Vertical helicity flux as an index of interannual atmospheric variability

Abstract

As an index of interannual climatic variability of the atmosphere, it is proposed to calculate the weighted average over the hemisphere area (to the pole from latitude 20°) of the vertical helicity flux across the upper boundary of the planetary boundary layer, which is determined by the product of the Coriolis parameter and the square of the wind speed at this boundary. In practical calculations, the data of reanalyses ERA-Interim and ERA-20C on the wind speed at the isobaric level of 850 hPa were used. The statistical distribution of the Earth’s surface area on the helicity flux values, as well as the informational entropy of this distribution, are calculated. It is shown that the introduced index usefully characterizes the interannual climate variability of the atmosphere in both hemispheres.