Decadal Amplitude Modulations of the Stratospheric Quasi-biennial Oscillation

Abstract

 Decadal variations of the quasi-biennial oscillation (QBO) in the equatorial stratosphere are investigated, using the Singapore data and reanalysis data from 1950s to 2019/2020. It is found that the QBO is decadally modulated in the amplitude as well as in the period. These two decadal variations are positively correlated with each other after 1980s, while they show approximately negative correlation before 1980s. In the time series of the QBO amplitude from 1950s to 2014, there are four maxima (QBO_max) around 1967, 1983, 1995, and 2005, and three minima (QBO_min) around 1973, 1988, and 2000. Composite analyses of QBO_max and QBO_min based on these extrema reveal that the decadal amplitude variations have maximum amplitude of about 3 m s⁻¹ at 20 hPa in the vertical. In the horizontal structure there appear off-equator extrema of about 3.5 m s⁻¹ around 5°N at 20 hPa, while at 50 hPa extrema of about 1.8 m s⁻¹ are situated around 5°S. The decadal amplitude variations of the QBO are closely and positively correlated with the decadal components of Niño 3.4 sea surface temperature anomalies (SSTa) and Pacific decadal oscillation (PDO) index, suggesting that the tropical SSTa in the central Pacific substantially influences the QBO in the decadal time-scales.