The role of ocean dynamics for low-frequency fluctuations of the NAO in a coupled ocean-atmosphere GCM

• Main Authors: Christoph, M., Ulbrich, U., Oberhuber, J., Roeckner, E.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2000
• Subjects: North Atlantic; North Atlantic oscillation
• Online Access: http://hdl.handle.net/21.11116/0000-0005-8392-1; http://hdl.handle.net/21.11116/0000-0005-8394-F; http://hdl.handle.net/21.11116/0000-0005-8395-E

Variability at all timescales, including low-frequency variability, is found in the North Atlantic sector in a 300-yr control integration of the coupled ocean-atmosphere general circulation model (CGCM) ECHAM4/OPYC3. The atmospheric variability is dominated by the North Atlantic Oscillation (NAO). Only rather weak spectral peaks are superimposed on the "white noise" power spectrum of sea level pressure and on the essentially "red noise" spectrum of SST in highly active regions. Replacing the full ocean model with a 50-m fixed-depth mixed layer ocean (MLO) and coupling it to the atmosphere yields qualitatively and quantitatively very similar power spectra of the NAO index. Net surface heat fluxes, describing the coupling between the ocean and the atmosphere For the long-term variations (>10 yr) of the NAO are much weaker in the MLO model, but show general agreement in both simulations regarding spatial distributions. This spatial agreement with respect to NAO variability occurs even though the associated SST anomaly pattern in the CGCM is shifted northward by about 10 degrees relative to its position in the run without the dynamical ocean. This fact is mainly attributed to advection in the full ocean model. There is evidence for the existence of ocean-cryosphere-atmosphere coupling in the CGCM. From the fact that we found only weak spectral peaks it appears that the role of a fully coupled ocean with respect to long-term NAO variability is limited to a shift in SST variability and to a moderate increase of the atmosphere's long-term variability over most parr of the domain. In view of the subordinate relevance of ocean-atmosphere coupling for the NAO it is suggested that the CGCM presented in this study mainly follows the stochastic climate model concept, that is, the ocean integrates over the chaotic forcing imposed by the atmosphere, leaving the NAO rather unpredictable on decadal and longer timescales.