Scaling of the Atlantic meridional overturning circulation in a global ocean model

Dijkstra, Henk A.

doi:10.1111/j.1600-0870.2008.00326.x

Cited by 13 publications

(18 citation statements)

References 26 publications

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“…In this work, we extend recent process‐oriented studies (Dalan et al, 2005; Dijkstra, 2008) that show that the scaling of the strength of the meridional overturning circulation (MOC) with vertical (or diapycnal) diffusivity ( K V ) is different between the Pacific and Atlantic Oceans. The question of the circulation's sensitivity to K V is rooted in the now long standing thermocline problem (Welander, 1959; Robinson and Stommel, 1959; Welander, 1971).…”

Section: Introductionsupporting

confidence: 51%

“…The power law exponent for the Pacific overturning is thus quite close to the classical value of 2/3, but the exponent for Ψ A is clearly smaller. More recently, Dijkstra (2008) analysed the scaling behaviour of the MOC in a fully implicit global ocean‐only model. Using restoring conditions for salinity and a simple energy balance atmospheric model to control upper ocean temperatures, he found that Ψ A scales as K 1/3 V , whereas Ψ P scales as K 2/3 V .…”

Section: Introductionmentioning

confidence: 99%

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Scaling of the strength of the meridional overturning with vertical diffusivity in an idealized global geometry

Toom

Dijkstra

2011

Tellus A: Dynamic Meteorology and Oceanography

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An important expression of the non‐linear character of the ocean's meridional overturning circulation (MOC) is the scaling of its amplitude with the magnitude of the vertical mixing coefficient (diffusivity) of heat and salt. This paper extends recent work that indicated that the Atlantic and Pacific MOC exhibit different scaling behaviour. An idealized two‐basin model configuration is used to study the meridional overturning circulation under restoring boundary conditions. In particular, the effects of wind forcing and the choice of the parametrization of lateral mixing are examined. Without wind, the scaling is similar in the two basins and consistent with theoretical predictions, provided that the diffusivity is small enough. Towards higher diffusivities the scaling of the overturning diverges. With non‐zero wind, the sensitivity is strongly determined by the choice of the lateral mixing scheme. In case of traditional horizontal diffusion, the scaling behaviour is asymmetric due to spurious diapycnal mixing. With the Gent‐McWilliams parametrization, the scaling is symmetric for the lower range of diffusivities, where results agree partly with theoretical scaling relations. In all cases considered, the pycnocline depth has the same sensitivity to diffusivity in both basins, implying that there is no universally valid relation between overturning strength and pycnocline depth.

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Section: Introductionsupporting

confidence: 51%

Section: Introductionmentioning

confidence: 99%

Scaling of the strength of the meridional overturning with vertical diffusivity in an idealized global geometry

Toom

Dijkstra

2011

Tellus A: Dynamic Meteorology and Oceanography

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“…The zonal overturning stream function shows a maximum value of > 4.5 Sv in the basin center at ∼300 m depth. We can compute the formation rate of the intermediate water from the zonal stream function (following Dijkstra [2008], Ezer and Mellor [1997], and Rahmstorf and England [1997]), by considering the volume of the downwelling water east of 25°E. We find that the model predicts a formation rate of ∼2.5 Sv (Figure 7c).…”

Section: General Circulationmentioning

confidence: 96%

Sensitivity of Mediterranean thermohaline circulation to gateway depth: A model investigation

2010

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[1] The Neogene sedimentary record of the Mediterranean Sea holds evidence for changes in water properties and circulation. These paleoceanographic changes have been attributed to changes in the flow through the ocean gateway between the Mediterranean Sea and the Atlantic Ocean. We use an oceanic general circulation model to achieve quantitative, physics-based understanding of the effect of changes in the depth of the gateway such as may result from tectonic activity or variation in sea level. To isolate these effects we use idealized basin geometry and impose simplified atmospheric forcing. A reference experiment with present-day sill depth demonstrates that the model simulates reasonably well the main features of the present-day Mediterranean thermohaline circulation as inferred from observations and previous numerical studies. Subsequently, a series of sensitivity simulations is performed with different sill depths. The model results show that Mediterranean temperature, salinity, and thermohaline circulation depend strongly on sill depth. As the sill depth decreases, the upper overturning cell is quasi-linearly reduced in strength, while, contrary to what one would expect, the deep cell intensifies and does so in a nonlinear way. We find that a shoaling of the sill depth induces a "blocking effect" on the outflow waters, which creates a strong recirculation in the deep layers, strengthening the deep cell. Nevertheless, deep-water formation is reduced, and, as a consequence, the ventilation of the deep layers diminishes. We identify three different circulation modes of the Mediterranean thermohaline circulation depending on the sill depth: "shallow sill," "moderate sill," and "deep sill" modes coupled with strong, weak, and negligible blocking effects, respectively. Our results are consistent with the pre-Messinian paleoceanographic record of the Mediterranean Sea and may be useful to understanding the behavior of other land-locked basins, both extant and from the geological past.Citation: Alhammoud, B., P. T. Meijer, and H. A. Dijkstra (2010), Sensitivity of Mediterranean thermohaline circulation to gateway depth: A model investigation, Paleoceanography, 25, PA2220,

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“…Forest et al , ; Urban and Keller, ; Olson et al , ), including AMOC strength projections (cf. Dijkstra, ). The results shown in Figure demonstrate that the model hindcasts and projections of AMOC strength are strongly affected by the K v parameter.…”

Section: Resultsmentioning

confidence: 99%

“…Reducing the uncertainty of K v may also result in better predictions of AMOC strength (Dijkstra, ) as well as other climate predictions (Schmittner et al , ). Our main objective is to characterize and (if possible) reduce uncertainty about K v .…”

Section: Introductionmentioning

confidence: 99%

Inferring likelihoods and climate system characteristics from climate models and multiple tracers

et al. 2012

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An important potential outcome of anthropogenic climate change is a possible collapse of the Atlantic meridional overturning circulation (AMOC). Assessing the risk of an AMOC collapse is of considerable interest since it may result in major temperature and precipitation changes and a shift in terrestrial ecosystems. One key source of uncertainty in AMOC predictions is uncertainty about background ocean vertical diffusivity (Kv), a key model parameter. Kv cannot be directly observed but can be inferred by combining climate model output with observations on the oceans (so called "tracers"). In this work, we combine information from multiple tracers, each observed on a spatial grid. Our two stage approach emulates the computationally expensive climate model using a flexible hierarchical model to connect the tracers. We then infer Kv using our emulator and the observations via a Bayesian approach, accounting for observation error and model discrepancy. We utilize kernel mixing and matrix identities in our Gaussian process model to considerably reduce the computational burdens imposed by the large data sets. We find that our approach is flexible, reduces identifiability issues, and enables inference about Kv based on large data sets. We use the resulting inference about Kv to improve probabilistic projections of the AMOC.

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Scaling of the Atlantic meridional overturning circulation in a global ocean model

Cited by 13 publications

References 26 publications

Scaling of the strength of the meridional overturning with vertical diffusivity in an idealized global geometry

Scaling of the strength of the meridional overturning with vertical diffusivity in an idealized global geometry

Sensitivity of Mediterranean thermohaline circulation to gateway depth: A model investigation

Inferring likelihoods and climate system characteristics from climate models and multiple tracers

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