Origin of the Seasonal Variation of Total Column Ozone in High Northern Latitudes

The total column ozone at high northern latitudes has a well-documented seasonal variation, with high values toward the end of winter decreasing to a minimum value in early autumn. Data from the High Resolution Dynamics Limb Sounder (HIRDLS) have been used to study the descent of high-ozone air in the Brewer-Dobson circulation to create the winter maximum, and the subsequent isentropic mixing to lower latitudes leading to the decrease. These show that a large part of the total seasonal variation is due to the 330-450K levels in the UTLS. This is because a barrier to rapid mixing to low latitudes exists during the winter, causing a large build-up poleward of 40°N. In spring the barrier weakens and later disappears, allowing rapid mixing. Summing the HIRDLS observations of contributions by the different atmospheric layers gives excellent agreement with OMI total column measurements.