An oscillating wind system in the tropical stratosphere: future evolution and new projections
The quasi-biennial oscillation: Why we should care about winds 17 to 40 km above our heads
The quasi-biennial oscillation (QBO) is a wind system located in the equatorial stratosphere (locating between 17 km and 40 km). In this altitude range, westerly and easterly jets alternately propagate down with time, so that at a given altitude the winds switch between westerlies and easterlies roughly every other year. Despite being high up, the QBO winds have been shown to impact weather we experience at the surface, both in the tropics and midlatitudes. Due to this surface impact and given its comparatively long timescale, the QBO constitutes an important source of long-term predictability on timescales from a few weeks up to three years.
The uncertain future of the QBO
Estimates of how the QBO may change under global warming have not been robust, as the types of models used to predict them rely on poorly represented processes to represent the basic physics of the QBO wind system. This is caused by the fact that an important driving mechanism of the QBO, small-scale atmospheric waves due to gravity, ‘gravity waves’, are not resolved in these models. Instead, it is attempted to represent their effects empirically, through parameterizations. The inherent uncertainty associated with the parameterization of gravity waves feeds back into the QBO projections and causes huge uncertainty there. As a consequence, projections of the QBO vary greatly from model to model.
Using global storm-resolving simulations with ICON to shed light into this uncertainty
To overcome this uncertainty in QBO projections, Franke and his co-authors used an obvious yet novel strategy, which was to ‘simulate’ the QBO directly: Simply do not use an uncertain parameterization of gravity waves anymore, but instead increase the model resolution until the waves become explicitly resolved, and hence simulated. Additionally, this model resolution also explicitly resolves deep tropical convection which generates the gravity waves, which is also a new and more physical approach to representing the QBO. Using the ICON model in this “storm-resolving” setup, the authors performed a series of short simulations of idealized warming scenarios. These simulations showed that the momentum flux associated with the gravity waves that enter the stratosphere increase substantially in the warmer climate states. As a consequence, the downward propagation of the QBO wind jets speeds up. Additionally, the QBO wind jets get stronger with warming, something the authors explain by a warming-induced increase of the speed at which the gravity waves propagate horizontally. Ultimately, the authors argue, both the increase in gravity wave momentum flux as well as the increase in gravity wave propagation speed can be attributed to changes in the tropical convection generating the waves.
Towards more accurate QBO projections
The results presented by Franke et al. thus provide a clear target for modelers interested in improving parameterizations, to be used in cheaper and more coarsely resolved models. An open question that their study does not resolve, is how other, statistical aspects of the QBO may respond to warming. This is due to the fact that the QBO is also forced by other processes than gravity waves which act on much longer timescales. The experiments of the present study are too short to reliably represent potential changes in these slow forcing processes of the QBO. Therefore, the researches plan to address this problem soon by performing much longer simulations of a full QBO cycle in different climate states.
Franke, H., Preusse, P. & Giorgetta, M. (2023) Changes of tropical gravity waves and the quasi-biennial oscillation in storm-resolving simulations of idealized global warming.Quarterly Journal of the Royal Meteorological Society, 1–23. Available from: https://doi.org/10.1002/qj.4534
Max Planck Institute for Meteorology