Low-frequency variability of the atmospheric circulation: a comparison of statistical properties in both hemispheres and extreme seasons

Abstract

A statistical investigation is presented of the main variables characterizing the tropospheric general circulation in both hemispheres and extreme seasons, Winter and Summer. This gives us the opportunity of comparing four distinct realizations of the planetary circulation, as a function of different orographic and thermal forcing conditions. Our approach is made possible by the availability of 6 years of global daily analyses prepared by ECMWF (European Centre for Medium-range Weather Forecast). The variables taken into account are the zonal geostrophic wind, the zonal thermal wind and various large-scale wave components, averaged over the tropospheric depth between 1000 and 200 hPa. The mean properties of the analysed quantities in each hemisphere and season are compared and their principal characteristics are discussed. The probability density estimates for the same variables, filtered in order to eliminate the seasonal cycle and the high-frequency «noise», are then presented. The distributions are examined, in particular, with respect of their unimodal or multimodal nature and with reference to the recent discussions in the literature on the bimodality which has been found for some indicators of planetary wave activity in the Northern Hemisphere Winter. Our results indicate the presence of nonunimodally distributed wave and zonal flow components in both hemispheres and extreme seasons. The most frequent occurrence of nonunimodal behaviour is found for those wave components which exhibit an almost vanishing zonal phase speed and a larger «response» to orographic forcing.