The role of mixing in the large-scale ocean circulation

Lavergne, Casimir de; Groeskamp, Sjoerd; Zika, Jan D.; Johnson, H. L.

doi:10.1016/b978-0-12-821512-8.00010-4

Cited by 23 publications

(24 citation statements)

References 226 publications

(275 reference statements)

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“…This method avoids the potential for large spurious dianeutral fluxes that comes with alternative slope clipping methods. In ACCESS‐OM2 the neutral diffusion operator is reduced to horizontal diffusion in the top surface layer (of thickness ∼2 m) and bottom topography grid cells (Ferrari et al., 2008; Treguier et al., 1997), meaning that the neutral diffusion parameterization can directly drive some dianeutral flux there, along with interactions with surface boundary layer turbulence and surface fluxes (de Lavergne et al., 2022). We also note that model implementations of rotated neutral diffusion are affected by various numerical discretization errors that can create spurious dianeutral fluxes that are nontrivial to quantify and are treated elsewhere (e.g., Beckers et al., 1998, 2000; Griffies et al., 1998; Groeskamp et al., 2019; Lemarié et al., 2012; Shao et al., 2020; Urakawa et al., 2020).…”

Section: Methodsmentioning

confidence: 99%

Sensitivity of a Coarse‐Resolution Global Ocean Model to a Spatially Variable Neutral Diffusivity

Holmes

Groeskamp

Stewart

et al. 2022

J Adv Model Earth Syst

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Motivated by recent advances in mapping mesoscale eddy tracer mixing in the ocean we evaluate the sensitivity of a coarse‐resolution global ocean model to a spatially variable neutral diffusion coefficient κn(x, y, z). We gradually introduce physically motivated models for the horizontal (mixing length theory) and vertical (surface mode theory) structure of κn along with suppression of mixing by mean flows. Each structural feature influences the ocean's hydrography and circulation to varying extents, with the suppression of mixing by mean flows being the most important factor and the vertical structure being relatively unimportant. When utilizing the full theory (experiment “FULL”) the interhemispheric overturning cell is strengthened by 2 Sv at 26°N (a ∼20% increase), bringing it into better agreement with observations. Zonal mean tracer biases are also reduced in FULL. Neutral diffusion impacts circulation through surface temperature‐induced changes in surface buoyancy fluxes and nonlinear equation of state effects. Surface buoyancy forcing anomalies are largest in the Southern Ocean where a decreased neutral diffusivity in FULL leads to surface cooling and enhanced dense‐to‐light surface water mass transformation, reinforced by reductions in cabbeling and thermobaricity. The increased water mass transformation leads to enhanced midlatitude stratification and interhemispheric overturning. The spatial structure for κn in FULL is important as it enhances the interhemispheric cell without degrading the Antarctic bottom water cell, unlike a spatially uniform reduction in κn. These results highlight the sensitivity of modeled circulation to κn and motivate the use of physics‐based models for its structure.

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Section: Methodsmentioning

confidence: 99%

Sensitivity of a Coarse‐Resolution Global Ocean Model to a Spatially Variable Neutral Diffusivity

Holmes

Groeskamp

Stewart

et al. 2022

J Adv Model Earth Syst

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“…Talley et al, 2003;Lumpkin & Speer, 2007) and the ECCO state estimate (Forget et al, 2015;Cessi, 2019) show a reduction in AMOC's transport by about 2-10 Sv between 24 • N and 32 • S, largely driven by downward diffusion of low-latitude surface heat gain (Talley, 2013). An important further contribution to such reduction may be effected by diapycnal mixing near the Atlantic's topographic boundaries, along which a substantial fraction of the AMOC transport occurs (de Lavergne et al, 2022). Several recent investigations of the connection between diapycnal mixing and the turbulent transformation of water masses, especially in regions of topographicallyenhanced turbulence, have hypothesized that diapycnal mixing induces diapycnal downwelling (i.e.…”

Section: Introductionmentioning

confidence: 99%

Significance of diapycnal mixing within the Atlantic Meridional Overturning Circulation

Cimoli

Mashayek

Johnson

et al. 2022

Preprint

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Diapycnal mixing shapes the distribution of climatically-important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation. The rate of water mass transformation in the Atlantic Ocean's interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW), with a diapycnal circulation of up to 4 Sv between 24N and 32S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to hundreds of metres in the vertical. This result is confirmed with a zonally-averaged numerical model of the AMOC and indicates that tracer mixing can lead to divergent global pathways and ventilation timescales following the upwelling of tracers in the Southern Ocean. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.

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“…However, observation‐based inverse models (e.g., Lumpkin & Speer, 2007; Talley et al., 2003) and the ECCO state estimate (Cessi, 2019; Forget et al., 2015) show a reduction in AMOC's transport by about 2–10 Sv between 24°N and 32°S, largely driven by downward diffusion of low‐latitude surface heat gain (Talley, 2013). An important further contribution to such reduction may be effected by diapycnal mixing near the Atlantic's topographic boundaries, along which a substantial fraction of the AMOC transport occurs (de Lavergne et al., 2022). Several recent investigations of the connection between diapycnal mixing and the turbulent transformation of water masses, especially in regions of topographically enhanced turbulence, have hypothesized that diapycnal mixing induces diapycnal downwelling (i.e., a densification of water masses) in the ocean interior, and diapycnal upwelling (i.e., a lightening of water masses) in the proximity of topographic boundaries (de Lavergne, Madec, Sommer, et al., 2016; Ferrari et al., 2016; Mashayek, Salehipour, et al., 2017; McDougall & Ferrari, 2017).…”

Section: Introductionmentioning

confidence: 99%

Significance of Diapycnal Mixing Within the Atlantic Meridional Overturning Circulation

et al. 2023

Self Cite

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The Atlantic Meridional Overturning Circulation (AMOC) constitutes a key component of the global ocean circulation, and plays a central role in the regulation, variability and anthropogenic change of our climate. The AMOC is the primary contributor to the redistribution of heat in the Atlantic Ocean, transporting heat northward in both hemispheres (Forget & Ferreira, 2019). Further, it exerts a profound influence on the sequestration of tracers, such as oxygen and anthropogenic carbon, that are taken up in the process of dense water formation (Gruber et al., 2019).The AMOC is an overturning cell, encompassing net southward transport of dense waters and net northward return of lighter waters (Figure 1a). The dense southward-flowing waters are produced through surface transformation of lighter waters in the sub-polar North Atlantic, and are additionally sourced by entrainment of Mediterranean Water, and by diapycnal exchanges with northward-flowing intermediate and abyssal waters (Reid, 1994;Talley, 1996). Together, these diverse sources give rise to North Atlantic Deep Water (NADW), which flows

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The role of mixing in the large-scale ocean circulation

Cited by 23 publications

References 226 publications

Sensitivity of a Coarse‐Resolution Global Ocean Model to a Spatially Variable Neutral Diffusivity

Sensitivity of a Coarse‐Resolution Global Ocean Model to a Spatially Variable Neutral Diffusivity

Significance of diapycnal mixing within the Atlantic Meridional Overturning Circulation

Significance of Diapycnal Mixing Within the Atlantic Meridional Overturning Circulation

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