QBO Modulation of Upper-stratospheric High-latitude Planetary Waves in the Northern Hemisphere in March

The development of large-amplitude planetary waves (PWs) in March in the upper stratosphere during the easterly phase of the equatorial quasi-biennial oscillation (QBO) was investigated using ERA-interim reanalysis data for 1979–2019. During the 10-hPa easterly QBO, the amplitude at 3 hPa was significantly larger than that during the westerly QBO for cases of large-amplitude PWs. Case studies were conducted for individual events of the wave number 1 (wave-1) PW growth: an easterly case in 1994 and a westerly case in 1995. During the easterly QBO in March 1994, a developing perturbation at middle latitudes moved rapidly northeastward to replace the decaying high-latitude wave. In the early stage, conversion from the zonal mean to eddy kinetic energy in the subtropical region was crucial for wave development. This energy conversion was dependent on the sign of the meridional shear of the zonal wind in the middle latitudes. Negative shear was produced by the secondary meridional circulation associated with the equatorial QBO. After the perturbation started to develop in the middle latitudes, it moved northeastward over a few days due to potential vorticity flux, and the growth of the high-latitude waves was enhanced. A composite analysis also showed that the meridional shear of the zonal wind in the middle latitudes was negative during the easterly QBO in March. This study improves our understanding of the dynamic mechanism underlying the equatorial-polar relationship in the stratosphere in March.