The QBO Dynamics and Gravity Waves Characteristics as Seen in ERA5 Reanalysis

Abstract

The quasi-biennial oscillation (QBO) is the primary mode of interannual variability in the tropical stratosphere. It is characterized by the downward propagation of successive westerly and easterly wind regimes with an average period of about 28 months. Although the QBO is a tropical stratospheric phenomenon, it influences the circulation and the interannual variability of the global atmosphere and has a discernible effect on the weather and climate in some parts of the troposphere.It is now well-established that the QBO is driven by dissipation of vertically propagating atmospheric waves. However, the simulation of the QBO in weather and climate models has proven to be challenging. In this study, we use ERA5, a state-of-the-art reanalysis dataset, to investigate the dynamics of the QBO, and the characteristics of atmospheric waves driving it. Because of ERA5’s higher spatial resolution compared to its predecessors, it is capable of resolving a broader spectrum of atmospheric waves and allows for a better representation of the wave–mean flow interactions, both of which are of crucial importance for QBO studies. In Chapter 1, I provide a brief description of the QBO and discuss some of the challenges in the QBO science, which motivated this thesis. In Chapter 2, I investigate the dynamics and momentum budget of the QBO using ERA5. The results show that half the required QBO wave forcing is provided by resolved waves during the descent of both westerly and easterly regimes. It is also shown that planetary-scale waves account for most of the resolved wave forcing of the descent of westerly shear zones and small-scale gravity (SSG) waves with wavelengths shorter than 2000 km account for the remainder. SSG waves account for most of the resolved forcing of the descent of the easterly shear zones. It is shown that the resolved wave forcing is approximately twice as strong in ERA5 as in its immediate predecessor, ERA-Interim (ERA-I). In Chapter 3, I further examine the stratospheric waves in ERA5 and evaluate the contributions of different types of waves to the driving of the QBO. I show that the eastward accelerations by the resolved waves are mainly due to Kelvin and SSG waves, whereas the westward accelerations are forced mainly by SSG waves, with smaller contributions from inertio-gravity and mixed Rossby–gravity waves. In Chapter 4, I further investigate the properties of convectively generated gravity waves in the tropical region using ERA5. Based on a novel representation of the two-sided zonal wavenumber-frequency spectrum, I show evidence of gravity wave signatures with phase speeds centered around ±35 m s-1 in a suite of atmospheric fields. The three-dimensional structure of these waves is also documented in composites of the temperature field relative to grid-resolved, wave-induced downwelling events at individual reference grid points along the equator. I show that the waves radiate outward and upward relative to the respective reference grid points, and their amplitude decreases rapidly with time. Within the broad continuum of gravity wave phase speeds there are preferred values around ±49 m s-1 and ±23 m s-1, the former associated with the first baroclinic mode in which the vertical velocity perturbations are of the same sign throughout the depth of the troposphere, and the latter with the second mode in which they are of opposing polarity in the lower and upper troposphere. In Chapter 5, I provide further specifics on how the QBO-related zonal wind profile in the equatorial stratosphere modulates the characteristics of vertically propagating gravity waves. I show that the QBO influences the spectrum and structure of the gravity waves, and as a result, the net upward flux of momentum is westward in the westerly phase of the QBO and eastward in the easterly phase, consistent with theory. In Chapter 6, I provide a summary of the results and discuss their implications.