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Using water vapor data from HALOE and SAGE II, an anti-correlation  between planetary wave driving (here expressed by the mid-latitude  eddy heat flux at 50 hPa added from both hemispheres) and tropical  lower stratospheric (TLS) water vapor has been obtained. This  appears to be a manifestation of the inter-annual variability of the  Brewer-Dobson (BD) circulation strength (the driving of which is generally measured in terms of the mid-latitude  eddy heat flux), and hence amount of water vapor  entering the stratosphere. Some years such as 1991 and  1997 show, however, a clear departure from the anti-correlation  which suggests that the water vapor changes in TLS can not be  attributed solely to changes in extratropical planetary wave activity (and its effect on the BD circulation).  After  2000 a sudden decrease in lower stratospheric water vapor has been  reported in earlier studies based upon satellite data from HALOE,  SAGE II and POAM III indicating that the lower stratosphere has  become drier since then. This is consistent with a sudden rise in  the combined mid-latitude eddy heat flux with nearly equal  contribution from both hemispheres as shown here and with the  increase in tropical upwelling and  decrease in cold point  temperatures  found by Randel et al. (2006). The low water vapor  and enhanced planetary wave activity (in turn strength of the BD circulation) has persisted until the  end of the satellite data records. From a multi-variate regression  analysis applied to 27 years of NCEP and HadAT2 (radiosonde) temperatures (up to  2005) with contributions from solar cycle, stratospheric  aerosols and QBO removed, the enhancement wave driving  after 2000 is  estimated to contribute up to 0.7 K cooling to the overall TLS  temperature change during the period 2001–2005 when compared to the  period 1996–2000. NCEP cold point temperature show an average  decrease of nearly 0.4 K from changes in the wave driving, which is  consistent with observed mean TLS water vapor changes of about −0.2 ppm after 2000

The relationship between tropospheric wave forcing and tropical lower stratospheric water vapor