Interdecadal variability in a global coupled model

Abstract

Interdecadal variations are studied in a 325-year simulation performed by a coupled atmosphere, ocean general circulation model. The patterns obtained in this study describe some of the dominant features of interdecadal variations in the model atmosphere and in the model Pacific Ocean. (1) Atmosphere. Interdecadal variations have no preferred time scale and can be generated by an AR(1)-process which has a longer characteristic time than that of the weather fluctuations. There are two dominant modes, one is related to variations of the tropical easterlies and the other to the Southern Hemisphere westerlies. The amplitude of the associated wind anomalies is largest in the upper troposphere. The associated temperature anomalies are in thermal-wind balance with the zonal winds and are out of phase in the troposphere and in the lower stratosphere. (2) Pacific Ocean. The dominant mode in the Pacific is basin-wide with a time scale of 10 to 20 years. Its signature propagates westward in the tropics and then northward (southwestward) in the North (South) Pacific. The evolution is related to changes in the upper ocean circulation and the upper ocean thermal structure. In the midlatitudes, the current changes are in Sverdrup relation with the wind changes whereas in the tropics air—sea interaction processes might be involved during a certain phase of the oscillation.