Lagrangian calculations of tropical troposphere-to-stratosphere transport and implications for seasonal, interannual and longer-term variation of stratospheric water vapor

We present trajectory calculations of tropical troposphere-to-stratosphere transport based on ERA-40 reanalysis wind fields and temperature data. These calculations are used to predict stratospheric water vapor concentrations on the basis of minimum saturation mixing ratios calculated along trajectories, and the predicted concentrations are shown to be in good agreement with space borne and ballon borne observations. The results suggest that the water vapor mixing ratio of air entering the stratosphere in the tropics is controlled by synoptic-scale temperatures and circulation. Longitudinal variation in circulation and temperatures plays an important role in determine absolute values of entry mixing ratios (on average about 3.5ppmv). Most notably, the cold anomaly over the tropical western Pacific at tropopause level accounts for the substantial lowering of mean mixing ratio compared to expected values from zonal mean tropopause temperatures. However, the results also indicate that interannual anomalies (of order 0.5ppmv) are well explained by zonal mean temperature anomalies, induced, for example, by the Quasi-Biennial Oscillation (QBO). More generally the good agreement between the trajectory predictions and satellite observations of stratospheric water vapour during periods when the ERA-40 temperature fields agree well with radiosondes, and when the various satellite observations are consistent among themselves (1995-2002), strengthens our confidence that stratospheric water vapor is tightly controlled by synoptic-scale temperatures and circulation, with only a secondary role for processes such as overshooting convection. The analysis of interannual variability further shows that anomalies of stratospheric water vapor mixing ratios are dominated by the anomalies of zonal mean temperature, and not by circulation anomalies. This emphasises the dilemma that arises from observations suggesting almost a doubling of water vapor in the stratospheric overworld since about 1960, without any apparent positive temperature trend at the tropical tropopause. We cannot reconcile the above mentioned dilemma, but tentatively suggest that the long-term trend may have been overestimated due to a combination of measurement uncertainties and too short continous measurements for a signal that shows variability of order +- 0.5ppmv over timescales of months to years.