High frequency variability of the Atlantic meridional overturning circulation

Balan Sarojini, B.; Gregory, J.M.; Tailleux, R.; Bigg, G.R.; Blaker, A.T.; Cameron, D.; Edwards, N.R.; Megann, A.P.; Shaffery, L.C.; Sinha, B.. 2011 High frequency variability of the Atlantic meridional overturning circulation. Ocean Science Discussions, 8. 219-246. 10.5194/osd-8-219-2011

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We compare the variability of the Atlantic meridional overturning circulation (AMOC) as simulated by the coupled climate models of the RAPID project, which cover a wide range of resolution and complexity, and observed by the RAPID/MOCHA array at about 26° N. We analyse variability on a range of timescales. In models of all resolutions there is substantial variability on timescales of a few days; in most AOGCMs the amplitude of the variability is of somewhat larger magnitude than that observed by the RAPID array, while the amplitude of the simulated annual cycle is similar to observations. A dynamical decomposition shows that in the models, as in observations, the AMOC is predominantly geostrophic (driven by pressure and sea-level gradients), with both geostrophic and Ekman contributions to variability, the latter being exaggerated and the former underrepresented in models. Other ageostrophic terms, neglected in the observational estimate, are small but not negligible. In many RAPID models and in models of the Coupled Model Intercomparison Project Phase 3 (CMIP3), interannual variability of the maximum of the AMOC wherever it lies, which is a commonly used model index, is similar to interannual variability in the AMOC at 26° N. Annual volume and heat transport timeseries at the same latitude are well-correlated within 15–45° N, indicating the climatic importance of the AMOC. In the RAPID and CMIP3 models, we show that the AMOC is correlated over considerable distances in latitude, but not the whole extent of the North Atlantic; consequently interannual variability of the AMOC at 50° N is not well-correlated with the AMOC at 26° N.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.5194/osd-8-219-2011
Programmes: CEH Topics & Objectives 2009 - 2012 > Water
CEH Sections: Billett (to November 2013)
ISSN: 1812-0806
Additional Information. Not used in RCUK Gateway to Research.: Ocean Science Discussions is an Open Access journal. Please click on the OFFICIAL URL to access full text.
NORA Subject Terms: Earth Sciences
Date made live: 14 Mar 2011 15:44

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