A possible influence of the tropical quasi-biennial oscillation on the variability of the extratropical circulation in the Southern Hemisphere

This study uses geopotential heights and zonal winds from the NCAR/NCEP Reanalysis, and the Naujokat tropical stratospheric zonal wind data set to evaluate the suggestion that the tropical quasi-biennial oscillation (QBO) influenced the observed amplitude increase in zonal harmonic wave 1 in the Southern Hemisphere winter after 1975. Fourier analysis was used to extract zonal wave 1 (ZW1) from the geopotential heights. ZW1 and the zonal winds were stratified by QBO phase and compared to those composited for years before and after 1975. The comparisons confirm that ZW1, during the easterly phase of the QBO, is larger than ZW1 during the westerly phase. The comparisons also suggest that the growth in ZW1 seen after 1975 took place preferentially during winters with an easterly tropical QBO and weak stratospheric winds. The change in ZW1 is associated with surface level changes over the Pacific and simultaneous upper level support by the easterly phase tropical QBO.