Thursday, 11 June 2009: 10:40 AM
Measured indicators of stratospheric circulation changes have been difficult to attain due to a lack of sufficient long-term stratospheric measurements. Several such datasets do exist however and each of them, such as stratospheric water vapor mixing ratios measured over Boulder, CO, suggest that changes have occurred in the stratosphere over the last 25 years. We show that the changes seen in the measured or measurement-based indicators of stratospheric circulation, including stratospheric water vapor measurements, a residual circulation calculation and age of air estimates, are consistent with each other. Collectively these datasets help to describe the two main features of recent stratospheric circulation changes: (1) a trend of increasing water vapor mixing ratios and age of air from 1980-2000, and (2) a large, persistent shift during the year 2000 towards increased mass flux in the lower stratosphere, decreased water vapor mixing ratios and decreased age of air. Global climate models have been unable to reproduce either the correct trend or the subsequent large shift in the stratospheric circulation. It is likely that these stratospheric changes are driven by changes in tropospheric wave activity that induces changes in both mean circulation and mixing between the tropics and extratropics. We will include a brief analysis of Eliassen-Palm flux divergence statistics calculated from the NCEP Reanalysis dataset as an indicator of wave driving of the stratosphere to begin to analyze the cause of the changes. We will also look for changes in measured stratospheric tracer-tracer correlations that can only occur due to circulation changes. These observed changes in the stratosphere provide a large constraint on global climate models and it is clearly important to more fully understand their causes.
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