Interannual Variation of Upper Stratospheric Ozone in the Northern Midlatitudes in Early Winter Caused by Planetary Waves

Ozone mixing ratios in the upper stratosphere, observed with a millimeter-wave radiometer at Rikubetsu (43.46 degrees N, 143.77 degrees E), Japan, from November 1999 to February 2017 showed both interannual and seasonal variation, which was characterized by a winter maximum and a summer minimum. During the study period, the summer minima were nearly constant whereas the winter maxima varied interannually and also displayed short-term variability. The observed ozone mixing ratios at 1 hPa were anticorrelated with temperature at 1 hPa from MERRA-2 data. The slope of the relationship between the logarithm of ozone concentration and the reciprocal of temperature differed between winter data and both summer and annual data. Therefore, we inferred that both chemistry and dynamics affect short-term variation of ozone mixing ratios in winter. We then examined the contribution of the polar vortex to interannual variations in ozone and temperature at 1 hPa. When the polar vortex was strong, wave number-1 planetary waves at high latitude propagated toward the midlatitudes instead of vertically. The vertical component of the wave number-1 Eliassen-Palm flux along 43 degrees N at 1 hPa was strongly correlated with zonal mean zonal wind along 60 degrees N at 50 hPa. When the zonal mean westerly wind was strong in December, upper stratospheric (similar to 1 hPa) temperatures over Rikubetsu and over a point on the opposite side of the globe (by longitude) were significantly lower and higher, respectively, than the climatological temperature. Thus, planetary wave propagation related to zonal mean westerly wind strength induced early winter interannual variation in upper stratospheric ozone in the midlatitudes.