Vertical structure and downward propagation mechanism in the seasonal cycle of stratospheric variability in the northern hemispheric polar region

The seasonal cycle of stratospheric variability during 150 days from November 17 through April 15 was extracted via cyclostationary EOF (CSEOF) analysis from the 20-year (1989-2009) ECMWF reanalysis and the 58-year (1950-2009) NCEP/NCAR reanalysis data. Using the 10-hPa air temperature as a target variable, the 4-dimensional structures of the seasonal cycle were derived based on air temperatures, geopotential heights, zonal winds, vertical velocity, and potential vorticity from 200 hPa to 10 hPa for the entire zonal band from 30°N-80°N. The seasonal cycle explains ~30% of the total variability of the target variable and exhibits a strong interannual variation in its strength. Due to the strong variability, the seasonal cycle contributes to a significant amount of wintertime variability each year.

The seasonal cycle depicts wintertime cooling in early winter replaced by warming in early spring. Vertical advection is clearly seen transmitting upper stratospheric cooling or warming downward. Physical relationships among different variables were derived. Based on these physical relationships, vertical structures of variability of various physical variables were derived and compared. The time scale and speed of vertical advection were also investigated as a function of pressure.