A recent JGR article by B. Kravitz and co-authors presents results from one of the GeoMIP model experiments (experiment G1). This experiment looks at the climate response of 12 models to an abrupt quadrupling of CO2 from preindustrial concentrations brought into radiative balance via a globally uniform reduction in insolation. The model results indicate that this reduction offsets global mean surface temperature to a large extent and also prevents 97% of Arctic sea-ice loss. However, compared to the preindustrial climate, the tropics are cooler (-0.3K) while the poles are warmer (+0.8K). Annual mean precipitation minus evaporation anomalies remain largely unchanged, except over some tropical regions where precipitation is slightly reduced. Global average net primary production increases by 120% above simulated preindustrial levels, mainly because of CO2 fertilisation, but also because of reduced plant heat stress compared to a world without geoengineering. Importantly, however, all models show that uniform solar geoengineering in the G1 experiment cannot simultaneously return regional and global temperature and hydrologic cycle intensity to preindustrial levels. The full abstract can be found here.

J. Kim and co-authors focus on the cold-point tropopause region as simulated by CMIP-5 models in a new JGR article. They look at the climatology, seasonality, and intraseasonal to interannual variability of the temperature field near the cold-point tropopause (CPT) in all CMIP-5 models for both historical simulations and future projections (using RCP8.5). The CPT temperature is estimated using both the 100-hPa and zero-lapse-rate (ZLR) temperatures. The historical simulations reproduce the spatio-temporal structure of the CPT temperature as well as the interannual variability associated with ENSO and the intraseasonal variability associated with equatorial waves successfully. However, the models show non-negligible biases in several aspects: 1) most models have a warm bias around the CPT; 2) large inter-model differences occur in the amplitude of the seasonal cycle in 100-hPa temperature; 3) several models overestimate lower stratospheric warming in response to volcanic aerosols; 4) temperature variability associated with the quasi-biennial oscillation and Madden-Julian oscillation is absent in most models; 5) equatorial waves near the CPT exhibit a wide range of variations among the models with unrealistically persistent Kelvin waves in several models. In terms of future projections, the models predict a robust warming at both the 100hPa and ZLR levels, but cooling at the 70hPa level. Most models also project a weakened seasonal cycle of temperature at both 100hPa and 70hPa levels. These findings may have important implications for cross-tropopause water vapour transport and related global climate change and variability. The full abstract can be found here.

Find the Future Earth July 2013 newsletter.

An ACP article by F. Jégou and co-authors investigates the emissions released into the stratosphere by the Sarychev volcano, on the Kuril Islands, northeast of Japan. The strongest sulphur dioxide emissions occurred from 15-16 April 2009 and an estimated 0.9Tg were injected into the UTLS. By the first week of July the aerosol plume had spread over much of the Arctic region, and was measured by several satellites (OSIRIS, CALIOP), at the surface (lidar), as well as by balloon-borne instruments during the STAC campaign. The OSIRIS stratospheric aerosol optical depth observations indicate an enhancement at 750nm of a factor of 6, with a value of 0.02 in late July compared to 0.0035 before the eruption. Model simulations of the eruption using the HadGEM2 and MIMOSA models agree well with the satellite, in situ, and ground-based observations. Aerosol concentrations returned to near-background levels by spring 2010. The full abstract can be found here.

F. Ploeger and co-authors compare MLS water vapour observations with simulations from the CLaMS model to investigate pathways of water vapour transport in the lower stratosphere during the boreal summer in a new JGR article. In the northern hemisphere (NH) mid- and high latitudes, a negative correlation is found between water vapour and ozone daily tendencies, indicative of frequent horizontal transport from the low latitudes. Analysis of the zonal mean tracer continuity equation shows that poleward of 50°N eddy mixing dominates the horizontal water vapour transport, while in the subtropics horizontal transport is largely due to the residual circulation. Model simulations with transport barriers confirm that most of the annual cycle in NH mid-latitude water vapour above 360K results from horizontal transport from low latitudes. The full abstract can be found here.

J. Liu and co-authors have produced a new ozone climatology utilising ozone soundings and trajectory techniques. The data cover the period 1965-2008, and are available as monthly means at a resolution of 5°x5° from the surface through to 26km. The dataset compares well with SAGE and OSIRIS seasonal and zonal means, with maximum differences of 20% found above 15km. Agreement is better in the northern hemisphere, where more ozone soundings are available, and in the mid- and high latitudes, where reanalysis winds are more accurate. The full abstract can be found here.

P. Baron and co-authors present stratospheric wind observations between 8-0.01hPa (~35-80km) from the SMILES instrument from October 2009 to April 2010. Good agreement between the ECMWF analysis and the observations is found, except for the zonal winds over the equator, where differences greater than 5m/s are found. The results demonstrate that the SMILES instrument and others like it have the potential to provide high quality observations of stratospheric winds. The full abstract can be found here.

D.M. Murphy and co-authors present results distinguishing various stratospheric aerosol particles and their components in a new article in the Quarterly Journal of the Royal Meteorological Society. Under background conditions, between major volcanic eruptions, they find that most stratospheric aerosol are either relatively pure sulphuric acid, sulphuric acid mixed with meteoritic material, or mixed organic-sulphate particles originating from the troposphere. Certain meteoritic elements are dissolved in the particles (e.g. iron and magnesium), while others are found as solid phase inclusions. These solid phases could have large but unknown implications in terms of the particles acting as freezing nuclei for polar stratospheric clouds. Find the full abstract here.

Using a new merged ozone data set, C.E. Sioris and co-authors investigate the trends and variability of ozone in the tropical lower stratosphere. The results presented in a recent ACPD article indicate a statistically significant negative trend at all altitudes from 18-25km over the period 1984-2012 in the merged SAGE-II/OSIRIS dataset. Trends reach up to -6.5% per decade at 18.5km, with underlying strong variations from ENSO, the QBO and tropopause height. The full abstract can be found here.

A recent GRL paper by K.M. Grise and co-authors looks at the effects of the poleward shift in southern hemisphere (SH) tropospheric circulation induced by the Antarctic ozone hole. Using climate model simulations in which only stratospheric ozone depletion is specified, they find that high and mid-level clouds follow the poleward shift of the SH mid-latitude jet, and that low-level clouds decrease over much of the Southern Ocean. The annual hemispheric mean radiation response to the cloud anomalies is estimated at approximately +0.25 W m-2, largely a response to the reduction of total cloud fraction in the SH mid-latitudes in austral summer. Find the full abstract here.