Abstract
Using the JRA-55 reanalysis datasets and radiosonde observations, the impact of the quasi-biennial oscillation (QBO) on the downward extension of stratospheric Northern Annular Mode (NAM) signals in its positive and negative phases (positive and negative NAM hereafter) is examined. The results show that the average number of days per month on which stratospheric negative NAM signals extend downward into the troposphere is more than twice as large during the easterly phase of the QBO (EQBO) as those during the westerly phase of the QBO (WQBO). The increasing likelihood of downward extension of stratospheric negative NAM signals during the EQBO is related to three factors: enhanced upward wave-1 fluxes entering the stratosphere, anomalous poleward wave fluxes below 100 hPa and EQBO-induced residual circulation. It is also revealed that the EQBO-induced subtropical tropospheric easterly anomalies lead to positive height anomalies over the subtropical North Atlantic, which are amplified by transient eddies and maintained by convection. These subtropical positive height anomalies further stimulate an anomalous wave train, forcing subpolar positive height anomalies over the Euro-Atlantic sector and enhancing wave 1. Over the Pacific, the EQBO could also induce positive height anomalies in the tropical troposphere, and these anomalies are related to convection and vertical motion. The positive height anomalies can induce anomalous poleward-propagating waves, forcing negative height anomalies over the North Pacific at mid-high latitudes and the enhancing wave 1. Fewer stratospheric positive NAM signals extend downward into the troposphere during the EQBO than during the WQBO, with mechanisms similar to those of negative NAM.
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Acknowledgements
This work is supported by the National Natural Science Foundation of China (41630421, 91837311) and the Fundamental Research Funds for the Central Universities (lzujbky-2018-ct03). We appreciate Matlab codes for the linearized barotropic vorticity equation model support provided by J. Shaman and E. Tziperman. We also thank the scientific teams at JRA-55 for providing the reanalysis data and thank the Free University of Berlin for providing QBO dataset. The JRA-55 datasets can be acquired from ftp://ds.data.jma.go.jp. The QBO dataset can be acquired from https://www.geo.fu-berlin.de/en/met/ag/strat/produkte/qbo/index.html.
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Zhang, R., Tian, W. & Wang, T. Role of the quasi-biennial oscillation in the downward extension of stratospheric northern annular mode anomalies. Clim Dyn 55, 595–612 (2020). https://doi.org/10.1007/s00382-020-05285-4
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DOI: https://doi.org/10.1007/s00382-020-05285-4