Vertical structure of stratospheric water vapour trends derived from merged satellite data.

Stratospheric water vapour is a powerful greenhouse gas. The longest available record from balloon observations over Boulder, Colorado, USA shows increases in stratospheric water vapour concentrations that cannot be fully explained by observed changes in the main drivers, tropical tropopause temperatures and methane. Satellite observations could help resolve the issue, but constructing a reliable long-term data record from individual short satellite records is challenging. Here we present an approach to merge satellite data sets with the help of a chemistry-climate model nudged to observed meteorology. We use the models' water vapour as a transfer function between data sets that overcomes issues arising from instrument drift and short overlap periods. In the lower stratosphere, our water vapour record extends back to 1988 and water vapour concentrations largely follow tropical tropopause temperatures. Lower and mid-stratospheric long-term trends are negative, and the trends from Boulder are shown not to be globally representative. In the upper stratosphere, our record extends back to 1986 and shows positive long-term trends. The altitudinal differences in the trends are explained by methane oxidation together with a strengthened lower-stratospheric and a weakened upper-stratospheric circulation inferred by this analysis. Our results call into question previous estimates of surface radiative forcing based on presumed global long-term increases in water vapour concentrations in the lower stratosphere.

[1]  M. Saier,et al.  Climate Change, 2007 , 2007 .

[2]  Veronika Eyring,et al.  Chemistry-Climate Model Simulations of Twenty- First Century Stratospheric Climate and Circulation Changes , 2010 .

[3]  W. Randel,et al.  Physical processes in the tropical tropopause layer and their roles in a changing climate , 2013 .

[4]  L. Polvani,et al.  The stratosphere : dynamics, transport, and chemistry , 2010 .

[5]  D. Seidel,et al.  Climatological characteristics of the tropical tropopause as revealed by radiosondes , 2001 .

[6]  J. Staehelin,et al.  Trends and variability of midlatitude stratospheric water vapour deduced from the re-evaluated Boulder balloon series and HALOE , 2007 .

[7]  D. Seidel,et al.  How well do we know recent climate trends at the tropical tropopause , 2012 .

[8]  P. Forster,et al.  Assessing the climate impact of trends in stratospheric water vapor , 2002 .

[9]  M. Toohey,et al.  Characterizing sampling biases in the trace gas climatologies of the SPARC Data Initiative , 2013 .

[10]  Corinne Le Quéré,et al.  Climate Change 2013: The Physical Science Basis , 2013 .

[11]  Fei Wu,et al.  Interannual changes of stratospheric water vapor and correlations with tropical tropopause temperatures , 2004 .

[12]  Colette Brogniez,et al.  Validation of ACE-FTS v2.2 methane profiles from the upper troposphere to the lower mesosphere , 2008 .

[13]  G. Diskin,et al.  Ice nucleation and dehydration in the Tropical Tropopause Layer , 2013, Proceedings of the National Academy of Sciences.

[14]  E. Ray,et al.  Stratospheric water vapor trends over Boulder, Colorado: Analysis of the 30 year Boulder record , 2011 .

[15]  P. Braesicke,et al.  Evidence for changes in stratospheric transport and mixing over the past three decades based on multiple data sets and tropical leaky pipe analysis , 2010 .

[16]  T. Shepherd,et al.  Simulated Anthropogenic Changes in the Brewer–Dobson Circulation, Including Its Extension to High Latitudes , 2009 .

[17]  M. Riese,et al.  Long‐term changes of methane and hydrogen in the stratosphere in the period 1978–2003 and their impact on the abundance of stratospheric water vapor , 2006 .

[18]  David A. Plummer,et al.  Technical Note: The CCCma third generation AGCM and its extension into the middle atmosphere , 2008 .

[19]  K. Cowtan,et al.  Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends , 2014 .

[20]  K. Rosenlof,et al.  Trends in the temperature and water vapor content of the tropical lower stratosphere: Sea surface connection , 2008 .

