Upwelling into the lower stratosphere forced by breaking tropical waves: evidence from chemical tracers

Abstract. Measurements from the Microwave Limb Sounder (MLS) on the 68 hPa pressure level from 1 January 2005 to 31 December 2010 are used to calculate the coherence between anomalies in the tropical mean mixing ratios of H₂O, CO, and N₂O, and 100 hPa temperature. We show that the fluctuations of lower stratospheric water vapor in the subseasonal and multiyear time windows are generated by different physical mechanisms. In the subseasonal time window, the spatial pattern of the coherence between 100 hPa temperature and water vapor, and the time lag, show that the variability in lower stratospheric water vapor is dominated by fluctuations in upwelling forced by the dissipation of tropical Rossby waves. In the multiyear time window, the variability of lower stratospheric water vapor is more strongly coherent with temperature fluctuations on the 100 hPa surface in regions where the annual mean temperature is colder than 194 K. In addition, the 68 hPa water vapor anomalies lag the 100 hPa temperature anomalies by roughly 140 days. In this time window, the variability of lower stratospheric water vapor is therefore dominated by changes in the temperature dependent dehydration efficiency which modulate the water vapor stratospheric entry mixing ratio. On subseasonal timescales, the spatial pattern of the coherence between 100 hPa temperature and 68 hPa CO anomalies is very similar to the pattern of coherence between 100 hPa temperature and the Real-time Multivariate MJO series 1 (RMM1) index of the Madden Julian Oscillation (MJO). The MJO therefore has a strong influence on the subseasonal variability of CO in the lower stratosphere. The subseasonal 68 hPa CO and H₂O anomalies lag the 100 hPa temperature anomalies by 3.16 and 2.51 days, respectively. The similarity between the two time lags suggests that the subseasonal CO anomalies can also be attributed to changes in upwelling. The multiyear variability in lower stratospheric N₂O appears to be dominated by the Quasi Biennial Oscillation (QBO).