Abstract
In order to simulate the quasi-biennial oscillation (QBO) with a realistic period and amplitude, general circulation models commonly include parameterizations of small scale gravity waves (GW). In this work, we explore how different GW parameterization setups determine the response of QBO properties to a warmer climate. Atmosphere-only experiments in both present day and warmer climate serve as testbed to analyze the effect of four different GW parameterization setups, active in the tropics. Having tuned the GW parameterizations to produce a realistic QBO in present day climate, we analyze changes of QBO properties in the warmer climate. The QBO period decreases in two parameterization setups by ~30 %, while the QBO period remains unchanged in the remaining two parameterization setups. In all parameterization setups, the QBO amplitude in the warmer climate weakens below 10 hPa but shows different behaviour above 10 hPa. We show that changes in QBO amplitude and changes in QBO period are inconsistent among experiments. In the chosen experimental design, the inconsistent future change in QBO properties among the suite of experiments depends solely on the choice of the GW parameterization setup.
Highlights
The quasi-biennial oscillation (QBO) of equatorial zonal winds is a prominent feature of stratospheric dynamics
In the lower stratosphere below the QBO relevant heights, the absolute amount of momentum flux for each of the four parameterization setups lies within the range of observations, with mean absolute momentum fluxes varying between 1 and 5 mPa (Sato and Dunkerton 1997; Piani et al 2000; Grimsdell et al 2010; Geller et al 2013)
In order to systematically analyze the QBO changes across the suite of parameterization setups, we focus on the two main QBO characteristics: The QBO period and the QBO amplitude
Summary
The quasi-biennial oscillation (QBO) of equatorial zonal winds is a prominent feature of stratospheric dynamics. The QBO is driven downward—against the general tropical upwelling—by waves which emanate from the troposphere and travel upwards into the stratosphere. At the alternating shear zones of the QBO, the waves break, deposit momentum and cause a downward propagation of the wind maxima. The waves driving the QBO range from large scale Kelvin and Rossby-gravity waves to smaller scale gravity waves (GW) with shorter horizontal wavelength. Dunkerton (1997) show that the contribution of intermediate inertiaGWs and mesoscale GWs is necessary to produce a QBO with realistic period and amplitude. For a more detailed description on the forcing and the physical mechanisms of the QBO (see the review paper by Baldwin et al 2001)
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