Climate variability in millennium integrations with coupled atmosphere-ocean GCMs: a spectral view

Abstract

Climate variations in four millennium integrations obtained with coupled GCMs are studied from a spectral point of view. It is shown that the bulk of these variations can be described by two distinctly different types of spectra. The type-I spectra, characterized by a high-frequency ω−2 slope (with ω being frequency) and a low-frequency plateau, indicate the dominance of short-term fluctuations in generating climate variations. They are obtained for many atmospheric variables and variables representing predominantly the upper ocean and the high-latitude part of the deep ocean. The time scale, at which the spectra level off, varies from a few days for grid-point time series of atmospheric variables, to a few months for time series of large-scale atmospheric patterns, several years for SST anomalies in the tropical Pacific, and a few decades for variables describing oceanic baroclinic waves. The type-II spectra are obtained in the ocean interior, which is shielded from the fluctuating forcing at the surface. Since the ocean model does not produce oceanic eddies, the disappearance of type-I spectra in the deep ocean indicates that the fluctuating surface forcing does not fully penetrate into the deep ocean. While type-I spectra are supported by observations, type-II spectra might describe a model specific phenomenon and the realism of these spectra is still a open question.