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

Abstract

Abstract. 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 BrewerDobson (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.

Analysis of HCl and ClO time series in the upper stratosphere using satellite data sets

“…1 is still present, implying that according to this analysis it is neither a related feature of seasonal or QBO behaviour. This feature may be related to possible anomalous atmospheric dynamics in the Southern Hemisphere, such as the Antarctic ozone hole split in 2002 (Manney et al, 2005;Charlton et al, 2005) and/or changes in tropical upwelling due to enhanced wave driving in (Dhomse et al, 2008. The tropics bin from 20 • S-20 • N shows a more negative trend of 5.8 % decade −1 since 1997 compared to the mid-latitudes.…”

Relationships between Brewer-Dobson circulation, double tropopauses, ozone and stratospheric water vapour

“…This increase of the mean tropical upwelling is consistent with a higher, colder and drier tropical tropopause leading to a decrease of water vapour in the lower stratosphere (Randel, 2010, Plate 6). Dhomse et al (2008) had also related the sudden decrease in lower stratospheric water vapour after 2001 with a sudden rise in the strength of the BDC. Those authors used the 50 hPa eddy heat flux averaged from 45 • to 75 • and added from both hemispheres to represent the strength of the BDC.…”

The Variations in Middle and Upper Stratospheric Water Vapour over the Past Two Decades

“…Understanding the observed SWV variations and determining the factors that control such variations have become subjects of great interest in recent years (Holton and Gettelman 2001;Rosenlof et al 2001;Randel et al 2006;Dhomse et al 2008;Wang et al 2014Wang et al , 2015. Observations have shown that SWV increased between 1981 and 2000, began to decrease after 2000, before increasing again after 2005 (Oltmans et al 2000;Hurst et al 2011).…”