2005 Volume 83 Issue 1 Pages 57-67
Over the subtropical northwestern Pacific, a prominent ozone minimum of less than 235 Dobson Units (DU) is observed in winter, with extremely low values of less than 200 DU occurring occasionally over this region for short time periods (days to weeks). In this study the vertical structure of this ozone minimum is examined using a 9 year (1991-2000) average of the total ozone mapping spectrometer (TOMS) and Halogen Occultation Experiment (HALOE) data. It is found that the ozone minimum is mainly stratospheric origin with two distinct low-ozone layers in the stratosphere, one in the middle stratosphere of 10-15 hPa, and the other in the lower stratosphere of 40-60 hPa. The mid-stratospheric low-ozone layer is attributed to southward transport of high-latitude low-ozone air by atmospheric circulation in wintertime associated with the Aleutian High, which is simulated successfu11y using a simple chemical transport model. The extremely low ozone was observed in December 2001 over the subtropical northwestern Pacific. This case is also examined by ozonesonde observations at Naha (26°12´N, 127°41´E) and the model. Both the results from the 9 year climatological field, and the December 2001 field, show that the wintertime total ozone minimum over the subtropical northwestern Pacific is substantially dynamical in origin.