Turbulence occurrence in the Tropical Tropopause Layer from superpressure balloon observations: distribution and sources

Abstract. Turbulence characteristics remain poorly documented in the Tropical Tropopause Layer (TTL), although turbulence may play a significant role in the vertical transport of tracers and momentum from the troposphere to the stratosphere. In the framework of the Strateole-2 project, we use pressure, temperature and GPS in situ measurements collected under stratospheric Super-Pressure Balloons (SPBs) during long-duration flights at quasi-constant altitude (~20 km) across the tropical belt. These quasi-Lagrangian observations are used to estimate high-resolution time series (~110 s) of the gradient Richardson number (Ri), allowing us to characterize the flow regime, turbulent or laminar, along the balloon trajectories.

Using a classical instability criterion (Ri<0.25), we estimate a mean turbulent fraction of about 0.18 in the tropical lower stratosphere and investigate its geographical variability. Turbulence occurrence is significantly enhanced in the vicinity of deep convection. Increased kinetic energy of short-period gravity waves is also observed close to convection, strongly suggesting that the breaking of gravity waves generated by convection constitutes an important source of turbulence in the TTL. We also identify turbulence events occurring far from convective regions, particularly over the Pacific Ocean, where they cluster in areas of enhanced vertical wind shear associated with the Quasi-Biennial Oscillation (QBO) amplified by the weakening with altitude of the upper branch of the Walker circulation.

These results highlight the important role of both convectively generated gravity waves and large-scale circulation patterns in controlling turbulence occurrence in the tropical lower stratosphere, with potential implications for tracer transport and mixing across the TTL.