Modified Ertel's potential vorticity as a climate variable

Abstract

Fundamental freedom existing in Ertel's general definition of potential voriticity (PV) is used to construct an essential invariant measure in (q, χ) space, where q is an 'optimal' modified PV and χ is a monotonic increasing function of potential temperature, which enters q. This novel measure is preserved in idealized climate processes, when external diabatic heating and frictional forcing, being applied to zonally oriented infinitely thin (q, χ) tubes each other. A reference stationary airmass distribution on (q, χ) values is introduced for both hemispheres, separately, which corresponds to a hypothetical atmospheric climate equilibrium state. This reference distribution, with no dependence on χ, is characterized by exponentially decaying function of the absolute value of q and has the same total amount of 'PV substance' (atmospheric vorticity charge) as the actual atmospheric state. Comparison of the actual monthly mean airmass distribution on q (calculated on the basis of 1980-89 European Centre for Medium-Range Weather Forecasts data) with this reference stationary distribution by their informational entropy values enables one to quantify a degree of nonequilibrium of real atmospheric climate processes, with respect to the introduced invariant measure.