Turbulence in the Tropical Stratosphere, Equatorial Kelvin Waves, and the Quasi-Biennial Oscillation

The tropical stratosphere is the gateway to the global stratosphere. The transport of air in this region is driven by waves, generated primarily by deep convection, and turbulent motions, superimposed on a background of large-scale ascent. The dynamics of this remote and difficult to observe region are poorly understood, particularly at turbulent length scales. Existing observational estimates of turbulence frequency and strength vary widely. Furthermore, the sources of turbulence and the relationship between turbulence and the mean flow are largely unknown. We assembled a 21-year database of high vertical resolution (10 m) radiosonde data from four equatorial sites in two ocean basins to study tropical stratospheric turbulence frequency, variability and sources. Turbulent layers thicker than 200 m are identified using subcritical Richardson number as a proxy for turbulence. We show that the turbulent fraction of the tropical stratosphere is strongly modulated by the Quasi-Biennial Oscillation (QBO). Turbulence is enhanced during the QBO phase shifts, and the atmosphere is most turbulent right before the QBO phase switches from negative to positive, where turbulent instabilities typically occur within specific phases of convectively generated Kelvin waves. Turbulence is less common when the QBO phase is well-established, and the atmosphere is least turbulent during the negative phase of the QBO. The turbulent fraction of the equatorial lower stratosphere varies over a factor of nine depending on QBO phase. During active Madden-Julian Oscillation (MJO) events, this dependence is even stronger and the turbulent fraction instead varies over a factor of 12. This is because there is enhanced Kelvin wave energy in the lower stratosphere during the negative to positive QBO phase shift when there is an active MJO and this leads to a 25% increase in the occurrence of turbulence there. The relationship between QBO phase and turbulence fraction provides a robust observational constraint on the multi-scale dynamics within this region, which is useful for evaluating atmospheric models, studying wave-mean flow interactions in the context of the QBO, and understanding the interactions between the QBO and tropical tropospheric modes of variability such as the MJO.