Forced and Free Variations of Weather and Climate

Edward N. Lorenz Department of Meteorology, Massachusetts Institute of Technology, Cambridge 02139

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Abstract

Variations of weather and climate are termed “forced” or “free” according to whether or not they are produced by variations in external conditions.

In many simple climate models, the poleward transport of sensible heat in the atmosphere has been treated as a diffusive process, and has been assumed to be proportional to the poleward temperature gradient. The validity of this assumption, for various space and time scales, is tested with 10 years of twice-daily upper level weather data. The space scales are defined by a spherical harmonic analysis, while the time scales are defined by a “poor man's spectral analysis.” The diffusive assumption is verified for the long-term average and the seasonal variations of the largest space scale, but it fails to hold for most of the remaining scales.

It is shown that diffusive behavior can be expected only for forced scales. It is suggested that most of the scales resolved by the data are free.

Abstract

Variations of weather and climate are termed “forced” or “free” according to whether or not they are produced by variations in external conditions.

In many simple climate models, the poleward transport of sensible heat in the atmosphere has been treated as a diffusive process, and has been assumed to be proportional to the poleward temperature gradient. The validity of this assumption, for various space and time scales, is tested with 10 years of twice-daily upper level weather data. The space scales are defined by a spherical harmonic analysis, while the time scales are defined by a “poor man's spectral analysis.” The diffusive assumption is verified for the long-term average and the seasonal variations of the largest space scale, but it fails to hold for most of the remaining scales.

It is shown that diffusive behavior can be expected only for forced scales. It is suggested that most of the scales resolved by the data are free.

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