Revisiting the Quasi Biennial Oscillation as Seen in ERA5.Part II: Evaluation of Waves and Wave Forcing

TL;DR

This study uses ERA5 reanalysis data to analyze the spectrum of stratospheric waves and their roles in driving the quasi-biennial oscillation, highlighting the contributions of different wave types.

Contribution

It provides a detailed evaluation of wave contributions to QBO driving using ERA5's higher resolution, emphasizing the roles of Kelvin, gravity, and Rossby waves.

Findings

Resolved waves contribute to both eastward and westward accelerations.

Kelvin and small-scale gravity waves mainly drive eastward acceleration.

Rossby waves influence westward acceleration and QBO irregularities.

Abstract

This paper describes stratospheric waves in ERA5 reanalysis and evaluates the contributions of different types of waves to the driving of the quasi-biennial oscillation (QBO). Because of its higher spatial resolution compared to its predecessors, ERA5 is capable of resolving a broader spectrum of waves. It is shown that the resolved waves contribute to both eastward and westward accelerations near the equator, mainly by the way of the vertical flux of zonal momentum. The eastward accelerations by the resolved waves are mainly due to Kelvin waves and small-scale gravity (SSG) waves with zonal wavelengths smaller than 2000 km, whereas the westward accelerations are forced mainly by SSG waves, with smaller contributions from inertio-gravity and mixed-Rossby-gravity waves. Extratropical Rossby waves propagate into the tropical region and impart a westward acceleration to the zonal flow. They appear to be responsible for at least some of the irregularities in the QBO cycle.