Physicists have created an experimental device capable of reproducing on a small scale the quasi-biennial oscillation of the equatorial stratosphere. This is a rare example of an experimental study of a climate phenomenon, where mostly numerical simulations are available.
Most periodic weather phenomena have a period related to some astrophysical forcing. For example, the season cycle lasts one year because it corresponds to the duration of a terrestrial revolution around the Sun. However, one phenomenon is famous for its different behavior : the quasi-biennial oscillation (QBO). It consists in an inversion of the direction of the East-West dominant wind in the equatorial stratosphere, which oscillates with a period of about 28 months. The quasi-biennial oscillation affects hurricanes in North America and the winter climate in Europe. It derives its energy from waves in the stratosphere, in particular internal gravity waves (similar to waves in the ocean), which generate a mean flow corresponding to the observed wind.
This atypical atmospheric phenomenon requires special conditions that are difficult to recreate in the laboratory. Two experiments (1978 and 1998) succeeded in qualitatively reproducing the phenomenon, but a quantitative study of the underlying mechanism was missing. While an anomaly has recently been detected in the oscillation (in 2015-2016), it is valuable to add to the climate simulation models the rare experimental studies of climatic phenomena.
The nonlinear physics team of the Laboratory of Statistical Physics of the ENS (LPS, CNRS / Paris ENS / University Paris Diderot / Sorbonne University) has built an experiment consisting in placing a stratified fluid between two concentric cylinders (diameter and height of the order of 0.5 m). The fluid, salty water, has a lower density at the top than at the bottom, a situation similar to that observed in the stratosphere. Internal waves are forced into the liquid, using a system of 16 oscillating membranes in contact with the surface of the water.
The researchers showed that if the amplitude of the motion of the membranes (and thus that of the emitted waves) is sufficient, these waves of short period (15 s) generate a mean flow which displays reversals at a much longer period (around 3 000 s).
The study published in Physical Review Letters is the first to detail the nature of the bifurcation, that is to say the way in which one goes from a state without mean flow (at low amplitude of forcing) to a state where the mean flow is non-zero (high forcing amplitude).
According to the dominant mechanism of wave dissipation, the scenario is different : the transition from a state with a strong average flow to a state without flow can be brutal or continuous.
This study of a model system shows that different scenarii of evolution of the quasi-biennial oscillation are possible ; scenarii that could lead to a better understanding of climate phenomena. This approach would also apply to the study of oscillations comparable to the QBO occurring in the atmosphere of giant planets like Saturn or Jupiter, or even inside the stars.
Figure : Spatio-temporal diagram showing the mean flow of a liquid confined between the two cylinders of the experimental device (height 404 mm), subjected to forcing waves. The color corresponds to the mean flow velocity (in mm / s). We observe that it reverses every 3000 seconds, a much longer period than the one of the forcing waves (15 s). @ 2018 American Physical Society
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Laboratoire de physique statistique de l’ENS (LPS, CNRS/ENS Paris/Univ. Paris Diderot/Sorbonne Université)
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