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Ben Marzeion, Anders Levermann, and Juliette Mignot

Abstract

Using the “CLIMBER-3α” coupled climate model of intermediate complexity, the effect of a stratification-dependent vertical diffusivity on the sensitivity of the Atlantic Ocean meridional overturning circulation to perturbations in freshwater forcing is investigated. The vertical diffusivity κ is calculated as κN α, where N is the local buoyancy frequency and the parameter α is a measure of the sensitivity of the vertical diffusivity to changes in stratification. Independent of α, the stratification of the deep ocean is weakly increased as a response to an anomalous freshwater flux in the North Atlantic in these experiments. In the region of freshwater forcing and north of it this is caused by the freshwater anomaly itself, but farther south it is associated with anomalously warm surface waters caused by a reduction of the northward oceanic heat transport. Subsequently, and in opposition to results from previous studies, the overturning is reduced by the anomalous freshwater flux, independent of the choice of α. However, the amount of reduction in overturning following a freshwater perturbation is found to depend critically on the choice of the mixing sensitivity α. If α < α cr, the response is similar to the model’s response using constant vertical diffusivity (α = 0). For α > α cr, a sharp increase of the sensitivity is found. The value of α cr is found to be between 0.5 and 1. A general feedback is proposed explaining this threshold behavior: if α is large, both positive and negative perturbations of stratification are amplified by associated changes in diffusivity. In the experiments presented here, this enhances the initial positive stratification anomaly in northern high latitudes, which is created by the anomalous freshwater flux. As a result, convection is strongly reduced, and the overturning is significantly weakened.

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Juliette Mignot, Anders Levermann, and Alexa Griesel

Abstract

The sensitivity of the Atlantic Ocean meridional overturning circulation to the vertical diffusion coefficient κ in the global coupled atmosphere–ocean–sea ice model CLIMBER-3α is investigated. An important feature of the three-dimensional ocean model is its low-diffusive tracer advection scheme. The strength M max of the Atlantic overturning is decomposed into three components: 1) the flow MS exported southward at 30°S, 2) the large-scale upward flow that balances vertical diffusion in the Atlantic, and 3) a wind-dependent upwelling flux W bound along the Atlantic boundaries that is not due to vertical diffusion. The export of water at 30°S varies only weakly with κ, but is strongly correlated with the strength of the overflow over the Greenland–Scotland ridge. The location of deep convection is found to be mixing dependent such that a shift from the Nordic seas to the Irminger Sea is detected for high values of κ. The ratio R = MS/M max gives a measure of the interhemispheric overturning efficiency and is found to decrease linearly with κ. The diffusion-induced upwelling in the Atlantic is mostly due to the uniform background value of κ while parameterization of enhanced mixing over rough topography and in stratified areas gives only a weak contribution to the overturning strength. It increases linearly with κ. This is consistent with the classic 2/3 scaling law only when taking the linear variation of the density difference to κ into account. The value of W bound is roughly constant with κ but depends linearly on the wind stress strength in the North Atlantic. The pycnocline depth is not sensitive to changes in κ in the model used herein, and the results suggest that it is primarily set by the forcing of the Southern Ocean winds. The scaling of the total overturning strength with κ depends on the combined sensitivity of each of the terms to κ. In the range of background diffusivity values in which no switch in deep convection sites is detected, M max scales linearly with the vertical diffusivity. It is argued that scalings have, in general, to be interpreted with care because of the generally very small range of κ but also because of possible shifts in important physical processes such as deep convection location.

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