Interannual variations of stationary planetary wave activity in the northern winter troposphere and stratosphere and their relations to NAM and SST

With the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis data from 1958 to 1998, two teleconnection patterns in the Eliassen-Palm (EP) flux divergence field due to stationary planetary waves with wavenumbers (WNs) 1 to 3 are studied in the Northern Hemisphere winter. Both patterns appear as dipole structures with divergence of EP flux in the north and convergence in the south, or vice versa. One dipole evolves mainly in the stratosphere and is referred to as the stratospheric interannual oscillation (SIO). The other evolves mostly in the troposphere and is referred to as the tropospheric interannual oscillation (TIO). The TIO and the SIO are shown to be associated with anomalous meridional planetary wave propagation in the troposphere and stratosphere, respectively. The upward and poleward refraction of planetary waves into the polar waveguide across the tropopause is closely linked to the TIO. Examination of individual waves indicates that the tropospheric dipole of EP flux divergence comes basically from WN 3, whereas the stratospheric one comes totally from WNs 1 and 2. The reduced refraction of planetary waves into the polar waveguide across the tropopause associated with positive TIO index is dominantly contributed from both WNs 1 and 2. Regression and correlation analyses suggest that the TIO is closely related to the Northern Annular Mode (NAM). Its relation to the sea surface temperature (SST) is consistent with previous studies on the relation of NAM to SST. On the other hand, the SIO is shown to be closely associated with anomalous zonal mean winds in the subtropics of the midstratosphere and has a moderate correlation with an augmented Pacific-North American (PNA) pattern in 500 hPa geopotential height. For the leading EOF of stratospheric (u) over bar variability the SIO accounts for a smaller fraction than the TIO. However, with the teleconnectivity in stratospheric EP flux divergence one can distinguish a stratospheric variation of anomalous north-south planetary wave propagation from the total amount of wave activity entering the stratosphere, which may help us to understand the variability of transport circulation in the stratosphere. Significant correlation of SIO with tropical SST reveals that tropical SST is leading the SIO by up to around 9 months, which suggests a strong impact of El Nino/Southern Oscillation (ENSO) on the SIO. Planetary waves tend to be bent poleward in the midstratosphere when there is a warm event. The situation tends to be reverse during a cold event.