Dynamic formation of extreme ozone minimum events over the Tibetan Plateau during northern winters 1987-2001

Wintertime extreme ozone minima in the total column ozone over the Tibetan Plateau (TP) between 1978 and 2001 are analyzed using observations from the Total Ozone Mapping Spectrometer (TOMS), Global Ozone Monitoring Experiment (GOME), and reanalysis data from both National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasts. Results show that total column ozone reduction in nine persistent (lasting for at least 2 days) and four transient events can be substantially attributed to ozone reduction in the upper troposphere and lower stratosphere region (below 25 km). This reduction is generally caused by uplift of the local tropopause and northward transport of tropical ozone-poor air associated with an anomalous anticyclone in the upper troposphere. These anticyclonic anomalies are closely related to anomalous tropical deep convective heating, which is, however, not necessarily phase locked with the tropical Madden-Julian Oscillation as in our earlier case study. Considering stratospheric processes, the selected 13 events can be combined into nine independent events. Moreover, five of the nine independent events, especially the persistent events, are coupled with contributions from stratospheric dynamics between 25 and 40 km, i.e., 15%-40% derived from GOME observations for events in November 1998, February 1999, and December 2001. On the basis of these events, stratospheric column ozone reduction over the TP region can be attributed to the dynamics (development and/or displacement) of the two main stratospheric systems, namely, the polar vortex and the Aleutian High. The effect of a low-ozone pocket inside the Aleutian High on the total column ozone in East Asia requires further study.