We address the cause of the largely natural total ozone decline in the stratosphere from its spring maximum to fall minimum in the northern hemisphere and show that this is mainly due to NOchi-catalyzed ozone destruction. While in an earlier paper, using HALOE satellite observations, an analysis was made only for the year 1993, here we also analyze later years, yielding very similar results. We emphasize the role of the subtropical meridional transport barrier in isolating the chemistry at middle to high latitudes from that over the tropics. We find that for all years net ozone production takes place between the "subtropical barrier", at about 30 degrees N, and 50 degrees N. Nevertheless, also in this latitude region the ozone content declines due to transport to higher latitudes where very strong chemical ozone loss takes place due to summer time NO, activation. An interesting result of the analysis is that, rather irrespective of the starting values of total ozone in early spring, chemical ozone loss yields about the same minimum total ozone amounts in early fall, The result of our study strongly depends on improved determinations of rate coefficients of several. reactions in the stratospheric ozone budget.