Equatorial quasi-biennial oscillation influence on northern winter extratropical circulation
This study investigated the Holton-Tan relationship between the equatorial quasi-biennial oscillation (QBO) and the polar vortex as seen in the ERA-40 reanalysis and ERA-interim analysis (1980–2004, 25 years) data sets and in ensemble simulated data sets (five members covering the period 1980–2004, 125 years) by using the Meteorological Research Institute chemistry climate model (MRI-CCM) and focusing on the Northern Hemisphere winter. The primary tools employed were composite differences in zonal wind, temperature, and Eliassen-Palm (EP) fluxes. Although not many global circulation models can simulate the QBO realistically, the MRI-CCM, which includes the interaction between QBO dynamics and the ozone (hence heating) distribution, reproduces a self-generated QBO that is fairly realistic. In agreement with the finding of previous studies, our results revealed that the conventional Holton-Tan explanation that the equatorial winds in the lower stratosphere act as a waveguide for midlatitude planetary wave propagation cannot explain the winter circulation in either the ERA-40 data or the simulation. Although the composite differences in zonal wind and temperature show a slight yet statistically significant Holton-Tan effect, i.e., the stratospheric polar vortex is weaker and more disturbed under easterly QBO conditions, the EP flux diagnostics do not show more poleward propagation in the midlatitude stratosphere. Rather, planetary waves propagate more equatorward as well as more upward during the easterly phase of the QBO.