The variability of the horizontal circulation in the troposphere and stratosphere - a comparison

The variability of the horizontal circulation in the stratosphere and tropo sphere of the Northern Hemisphere (NH) is compared by using various approac hes. Spatial degrees of freedom (dof) on different time scales were derived . Modes of variability were computed in geopotential height fields at the t ropospheric and stratospheric pressure levels by applying multivariate stat istical approaches. Features of the spatial and temporal variability of the winterly zonal wind were studied with the help of recurrence and persisten ce analyses. The geopotential height and zonal ly-averaged zonal wind at th e 50-, 500- and 1000-hPa level are used to investigate the behavior of the horizontal circulation in the lower stratosphere, mid-troposphere and at th e near surface level, respectively. It is illustrated that the features of the variability of the horizontal circulation are very similar in the mid-t roposphere and at the near surface level. Due to the filtering of troposphe ric disturbances by the stratospheric and upper tropospheric zonal mean flo w, the variability of the stratospheric circulation exhibits less spatial c omplexity than the circulation at tropospheric pressure levels. There exist enormous differences in the number of degrees of freedom (or free variabil ity modes) between both atmospheric layers. Results of the analyses clearly show that the concept of a zonally symmetric AO with a simple structure in the troposphere similar to the one in the stratosphere is not valid. It is concluded that the spatially filtered climate change signal can be detecte d earlier in the stratosphere than in the midtroposphere or at the near sur face level.