Between 20 and 35 km above sea level, in the lower stratosphere, air masses circulate carrying ozone, volcanic aerosols and atmospheric pollutants. These movements depend on large planetary winds but also on smaller-scale eddies: turbulence. Invisible but powerful, turbulence affects many physical and chemical processes and plays a role in the dynamics of the climate system. However, detailed observations of this turbulence, on a scale of a few metres, remain rare to date.

The stratosphere, which is often stable and calm, is a strategic location for many applications: Earth observation, environmental monitoring (such as forest fires), telecommunications, and even geoengineering techniques.

Unpublished data from weather balloons

To better understand the dynamics of the stratosphere, researchers analysed data from weather balloons launched from four equatorial sites over a period of twenty years. This data, available at a resolution of 10 metres, makes it possible for the first time to explore the fine structures of atmospheric flows in this region of the globe.

Monthly average zonal wind (top graph, red: easterly winds, blue: westerly winds) showing the OQB and the fraction of turbulent atmosphere (bottom) as a function of time and altitude based on weather balloon observations. The solid and dotted lines represent the transitions between easterly and westerly wind phases. The periods of activity at the weather balloon launch sites are indicated by the coloured lines at the bottom. D. R.

Monthly average zonal wind (top graph, red: easterly winds, blue: westerly winds) showing the OQB and the fraction of turbulent atmosphere (bottom) as a function of time and altitude based on weather balloon observations. The solid and dotted lines represent the transitions between easterly and westerly wind phases. The periods of activity at the weather balloon launch sites are indicated by the coloured lines at the bottom.

This long-term work has revealed the extreme variability of turbulence, fluctuating by a factor of ten over a period of approximately two years. This intriguing regularity corresponds to that of the quasi-biennial oscillation, a cycle of wind reversal alternating between westerly and easterly winds in the equatorial stratosphere.

Quasi-biennial oscillation and Kelvin waves: a key link revealed

In-depth data analysis has revealed the origin of this variability. The researchers show that equatorial Kelvin waves, generated by tropical storms and propagating vertically, interact with changes in wind patterns. When they encounter transitions to westerly winds, they break, like a wave on a beach, triggering small-scale turbulence.

This discovery highlights a direct link between large-scale horizontal phenomena (several thousand kilometres) and highly localised vertical effects (a few metres to kilometres), a connection between different aspects of atmospheric dynamics that is rarely observed with such precision.

Prospects for climate and stratospheric technologies

By providing unprecedented observations of equatorial stratospheric turbulence, this research represents a significant step forward in constraining climate models, which still struggle to accurately incorporate the fine dynamics of the stratosphere. It should also fuel prospects for the use of solar-powered stratospheric aircraft.

Weather balloons. D. R.

More

Turbulence in the tropical stratosphere, equatorial Kelvin waves, and the quasi-biennial oscillation. PNAS, 2025.

Contacts
– Rachel Atlas, LMD-IPSL •
– Aurélien Podglajen, LMD-IPSL •
– Richard Wilson, LATMOS-IPSL •

Source: CNRS Terre & Univers.