On decadal-scale ocean-atmosphere interactions in the extended ECHAM1/LSG climate simulation

The last 810 years of a control integration with the ECHAM1/LSG coupled mod el are used to clarify the nature of the ocean-atmosphere interactions at l ow frequencies in the North Atlantic and the North Pacific. To a first appr oximation, the atmosphere acts as a white noise forcing and the ocean respo nds as a passive integrator. The sea surface temperature (SST) variability primarily results from short time scale fluctuations in surface heat exchan ges and Ekman currents, and the former also damp the SST anomalies after th ey are generated. The thermocline variability is primarily driven by Ekman pumping. Because the heat, momentum, and vorticity fluxes at the sea surfac e are correlated in space and time, the SST variability is directly linked to that in the ocean interior. The SST is also modulated by the wind-driven geostrophic fluctuations, resulting in persistent correlation with the the rmocline changes and a slight low-frequency redness of the SST spectra. The main dynamics are similar in the two oceans, although in the North Pacific the SST variability is more strongly influenced by advection changes and t he oceanic time scales are larger. A maximum covariance analysis based on s ingular value decomposition in lead and lag conditions indicates that some of the main modes of atmospheric variability in the two oceans are sustaine d by a very weak positive feedback between the atmosphere, SST, and the str ength of the subtropical and subpolar gyres. In addition, in the North Atla ntic the main surface pressure mode has a small quasi-oscillatory component at 6-year period, and advective resonance occurs for SST around 10-year pe riod, both periods being also singled out by multichannel singular spectrum analysis. The ocean-atmosphere coupling is however much too weak to redden the tropospheric spectra or create anything more than tiny spectral peaks, so that the atmospheric and oceanic variability is dominated in both ocean sectors by the one-way interactions.