Modeled Arctic ozone depletion in winter 1997/1998 and comparison with previous winters

We have used a three-dimensional stratospheric model to investigate Arctic ozone depletion in the three winters from 1995/1996 to 1997/1998, winters w ith quite different dynamical conditions. We have diagnosed the chemical de pletion from the model and compared the model ozone with ground-based and s onde measurements. In winter 1997/1998 the lower stratosphere was generally much warmer than the previous two cold winters. However, the occurrence of low temperatures in December 1997 did cause chlorine activation and ozone loss in the model. In fact, despite the very different meteorological condi tions for these three winters, by mid-February the model-calculated mean vo rtex loss was similar in each year at around 20% at 480 Ii. Larger differen ces in the calculated loss occurred when low temperatures persisted into Ma rch, this seems a prerequisite for very large loss. By late March the model -calculated mean vortex column loss was 53 Dobson units (DU) in 1998, 71 DU in 1996, and 80 DU in 1997. A first effort at a detailed day-to-day valida tion of a chemical transport model by comparison with ozone measurements is presented. The model reproduces the mean column observations to within abo ut +/-10%, although the bias varies in magnitude and changes sign in differ ent years. The agreement between the model and ozonesonde profiles in the l ower stratosphere varies from year to year and during the course of a parti cular winter/spring. In winter 1997/1998, the model agrees well with sonde profiles at the 480 K and 555 It levels with a positive mean difference (oz onesonde values minus model values) with amplitude of less than 6%. In cont rast, in winter 1996/1997, the mean difference is negative and the amplitud e is less than 15%. In winter 1995/1996 the mean difference is negative and the amplitude reaches 43%. The differences in 1995/1996 and 1996/1997 resu lt despite good agreement between model and observations in midwinter, and they develop following the large chemical losses which occurred in those wi nters. We suggest that a major contribution to the model/observation differ ences for those two winters is a model underestimation of the chemical ozon e loss.