Reconstructing extreme AMOC events through nudging of the ocean surface: a perfect model approach

Sébastien Nguyen,Pablo Ortega,Eric Guilyardi,Didier Swingedouw,Juliette Mignot

doi:10.1007/s00382-017-3521-4

Abstract

While the Atlantic Meridional Overturning Circulation (AMOC) is thought to be a crucial component of the North Atlantic climate, past changes in its strength are challenging to quantify, and only limited information is available. In this study, we use a perfect model approach with the IPSL-CM5A-LR model to assess the performance of several surface nudging techniques in reconstructing the variability of the AMOC. Special attention is given to the reproducibility of an extreme positive AMOC peak from a preindustrial control simulation. Nudging includes standard relaxation techniques towards the sea surface temperature and salinity anomalies of this target control simulation, and/or the prescription of the wind-stress fields. Surface nudging approaches using standard fixed restoring terms succeed in reproducing most of the target AMOC variability, including the timing of the extreme event, but systematically underestimate its amplitude. A detailed analysis of the AMOC variability mechanisms reveals that the underestimation of the extreme AMOC maximum comes from a deficit in the formation of the dense water masses in the main convection region, located south of Iceland in the model. This issue is largely corrected after introducing a novel surface nudging approach, which uses a varying restoring coefficient that is proportional to the simulated mixed layer depth, which, in essence, keeps the restoring time scale constant. This new technique substantially improves water mass transformation in the regions of convection, and in particular, the formation of the densest waters, which are key for the representation of the AMOC extreme. It is therefore a promising strategy that may help to better constrain the AMOC variability and other ocean features in the models. As this restoring technique only uses surface data, for which better and longer observations are available, it opens up opportunities for improved reconstructions of the AMOC over the last few decades.

Highlights

The ocean circulation, and in particular the Atlantic Meridional Overturning Circulation (AMOC), is an important driver of climate variability in the North Atlantic (Delworth et al 1993; McCarthy et al 2015) that can modulate regionally the global warming signal
Using a perfect model approach, this study explores the ability of different reconstruction techniques to reproduce the simulated AMOC variability in a target period of a preindustrial control simulation, selected to include an extreme positive AMOC event
We investigate the conditions leading to the exceptional dense water formation in the region of convection 5 years before the AMOC peak

Summary

Introduction

The ocean circulation, and in particular the Atlantic Meridional Overturning Circulation (AMOC), is an important driver of climate variability in the North Atlantic (Delworth et al 1993; McCarthy et al 2015) that can modulate regionally the global warming signal. Rapid and large decadal fluctuations in the AMOC strength emerge frequently in models as a result of internal climate variability, showing important widespread climate impacts (Allison et al 2014). Most of these results refer to the model world. A complementary altimetry-based reconstruction of the AMOC along the OVIDE section (from Greenland to Portugal) shows a decline in its strength between 1993 and 2010 (Mercier et al 2015) Both records are still too short to determine if this decadal trend emerges in response to global warming, or if it is compatible with natural ocean fluctuations

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