[21]  S. Oltmans,et al.  The increase in stratospheric water vapor from balloonborne, frostpoint hygrometer measurements at Washington, D.C., and Boulder, Colorado , 2000 .

[22]  E. Atlas,et al.  Age of stratospheric air unchanged within uncertainties over the past 30 years , 2009 .

[23]  J. Thepaut,et al.  The ERA‐Interim reanalysis: configuration and performance of the data assimilation system , 2011 .

[24]  S. Fueglistaler,et al.  Control of interannual and longer‐term variability of stratospheric water vapor , 2005 .

[25]  G. Dutton,et al.  An examination of the total hydrogen budget of the lower stratosphere , 1994 .

[26]  A. Gettelman.  Chapter 7 Upper Troposphere and Lower Stratosphere , 2010 .

[27]  E. Ray,et al.  On the structural changes in the Brewer-Dobson circulation after 2000 , 2010 .

[28]  Paul Poli,et al.  Estimating low‐frequency variability and trends in atmospheric temperature using ERA‐Interim , 2014 .

[29]  R. F. Strickler,et al.  Thermal Equilibrium of the Atmosphere with a Convective Adjustment , 1964 .

[30]  L. Oman,et al.  Temperature trends in the tropical upper troposphere and lower stratosphere: Connections with sea surface temperatures and implications for water vapor and ozone , 2013 .

[31]  H. Bovensmann,et al.  Evolution of stratospheric ozone and water vapour time series studied with satellite measurements , 2009 .

[32]  Adam A. Scaife,et al.  The Circulation Response to Idealized Changes in Stratospheric Water Vapor , 2013 .

[33]  W. D. Hypes,et al.  Dehydration in the lower Antarctic stratosphere during late winter and early spring, 1987 , 1989 .

[34]  S. Solomon,et al.  Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming , 2010, Science.

[35]  Douglas W. Nychka,et al.  Statistical significance of trends and trend differences in layer-average atmospheric temperature time series , 2000 .

[36]  J. Zawodny,et al.  A revised water vapor product for the Stratospheric Aerosol and Gas Experiment (SAGE) II version 6.2 data set , 2004 .

[37]  David A. Plummer,et al.  The CCCma third generation AGCM and its extension into the middle atmosphere , 2008 .

[38]  S. Fueglistaler.  Stepwise changes in stratospheric water vapor , 2012 .

[39]  R. Garcia,et al.  The role of molecular hydrogen and methane oxidation in the water vapour budget of the stratosphere , 1988 .

[40]  Robert Damadeo,et al.  SAGE version 7.0 algorithm: application to SAGE II , 2013 .

[41]  Masson-Delmotte,et al.  The Physical Science Basis , 2007 .

[42]  Piers M. Forster,et al.  Stratospheric water vapour changes as a possible contributor to observed stratospheric cooling , 1999 .

[43]  Qiang Fu,et al.  Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends , 2004, Nature.

[44]  A. Lambert,et al.  Validation of Aura Microwave Limb Sounder stratospheric water vapor measurements by the NOAA frost point hygrometer , 2013, Journal of geophysical research. Atmospheres : JGR.

[45]  C. Brühl,et al.  Multimodel assessment of the upper troposphere and lower stratosphere: Extratropics , 2010 .

[46]  Martin Riese,et al.  Impact of uncertainties in atmospheric mixing on simulated UTLS composition and related radiative effects , 2012 .

[47]  P. Bernath,et al.  The relation between atmospheric humidity and temperature trends for stratospheric water , 2013 .

[48]  H. Vömel,et al.  Accuracy of tropospheric and stratospheric water vapor measurements by the cryogenic frost point hygrometer: Instrumental details and observations , 2007 .

[49]  Masato Shiotani,et al.  Seasonal to Decadal Variations of Water Vapor in the Tropical Lower Stratosphere Observed with Balloon-Borne Cryogenic Frost Point Hygrometers , 2010 .