Attribution of variations in the quasi-biennial oscillation period from the duration of easterly and westerly phases

Abstract

This study reports the main features of quasi-biennial oscillation (QBO) period variability at stratospheric levels from 70 to 10 hPa and its attribution from the duration variability of westerly and easterly phases using monthly mean zonal wind data from August 1956 to July 2013, archived by Free University of Berlin. A total of 24 QBO events have been distinguished based on the zonal wind field and wavelet analysis for it. The QBO period varies in phase at various stratospheric levels and shows no significant long-term trend but decadal to multi-decadal variability. The noted case-to-case variations in QBO period are due to variations in durations of the westerly and easterly phases at the same level. The highly coupled variability of the easterly duration in the upper levels above 30 hPa and westerly durations in the lower levels below, which manifests the stalling or accelerating of the descent rate of easterly wind regimes around 30 hPa, is found to be the dominant variability of the easterly and westerly durations at various stratospheric levels. Accordingly, the period of QBO in the lower levels below 40 hPa/upper levels above 20 hPa is determined by the westerly/easterly durations there in about 75 % of the 24 QBO events; and at 30 hPa, variations in the durations of both easterly and westerly phases contribute to the QBO period variability. On the contrary, in only 4 out of 24 QBO events, the variations of the westerly/easterly durations in the upper/lower levels are greater than the variations of the easterly/westerly durations in the upper/lower levels, making deterministic contributions to the QBO period variability.