The equatorial QBO influence on the northern winter extratropical circulation

Based on both 44 years of the ERA-40 reanalysis dataset (1958-2001) and 125 years of ensemble simulated dataset under the REF1 common scenario, the effects of the equatorial quasi-biennial oscillation (QBO) on the extratropical circulation are investigated with statistical methods, focused on the Northern Hemisphere winter. Statistically significant differences are tested by the large sample method. The independence of serial daily data is examined by evaluating an effective sampling time. It is found that daily data are useful especially for effective sampling times being of order of 10 days or less. The model realistically reproduces the equatorial QBO and extratropical influences by the QBO. The simulated QBO power, however, is reduced to about 80 % of the observed one, and the amplitudes of extratropical QBO signals such as zonal winds and temperatures are also underestimated to 70-80%.

We investigate the Holton and Tan relationship and indicate that the planetary waves can propagate more upward as well as more equatorward during the easterly QBO. This result cannot be explained by the conventional hypothesis of the Holton and Tan relationship that planetary waves would be penetrate into the tropics during the westerly QBO whereas during the easterly QBO these waves would encounter the critical line and be refracted poleward, which means that a ‘new' hypothesis is needed in order to explain our presented result.