The quasi-biennial oscillation in a changing climate (QUBICC)

Project partners

  • Marco Giorgetta,, +49 40 411 73-358
    Max Planck Institute for Meteorology (MPI-M), Bundesstr. 53, 20144 Hamburg
  • Ulrich Achatz,, +49 69 79840243
    Johann Wolfgang Goethe-Universität Frankfurt/Main (GUF), Institut für Atmosphäre und Umwelt, Altenhöferallee 1, 60438 Frankfurt/Main
  • Manfred Ern,, +49 2461 61 3532, und
    Peter Preusse,, +49 2461 61 3532
    Forschungszentrum Jülich (FZJ), Institut für Energie- und Klimaforschung: Stratosphäre (IEK-7), Wilhelm-Johnen-Straße, 52425 Jülich
  • Oliver Reitebuch,, +49 8153 281321
    Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR), Institut für Physik der Atmosphäre, Münchener Straße 20, 82234 Weßling

Project description

The tropical quasi-biennial oscillation (QBO) is one of the most prominent coupling mechanisms from the stratosphere into the troposphere and evidence for the QBO influence is found in surface observations at all latitudes. As changes of the QBO are expected for a future warmer climate, due to changes in the tropical weather and the related atmospheric wave fields, which drive the QBO directly and indirectly, also QBO effects on the tropospheric climate are expected to change.
The overarching goal of the proposed research is therefore to develop a better understanding of (1) the responses of the tropical gravity waves (GWs) and other tropical wave modes, (2) the QBO, which is driven by tropical waves, and (3) the QBO signal in the tropical deep convection to a changing climate. The main tools to be developed and used are new deep-convection resolving versions of the ICON model for new baseline simulations of the QBO, improved GW parameterizations, and satellite observations of tropical waves and winds. The simulations will be validated by the observations, and the GW parameterizations will be calibrated against the deep convection resolving simulations. The related research will include five subordinate goals.

New baseline simulations of the QBO in deep convection resolving models (MPI-M)

A new version of the ICON general circulation model will be developed that excludes parameterizations of deep convection and GW drag, so that the two leading causes of uncertainty in the QBO forcing are avoided. These uncertainties currently impede any reliable statement on the response of the QBO to a changing climate. The development will include idealized small domain model setups for process studies and a realistic full domain setup for a comparison of the simulated and observed QBO in present conditions. These baseline simulations will be used to quantify in detail the forcing mechanism of the QBO.

Gravity wave parameterization for current and future climate states (GUF)

Based on the convection resolving QBO simulations the new gravity-wave parameterization MS-GWaM, allowing for the first time for transient wave-mean-flow interactions, and presently being developed and implemented into ICON within the DFG FOR MS-GWaves, will be adapted and calibrated for current and future climate conditions, with the goal to obtain comparable parameterized QBO simulations in coarser resolved ICON model setups, and thus a GW parameterization that works robustly in different climates. MS- GWaM will be coupled to a parameterization for convective sources of GWs. For the gravity-wave propagation two versions will be considered: a one-dimensional version that accounts only for the vertical propagation of GWs, and a three-dimensional that in addition accounts for the lateral propagation of GWs.

Future changes in GWs, the QBO and deep convection (MPI-M, GUF, FZJ)

Here the new model setup will be employed for current and future climate conditions in order to develop a better understanding of the changes in (1) the wave field that forces the QBO, (2) the QBO itself, and (3) the QBO modulation of the deep convection. For this purpose, deep-convection resolving QBO simulations will be analyzed for idealized current and future climate conditions on small domains. Properties of the excited GWs and tropical wave modes will be validated against observations. The idealized climate modifications will include the effects of the tropospheric warming that will be most important for the convectively triggered waves that can drive the QBO, as well as the effects of composition changes on the radiative wave damping that is important for the wave meanflow interaction.

Observations of waves and the QBO (FZJ with DLR support for Aeolus)

Observational estimates of vertical fluxes of zonal momentum carried by equatorial waves are derived from temperature measurements by the SABER and HIRDLS satellite instruments. The investigation will include GWs and tropical wave modes such as Kelvin waves. It will be investigated whether this is also possible for mixed-Rossby-gravity waves and tropical Rossby waves. We will use Aeolus wind observations to derive directly the QBO winds and compare them to ERA-interim or ERA-5 re-analyses. Further, we will validate the high resolution ICON simulations and assess the representation of tropical wave modes in the re-analysis data which allows to extend data series beyond the period of stratospheric satellite observation and estimate changes in the wave driving from past to present.


Comparison of observed and simulated QBO (MPI-M, GUF, FZJ)

We will compare the observed wave fields and momentum fluxes, and the observed QBO structure with the simulated wave field and the QBO in initialized seasonal simulations with start dates in periods with either SABER or Aeolus observations. The simulations will extend over six months to one year. The initial states will be chosen for different QBO phases. The skill in these simple hindcasts will be assessed both for the convection resolving setup and the lower resolution setup with parameterized GW sources and drag.