Turbulence in the tropical stratosphere, Kelvin waves, and the quasi-biennial oscillation

Turbulence in the tropical stratosphere affects the vertical transport of aerosols and gases, with implications for global atmospheric chemistry and the radiative budget of the Earth. However, it is unresolved in global models of the atmosphere, and turbulence parameterizations have not been evaluated within this region. Observational estimates of vertical mixing, including turbulent mixing, in the tropical stratosphere vary widely. We use two decades of high vertical resolution (10 m) radiosonde data from three near-equatorial sites in the tropical West Pacific to quantify the occurrence of stratospheric turbulent layers of at least 200 m thickness, and investigate its temporal and spatial variability, using subcritical Richardson number as a proxy for turbulence. Our estimates of upper tropospheric and lower stratospheric turbulence frequency agree well with published estimates from aircraft data in the same region. We find that stratospheric turbulence typically occurs within downward propagating Kelvin waves, and is most common (3.3% occurrence) right before the quasi biennial oscillation (QBO) phase switches from negative to positive, which coincides with a maximum in Kelvin wave activity. It is least common (0.3% occurrence) during the negative phase of the QBO. Thus, the frequency of tropical stratospheric turbulence varies over a factor of ten depending on the phase of the QBO.