Insights from Idealized Modelling into the Quasi-Biennial Oscillation

About This Grant

This work is focused on advancing understanding of the Quasi-Biennial Oscillation (QBO), a prominent and persistent mode of variability in the tropical stratospheric winds with a period of approximately 28 months. The QBO influences higher latitude phenomena such as the stability of the stratospheric polar vortex (the “Holton-Tan effect”) that subsequently impacts the tropospheric mid-latitude jet stream. As the QBO vertically propagates from the middle stratosphere to the tropopause, it also influences the position of the North Pacific jet as well as the variability in the tropical troposphere through its interactions with deep convection and the Madden-Julian oscillation. The persistence and quasi regular nature of the QBO offers the potential for predictability out to several years if well represented in global models. This study aims to provide insight into the longstanding model bias in QBO strength in the lower stratosphere, with the QBO being too weak in the models, and the inability of models to correctly capture the QBO modulation of the stratospheric polar vortex. The project includes the training of two graduate students in climate dynamics and provides research opportunities for undergraduate students in a STEM field. It additionally includes creating outreach materials for the University of Utah and the Natural History Museum of Utah on the QBO and its role in the climate system. The main goals of the project, to better understand the weak QBO bias in the lower stratosphere in fully coupled models and the inability of these models to capture the Holtan-Tan effect, will be addressed using a simplified model of the stratosphere with a self-generating QBO. To test the leading theories regarding the weak QBO bias in the lower stratosphere, experiments isolating the effects of vertical resolution, varying tropical upwelling speeds, varying imposed tropical latent heating and gravity wave drag, and adjusting gravity wave sources will be performed. To better understand the interactions of the QBO with the stratospheric polar vortex additional experiments will be conducted focusing on the sensitivity of the Holton-Tan effect to polar vortex strength and the sensitivity of the polar vortex strength to the mean equatorial zonal wind vertical structure. The use of the simplified model offers the ability more clearly interpret the results compared to a fully coupled climate model and to perform a greater number of experiments at low computation cost. The graduate students will lead these experiments, with guidance from the lead investigator, offering training for the next generation of atmospheric scientists capable of addressing societally relevant issues. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.