MODES OF NORTH-ATLANTIC DECADAL VARIABILITY IN THE ECHAM1 LSG COUPLEDOCEAN-ATMOSPHERE GENERAL-CIRCULATION MODEL/

The climate variability in the North Atlantic sector is investigated i n a 325-yr integration of the ECHAM1/LSG coupled ocean-atmosphere gene ral circulation model. At the interannual timescale, the coupled model behaves realistically and sea surface temperature (SST) anomalies ari se as a response of the oceanic surface layer to the stochastic forcin g by the atmosphere, with the heat exchanges both generating and dampi ng the SST anomalies. Ln the ocean interior, the temperature spectra a re red up to a period of about 20 years, and substantial decadal fluct uations are found in the upper kilometer or so of the water column. Us ing extended empirical orthogonal function analysis, two distinct quas i-oscillatory modes of ocean-atmosphere variability are identified, wi th dominant periods of about 20 and 10 years, respectively. The oceani c changes in both modes reflect the direct forcing by the atmosphere t hrough anomalous air-sea fluxes and Ekman pumping, which after some de lay affects the intensity of the subtropical and subpolar gyres. The S ST is also strongly modulated by the gyre currents. In the thermocline , the temperature and salinity fluctuations are in phase, as if caused by thermocline displacements, and they have no apparent connection wi th the thermohaline circulation. The 20-yr mode is the most energetic one; it is easily seen in the thermocline and can be found in SST data , but it is not detected in the atmosphere alone. As there is no evide nce of positive ocean-atmosphere feedback, the 20-yr mode primarily re flects the passive response of the ocean to atmospheric fluctuations, which may be in part associated with climate anomalies appearing a few years earlier in the North Pacific. The IO-yr mode is more surface tr apped in the ocean. Although the mode is most easily seen in the tempe rature variations of the upper few hundred meters of the ocean, it is also detected in the atmosphere alone and thus appears to be a coupled ocean-atmosphere mode. In both modes, the surface heat flux acts neut rally on the associated SST anomalies once they have been generated, s o that their persistence appears to be due in part to an overall adjus tment of the air-sea heat exchanges to the SST patterns.