Station versus reanalysis-based proxies for the Quasi-Biennial Oscillation (QBO): How do they differ – and does it matter?

The Quasi-Biennial Oscillation (QBO) has long been recognized as an important factor shaping hemispheric large-scale dynamics, in particular serving as a sort of switch for polar vortex dynamics in the Northern Hemisphere (NH). When calculating (extratropical) correlations or composites for different phases of the QBO some subjective choices have to be made, including “Which timeseries should be used?” or “How to define the phasing?”. Here, we will examine - from a historical perspective - the differences between a traditional station based QBO timeseries (specifically, the “Singapore” zonal wind timeseries provided by the Free University of Berlin and currently continued at the Karlsruhe Institute of Technology) in contrast to reanalysis based timeseries (specifically, zonal mean zonal wind timeseries from ERA5, and near station profiles from ERA5 corresponding to the station’s location). We explore the climatological properties of the QBO, including composites and phase transitions. Additionally, we examine how the choice of QBO proxy relates to and influences the perception and interpretation of the Holton-Tan relationship, which describes the potential link between the phases of the QBO and the strength of the stratospheric polar vortex – in particular during NH winter. Data and tools are accessible via the ATMOHub at https://www.atmohub.kit.edu/.