Observed interannual oscillations of planetary wave forcing in the Northern Hemisphere winter

Using NCEP-NCAR reanalysis data from 1958 to 1998, two interannual oscillations in the eddy forcing due to planetary waves (wavenumbers 1-3) are found in the Northern Hemisphere winter, called as the stratospheric interannual oscillation (SIO) and the tropospheric interannual oscillation (TIO). The temporal evolutions are different between the SIO and the TIO, suggesting different dynamic mechanisms. The planetary waves tend to be bent equatorward in the troposphere during a positive TIO phase, and in the mid- and upper stratosphere during a positive SIO phase. The result also suggests that upward propagation of planetary waves into the polar waveguide across the tropopause is closely associated with the TIO. Further regression and correlation analyses suggest that the TIO is closely linked to the Northern Annular Mode (NAM), whereas the SIO is closely associated with the El Nino/Southern Oscillation (ENSO).