilation, including subduction and obduction, for the global oceans was examined using Simple Ocean Data Assimilation (SODA) outputs. The global subduction rate averaged over the period from 1959 to 2006 is estimated at 505.8 Sv (1 Sv [ 10 6 m 3 s 21 ), while the corresponding global obduction rate is estimated at 482.1 Sv. The annual subduction/obduction rates vary greatly on the interannual and decadal time scales. The global subduction rate is estimated to have increased 7.6% over the past 50 years, while the obduction rate is estimated to have increased 9.8%. Such trends may be insignificant because errors associated with the data generated by ocean data assimilation could be as large as 10%. However, a major physical mechanism that induced these trends is primarily linked to changes in the Southern Ocean. While the Southern Ocean plays a key role in global subduction and obduction rates and their variability, both the Southern Ocean and equatorial regions are critically important sites of water mass formation/erosion.

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Diagnosing Water Mass Formation from Air–Sea Fluxes and Surface Mixing

Cited by 110 publications

References 34 publications

Water masses as a unifying framework for understanding the Southern Ocean Carbon Cycle

Water masses as a unifying framework for understanding the Southern Ocean Carbon Cycle

Geothermal heating, diapycnal mixing and the abyssal circulation

The Global Subduction/Obduction Rates: Their Interannual and Decadal Variability

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