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High frequency variability of the Atlantic meridional overturning circulation

High frequency variability of the Atlantic meridional overturning circulation
High frequency variability of the Atlantic meridional overturning circulation
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, from five-daily to interannual. 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 time-mean is within about 10% of the observational estimate. The amplitude of the simulated annual cycle is similar to observations, but the shape of the annual cycle shows a spread among the models. 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. The timemean of the western boundary current near the latitude of the RAPID/MOCHA array has a much wider model spread than the AMOC does, indicating large differences among models in the simulation of the wind-driven gyre circulation, and its variability is unrealistically small in the models. 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, where it is particularly
relevant to European climate, is not well-correlated with that of the AMOC at 26? N, where it is monitored by the
RAPID/MOCHA array.
1812-0792
471-486
Balan Sarojini, B.
28d3bd2c-0206-42b8-890d-0f36d6995c7a
Gregory, J M.
a38898dc-275d-438f-8e82-0eb7708a6b20
Tailleux, R.
f29f3572-63b7-477f-8ecc-20752027a86e
Bigg, G.R.
c48f5e24-e49d-485c-82ea-f78736789867
Blaker, A.T.
94efe8b2-c744-4e90-87d7-db19ffa41200
Cameron, D.R.
e514368d-0e41-4a42-9c9e-a3d17eae3961
Edwards, N.R.
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Megann, A.P.
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Shaffrey, L.C.
002fe5da-d0a5-4416-81a1-9bfb81dea806
Sinha, B.
544b5a07-3d74-464b-9470-a68c69bd722e
Balan Sarojini, B.
28d3bd2c-0206-42b8-890d-0f36d6995c7a
Gregory, J M.
a38898dc-275d-438f-8e82-0eb7708a6b20
Tailleux, R.
f29f3572-63b7-477f-8ecc-20752027a86e
Bigg, G.R.
c48f5e24-e49d-485c-82ea-f78736789867
Blaker, A.T.
94efe8b2-c744-4e90-87d7-db19ffa41200
Cameron, D.R.
e514368d-0e41-4a42-9c9e-a3d17eae3961
Edwards, N.R.
e41b719b-784e-4748-acc4-6ccbc4643c7d
Megann, A.P.
4fd92e77-5512-487d-bb28-8476afafdeaf
Shaffrey, L.C.
002fe5da-d0a5-4416-81a1-9bfb81dea806
Sinha, B.
544b5a07-3d74-464b-9470-a68c69bd722e

Balan Sarojini, B., Gregory, J M., Tailleux, R., Bigg, G.R., Blaker, A.T., Cameron, D.R., Edwards, N.R., Megann, A.P., Shaffrey, L.C. and Sinha, B. (2011) High frequency variability of the Atlantic meridional overturning circulation. Ocean Science, 7 (4), 471-486. (doi:10.5194/os-7-471-2011).

Record type: Article

Abstract

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, from five-daily to interannual. 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 time-mean is within about 10% of the observational estimate. The amplitude of the simulated annual cycle is similar to observations, but the shape of the annual cycle shows a spread among the models. 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. The timemean of the western boundary current near the latitude of the RAPID/MOCHA array has a much wider model spread than the AMOC does, indicating large differences among models in the simulation of the wind-driven gyre circulation, and its variability is unrealistically small in the models. 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, where it is particularly
relevant to European climate, is not well-correlated with that of the AMOC at 26? N, where it is monitored by the
RAPID/MOCHA array.

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Published date: 2011
Organisations: Marine Systems Modelling

Identifiers

Local EPrints ID: 197397
URI: https://eprints.soton.ac.uk/id/eprint/197397
ISSN: 1812-0792
PURE UUID: ea9bc727-66e0-4560-a784-3da60e56262e

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Date deposited: 22 Sep 2011 14:48
Last modified: 18 Jul 2017 11:20

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Contributors

Author: B. Balan Sarojini
Author: J M. Gregory
Author: R. Tailleux
Author: G.R. Bigg
Author: A.T. Blaker
Author: D.R. Cameron
Author: N.R. Edwards
Author: A.P. Megann
Author: L.C. Shaffrey
Author: B. Sinha

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