The unified internal dynamics and global interactions of the Quasi-Biennial Oscillation

Abstract

Tropical stratospheric dynamics are dominated by the Quasi-Biennial Oscillation (QBO), an alternating, descending pattern of zonal winds with a period averaging 28 months. The QBO is driven by dissipation of vertically propagating atmospheric waves. Although its internal dynamics are driven by waves, the QBO vanishes several kilometers above the wave source, forming a buffer zone. Based on foundational 1D model experiments, it was previously accepted that the buffer zone formed where upwelling opposed the descent of the QBO. This dissertation overturns the upwelling hypothesis by demonstrating that those foundational experiments implicitly prescribed the buffer zone through the model boundary conditions. New 1D model experiments show that the only general way to form the buffer zone is to damp QBO angular momentum towards zero. Angular momentum damping of sufficient magnitude and vertical structure to form the buffer zone is diagnosed in atmospheric reanalyses, and is found to result from horizontal momentum flux divergence, which exports QBO angular momentum anomalies to the far field atmosphere. Exported QBO angular momentum anomalies are known to reach the solid Earth, leading to a correlation between the QBO and O(0.1 millisecond) variations in the length of day.

The upwelling hypothesis has previously been used to attribute decreasing trends in QBO amplitude in the lower stratosphere to global warming-induced increases in residual upwelling. These trends depended on record-low QBO amplitudes in the mid-2000s, from which the QBO has since recovered. An updated analysis reveals no significant trends in QBO amplitude in the lower stratosphere. Rather, decadal variability in QBO amplitude is reinterpreted as large internal variability of QBO amplitude in the buffer zone.

In 2016, the QBO exhibited an unprecedented disruption, during which easterly winds formed unexpectedly around 40 hPa, and wind anomalies stagnated and ascended above 40 hPa. Previously, each feature of the disruption had been explained by a different dynamical mechanism, without accounting for how these proposed mechanisms might be linked through the internal dynamics of the QBO. The 1D model of the QBO is used to demonstrate that all features of the disruption follow from the internal dynamics of the QBO in response to a localized incursion of momentum from the far field.