Abstract
The quasi-biennial oscillation (QBO) is an important climate phenomenon in the tropical stratosphere with impacts worldwide, and thus reasonably representing the QBO properties, with its vertical propagation considered, is the key to clearly understanding its fundamental effects. This study proposes a new method that the QBO spatio-temporal characteristics can be reasonably captured by the first complex empirical orthogonal function (CEOF) and its derived time series. With this method, alternate variations of the typical barotropic (easterly/westerly wind) and baroclinic (easterly/westerly wind shear) components in the QBO can be adequately reflected by decomposing the QBO downward propagation structure in terms of the CEOF-based diagnostics. The QBO properties (amplitude and downward propagation speed) have a clear phase asymmetry and are larger in the westerly downward propagating phase than the easterly one. They show the largest difference between the two shear phases. These properties also feature a clear seasonality and are larger in boreal spring–summer but smallest in winter, which is much truer for the baroclinic component of the QBO than the barotropic one. Compared to those existing QBO indices, the CEOF-derived index can provide the representative QBO stages, much closer to realistic values, and thus better obtain the stable typical spatial structure of the QBO, contributing to a unified phase division for various QBO stages. In terms of the CEOF-based phase division, the QBO shows a strong modulation on the Arctic Oscillation and Madden–Julian Oscillation activities. This study provides a useful tool for diagnosing the QBO and a beneficial addition to related QBO monitoring and prediction.
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Data availability
ERA5 data during period 1956–1978 and 1979–2020 provided by ECMWF are available through the Copernicus Climate Data Store via https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels-monthly-means-preliminary-back-extension?tab=overview and https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-single-levels?tab=overview, interpolated to standard pressure levels and 2.5° × 2.5° horizontal resolution. The zonal wind of radiosonde observation from Free University of Berlin is publicly available via https://www.geo.fu-berlin.de/met/ag/strat/produkte/qbo/qbo.dat. The AO index from Climate Prediction Center is publicly available via https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao.shtml. The MJO amplitude based on RMM index is publicly available via http://www.bom.gov.au/climate/mjo/.
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Acknowledgements
We are grateful to the two anonymous reviewers whose comments and suggestions have greatly improved the manuscript and thankful to ECMWF, Climate Prediction Center, Free University of Berlin, and Australian Bureau of Meteorology for providing the data.
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This study was jointly supported by the National Natural Science Foundation of China (Grant No. U2242206), the National Key Research and Development Program on monitoring, Early Warning and Prevention of Major Natural Disaster (Grant No. 2018YFC1506000) and the Basic Research and Operational Special Project of CAMS (Grant No. 2021Z007).
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The material and datasets were prepared by WX. The study was conceived and designed by HLR and WX. The first draft of the manuscript was written by WX, and HLR reviewed and commented on the various previous versions of the manuscript.
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Xu, W., Ren, HL. A CEOF-based method for measuring amplitude and phase properties of the QBO. Clim Dyn 61, 923–937 (2023). https://doi.org/10.1007/s00382-022-06625-2
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DOI: https://doi.org/10.1007/s00382-022-06625-2