Frequency Domain Structure and Feedbacks in QBO Time Series

Abstract

QBO is studied here mostly in the frequency domain as a set of six bivariate time series at atmospheric pressure levels from 10 hPa to 70 hPa, or from 31 km to 18 km above sea level. It is found to have an exceptionally stable basic frequency of 0.43 cpy with the spectra which keep their shape through the entire layer. The coherence between the time series at 10 hPa level and the other six time series down to 70 hPa is very high so that the values of coherence spectra are close to values of respective spectra especially at the peak frequency. The gain factors show the strengthening of QBO oscillations from 10 hPa to 15 hPa and 20 hPa, and then, the amplitude slowly decreases and becomes much smaller at 70 hPa. The time delay between the highest and lower levels increases from 0.2 year to 1.1 year, and the rate of downward motion decreases from 1.7 m/h to 1.3 m/h. The feedback between the levels means the existence of upward flows within the QBO system at the monthly time scales.