An investigation of dynamical contributions to midlatitude ozone trends in winter

On timescales less than a month, negative (positive) 100 hPa temperature anomalies (deviations from seasonal means) and positive (negative) 100 hPa height anomalies at northern midlatitudes are associated with negative (positive) total ozone anomalies owing to vertical and horizontal advective transport of ozone. We apply linear regression relationships between total ozone and 100 hPa temperature and height together with observed monthly temperature and height anomalies to estimate the component of total ozone variability at northern midlatitudes that results from month-to-month differences in advective transport. At 45°N, this empirical method simulates a large part of the observed monthly zonal mean total ozone variability over the period 1979 - 1991. In order to estimate the advective transport contribution to total ozone trends, we apply a multiple regression statistical model to empirical model time series for latitudes of 10°N to 60°N. These empirical model trends result from long-term changes in ozone advective transport which are calculated on a month-to-month basis. The resulting model trend component at northern midlatitudes in February has a longitude dependence consistent with that of the observed ozone trends and has a latitude dependence that also agrees with that of the observed trends, peaking at midlatitudes and decreasing at higher latitudes. Subtracting the estimated advective contribution from the observed trends yields a residual meridional trend profile that agrees more closely in latitude dependence and amplitude with two-dimensional stratospheric model estimates.