Balan Sarojini, B., Gregory, J., Tailleux, R., Bigg, G.R., Blaker, A.T., Cameron, D.R., Edwards, N.R., Megann, A.P., Shaffrey, L. and Sinha, B.
High frequency variability of the Atlantic meridional overturning circulation.
Ocean Science, 7 (4).
To link to this article DOI: 10.5194/os-7-471-2011
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 26N. 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 time-mean 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 26N. Annual volume and heat
transport timeseries at the same latitude are well-correlated within 15--45N, 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 50N, where it is particularly
relevant to European climate, is not well-correlated with that of the AMOC at 26N, where it
is monitored by the RAPID/MOCHA array.
|Date Deposited:||05 Jul 2011 09:53|
|Last Modified:||17 Oct 2013 06:40|
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