2021 Volume 99 Issue 2 Pages 239-267
The history of observational studies regarding the influence of the stratospheric quasi-biennial oscillation (QBO) on the tropical and subtropical upper troposphere and lower stratosphere (UTLS) is reviewed. QBO phases of westerly (QBO W) and easterly (QBO E) winds are defined in the lower stratosphere. During 1960–1978, radiosonde data revealed QBO modulation of the UTLS with a warm anomaly during QBO W in the tropics and cool anomalies near 30°S and 30°N. This pattern agreed with theory of the QBO mean meridional circulation (MMC), which predicted a coherent, antiphased response between the tropics and subtropics. During 1978–1994, satellite observations of aerosol and temperature confirmed the existence of the QBO MMC. During 1994–2001, global data sets enabled analysis of zonal mean QBO variations in tropopause temperature. In 2001, National Centers for Environmental Prediction reanalyses for the 42-yr period 1958–2000 revealed seasonal and geographical variations in QBO W–E tropopause temperature, pressure, and zonal wind, which are presented here. An update using the 38-yr Modern-Era Retrospective Analysis for Research and Applications, Version 2, and the 40-yr European Centre for Medium Range Weather Forecasts Reanalysis (ERA-Interim) data sets provides a more complete view of seasonal and geographical variation.
The QBO range in tropical tropopause values is ∼ 0.5–2 K, ∼ 100–300 m, and ∼ 1–3 hPa, being colder and higher during QBO E, especially during boreal winter and spring. The QBO temperature signal tends to be larger near regions where deep convection is common. The QBO signal in the southern subtropics is enhanced during austral winter. During QBO W, the subtropical westerly jet is enhanced, while the Walker circulation is weaker, especially during boreal spring. A new climatology of the QBO MMC is presented. QBO E may enhance convection by reducing both static stability and wind shear in the UTLS.