No abstract

Microwave limb sounder observations of midstratospheric ozone during stratospheric warmings show tongues of high ozone drawn up from low latitudes into the developing anticyclone. Several days later, an isolated pocket of low ozone mixing ratios appears, centered in the anticyclone, and extending in the vertical from ≈15 to 5 hPa, with higher mixing ratios both above and below. These low ozone mixing ratios during northern hemisphere warmings are comparable to values well inside the vortex and are ≈3 parts per million by volume lower than typical midlatitude extra‐vortex mixing ratios. This type of feature is seen whenever the anticyclone is strong and persistent, including during relatively strong minor warmings in the southern hemisphere. Three‐dimensional back trajectory calculations indicate that the air in the region of the low‐ozone pockets originates at higher altitudes and low latitudes, where ozone mixing ratios are much higher. The air parcels studied here are typically confined together for 1 to 3 weeks before the lowest ozone mixing ratios are observed. The trajectory calculations and comparisons with passive tracer data confirm that the observed low‐ozone regions in the midstratosphere could not result solely from transport processes.

No abstract

Two stratospheric warmings during February and March 1993 are described using UKMO analyses, calculated PV and diabatic heating, and N2O observed by the CLAES instrument on the UARS. The first warming affected temperatures over a larger region, while the second produced a larger region of reversed zonal winds. Tilted baroclinic zones formed in the temperature field, and the polar vortex tilted westward with height. Narrow tongues of high PV and low N2O were drawn off the polar vortex, and irreversibly mixed. Tongues of material were drawn from low latitudes into the region between the polar vortex and the anticyclone; diabatic descent was also strongest in this region. Increased N2O over a broad region near the edge of the polar vortex indicates the importance of horizontal transport. N2O decreased in the vortex, consistent with enhanced diabatic descent during the warmings.

[1] The effect on stratospheric temperature of changing ozone is investigated by comparing two 5-member ensembles of 20-year Unified Model transient runs, one with a linear trend in ozone and one without. A significant stratospheric mean temperature trend of À0.17 K/ decade is attributed to ozone depletion. It is found that, although increasing the ensemble size to 20 members would have considerable benefits, increasing the ensemble size further would not dramatically improve confidence in the results. The timeslice approach to climate change modeling is found to produce similar temperature trends to the transient approach for this experiment.