First-time explicit simulation of a tropical wind system in the upper atmosphere
The weather unfolds in the lowest layer of the atmosphere, but is also strongly influenced by what happens in higher layers. Nevertheless, many climate models do not adequately simulate the phenomena that occur there, leading to considerable uncertainties. One example is the quasi-biennial oscillation (QBO) in the tropical stratosphere, i.e. at an altitude of around 17 to 35 kilometers. The manifestation of this phenomenon is a pattern of changing wind direction that propagates downward through the stratosphere. As a result, the wind sometimes blows strongly from the east and then again from the west. As the dominant wind direction changes from west to east and back to west again every 22 to 34 months, the oscillation is referred to as "quasi-biennial".
Although located in the tropics, the QBO can influence the weather in many parts of the globe. For example, easterly wind phases of the QBO tend to be associated with colder winter months in the eastern US and northern Europe, while westerly wind phases tend to bring milder winters. In addition, the QBO has an influence on the amount and distribution of rainfall in the tropics. The QBO is driven by atmospheric waves that form in the tropical troposphere, i.e. the lower part of the atmosphere near the equator. However, the processes that lead to the formation of these waves, as well as some of the waves themselves, are generally not resolved in climate models. The same holds for the way such waves can drive the QBO once they have reached the tropical stratosphere. The influence of many of these waves has so far only been taken into account in the form of so-called parameterizations, meaning that these processes are only modeled on the basis of simplified physical models, but the underlying physical mechanisms are not explicitly simulated. Different models therefore deliver very different results with regard to the QBO, making the prediction of how the QBO will change in the course of global warming difficult.
First explicit simulation of QBO-like winds
Henning Franke and Marco Giorgetta from the Max Planck Institute for Meteorology (MPI-M) have now succeeded for the first time in directly simulating QBO-like winds over a period of two years. This means that they represented the drivers of the QBO explicitly, i.e. physically, and no longer with the help of parameterizations. This was made possible by using the ICON climate model with a resolution of five kilometers in the horizontal and vertical steps of 350 to 560 meters in the stratosphere. The two scientists showed that this approach works.
"The results are really encouraging," says first author Henning Franke. "Our simulation does not yet reflect a closed cycle of the QBO or all the details. However, as this simulation was the very first of its kind, this was not necessarily expected." Weaknesses in the simulation of the QBO are probably due to weaknesses in the simulation of the tropospheric forcing of the QBO, in particular the lack of organization of tropical deep convection into large-scale atmospheric waves. Conversely, this means that a more accurate representation of deep convection in the troposphere should also improve the simulation of the QBO. This result once again underlines the importance and urgency of the MPI-M's cross-group efforts to better understand deep convection in the tropics and improve its representation in climate models.
Original publication
Franke, H., & Giorgetta, M. A. (2024). Toward the direct simulation of the quasi-biennial oscillation in a global storm- resolving model. Journal of Advances in Modeling Earth Systems, 16, e2024MS004381. https://doi.org/10.1029/ 2024MS004381
Contact
Dr. Henning Franke
Max Planck Institute for Meteorology
henning.franke@mpimet.mpg.de
Dr. Marco Giorgetta
Max Planck Institute for Meteorology
marco.giorgetta@mpimet.mpg.de