M. Schoeberl and co-authors use forward trajectory modelling to investigate the processes influencing upper tropospheric/lower stratospheric (UTLS) water vapour anomalies. Examining the pathways taken by parcels from the base of the tropical tropopause layer (TTL) upwards, they find that the belt of TTL parcel origins is much wider than the final dehydration zone near the top of the TTL. In the lower stratosphere, the driest air parcels originate from the Tropical West Pacific where they dehydrate in the cold upper troposphere as they move upwards, while the wettest air parcels originate from the edge of this region as well as in the American and Asian monsoon regions in summer. The full abstract can be found here.

Using high-precision measurements of the isotopic ratios of N2, O2, and Ar, as well as the mole fraction of Ar, S. Ishidoya and co-authors show that gravitational separation occurs in the stratosphere below the turbopause. Observed vertical profiles agree well with those expected theoretically from molecular mass differences. Simulations with a 2D model of the middle atmosphere indicate that a relationship between gravitational separation and stratospheric age-of-air would be significantly affected if the Brewer Dobson Circulation was enhanced. Therefore they suggest that gravitational separation could serve as a new indicator of changes in stratospheric circulation. The full abstract can be found here.

Some of the first results from the SPARC Data Initiative to be published, this new article by M.I. Hegglin and co-authors in JGR compares the water vapour climatologies from various satellite limb sounders for the period 1978-2010. Monthly zonal means from LIMS, SAGE II, UARS-MLS, HALOE, POAM III, SMR, SAGE III, MIPAS, SCIAMACHY, ACE-FTS, and Aura-MLS were calculated on a common latitude-pressure grid and then compared with the multi-instrument mean. Evaluations include comparisons of monthly or annual zonal mean cross-sections and seasonal cycles in the tropical and extra-tropical upper troposphere and stratosphere, comparisons of interannual variability, and the study of features such as the water vapour tape recorder. The instruments agree best in the mid-latitude and tropical middle and lower stratosphere, with a relative uncertainty of ±2–6% (as quantified by the standard deviation of the instruments’ multi-annual means). The uncertainty increases toward the polar regions (±10–15%), the mesosphere (±15%), and the upper troposphere/lower stratosphere below 100 hPa (±30–50%), where sampling issues add uncertainty due to large gradients and high natural variability in water vapour. The knowledge gained from these comparisons and regarding the quality of the individual data sets in different regions of the atmosphere will help to improve model-measurement comparisons (e.g., for diagnostics such as the tropical tape recorder or seasonal cycles), data merging activities, and studies of climate variability. The full abstract can be found here.

S. Oberländer and co-authors investigate the different processes affecting the Brewer Dobson Circulation in future using the EMAC chemistry-climate model in a new JGR article. Using several sensitivity simulations they isolate the effects of external forcings such as greenhouse gases, sea surface temperatures (SSTs) and ozone-depleting substances. They find that in boreal winter the tropical upward mass flux increases by about 1%/decade (2%/decade) in the upper (lower) stratosphere until the end of the 21^st century. The mean stratospheric age of air decreases by up to 60 and 30 days/decade, respectively. Changes in transient planetary and synoptic waves account for the strengthening of the BDC in the lower stratosphere, whereas upper stratospheric changes are due to improved propagation properties for gravity waves in future climate. The radiative impact of increasing GHG concentrations is detected only in the upper stratosphere, whereas the effect of increasing SSTs dominates the lower stratospheric signal. Changes in tropical SSTs influence not only the shallow but also the deep branch of the BDC as confirmed from both changes in residual circulation and mixing. Declining ODSs were found to slightly counteract the BDC increase in the Southern Hemisphere. The full abstract can be found here.

A recent GRL paper by M. McGillen and co-authors presents measurements of the CFC-11 (CFCl3) absorption spectrum over various wavelengths (184.95–230nm) and temperatures (216–296K). Uncertainty in the temperature dependence, particularly in the UV absorption spectrum, is a significant contributing factor of overall uncertainty in CFC-11’s global lifetime. They find that the spectrum temperature dependence is less than that currently in use and that this slightly reduces the CFC-11 lifetime calculated with a 2D model using a spectrum parameterization developed in this work. Find the full abstract here.

Using both satellite observations and chemistry-climate models, C.I. Garfinkel and co-authors examine the zonal structure of tropical lower stratospheric temperature, water vapour, and ozone trends in a recent JGR article. Trends in both the tropical upper troposphere (warming) and lower stratosphere (cooling) have been strongest over the Indo-Pacific warm pool region and much weaker over the western and central Pacific. The model simulations suggest that the sea surface temperatures (SSTs) drive this zonal asymmetry with warming SSTs in the Indian Ocean and warm pool region having led to enhanced moist heating in the upper troposphere, and in turn to a Gill-like response that extends into the lower stratosphere. This has led to a zonal structure in ozone and water vapour trends and subsequently to less water vapour entering the stratosphere. Projected future SSTs drive a similar zonally-structure response in temperature and water vapour, which, for the lower stratosphere are similar in strength to that due directly to projected future CO2, ozone, and methane. The full abstract can be found here.

At a press conference held today in Stockholm, Sweden, the Summary for Policymakers of the Working Group I contribution of AR5 on Climate Change 2013: The Physical Science Basis was presented by the Co-Chairs Dahe Qin and Thomas Stocker.

Find the presentation by Dahe Qin and Thomas Stocker, IPCC Working Group I Co-Chairs.

Find the Summary for Policy Makers and approved final draft of the Scientific-technical Report; of particular interest to the SPARC community are Chapter 2 on Observations: Atmosphere and Surface, and Chapter 7 on Clouds and Aerosols.

Find online registration and programme at http://royalsociety.org/events/2013/climatescience-next-steps/

Find full report at http://www.ncdc.noaa.gov/bams-state-of-the-climate/2012.php.

Impact of a potential 21st century “grand solar minimum” on surface temperatures and stratospheric ozone

A new GRL article by J. Anet and co-authors investigates the effects of recently proposed 21^st century Dalton-minimum-like decline of solar activity on the climate and ozone layer. Using the IPCC RCP 4.5 scenario and the SOCOL3-MPIOM coupled ocean-atmosphere model, they find that a future grand solar minimum reduces mean global surface warming by 0.2-0.3K and that the decrease in solar UV radiation leads to a significant delay in the recovery of stratospheric ozone by up to 10 years and longer. The full abstract can be found here.