The Generation of Ultralow-Frequency Variations in a Simple Global Model

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Abstract
Fluctuations in the global angular momentum of a two-layer zonally homogeneous global model of the atmosphere are dominated by very low frequencies, periods longer than 100 days. This variability is confined to the tropics, where it is associated with variations of approximately ±5 m s−1 in the zonal winds. Lagged correlations suggest that the variations in angular momentum are a response to variability in the zonally averaged eddy flux of vorticity in the tropics. A zonally symmetric model is constructed to determine the response to an impulsive rearrangement of vorticity across the tropics. The response of the angular momentum in the zonally symmetric model, in convolution with the tropical eddy flux of vorticity derived from the full model, provides a successful model of the variations in angular momentum in the full model. The relevance of the results to James and James' finding ultralow-frequency variability in a five-layer model and to the possibility of ultralow-frequency variability in the atmosphere is discussed.

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