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.

Over the last two decades of the 20^th century the southern hemisphere stratospheric stationary wave amplitude increased in late spring and early summer. Using the results from several chemistry-climate models, L. Wang and co-authors separate the effects of anthropogenic forcing from ozone-depleting substances (ODSs) and greenhouse gases (GHGs) on these changes. The increase in amplitude is reproduced in simulations with changing ODSs, a response related to changes in the strength and timing of the breakdown of the polar vortex. GHGs have little impact on the simulated stationary wave amplitude, but are projected to induce an eastward shift of the waves, which is linked to the strengthening of the subtropical jet. The full abstract can be found here.

A recent JGR article by A. Kunz and co-authors uses thirty years of balloon-borne measurements to investigate the water vapour trend in the tropopause region over Boulder, Colorado, USA. They apply two new concepts: trends are presented on a thermal tropopause (TP) relative coordinate system and sonde profiles are selected according to TP height. Although this should reduce the dynamically induced water vapour variability at the TP, their results suggest there is still significant uncertainty in trends at altitudes -2 to +4km around the TP. This uncertainty in turn has an influence on the uncertainty and interpretation of water vapour radiative effects at the TP, which are locally estimated for the 30-year period to be of uncertain sign. Their results also do not indicate any detectable decrease in water vapour at the beginning of 2001. However, on the lower stratospheric isentropes, the water vapour change for this period is stronger for extra-tropical than for tropical air mass types, suggesting a possible link to changing dynamics above the jet stream. The full abstract can be found here.

W. Yuan and co-authors investigate the influence of the El Niño/Southern Oscillation (ENSO) on the quasi-biennial oscillation (QBO) modulation of cold-point tropopause (CPT) temperatures. Using almost five decades of radiosonde data from eleven near-equatorial stations, they show that the ENSO influence on the QBO is quite zonally symmetric. The data indicate that the QBO has larger amplitude and longer period during La Niña conditions than during El Niño. Their results also indicate that the warmer CPT temperatures during QBO westerly shear conditions and colder temperatures during QBO easterly shear conditions, are larger during La Niña than during El Niño. This strengthens earlier findings that the greatest dehydration of air entering the stratosphere from the troposphere occurs during the winter under La Niña and easterly QBO conditions. The full abstract can be found here.

A new ACPD article by H. Zhang and co-authors investigates the effects of recovering stratospheric ozone on tropospheric chemistry and air quality using the global chemistry-transport model GEOS-Chem. They find that surface ozone photolysis rates decrease significantly while ozone lifetime in the troposphere increases by up to 7% and the tropospheric ozone burden increases slightly (0.78%). Perturbations of tropospheric and surface ozone show large seasonal and spatial variations, with increases of up to 5% for some regions. The full abstract can be found here.

Recent Arctic ozone loss has been linked with climate change resulting from increasing greenhouse gases. In a recent GRL paper, H. Rieder and co-authors provide evidence to the contrary, by focusing on the volume of polar stratospheric clouds (PSCs), a simple proxy for polar ozone loss. Analysis of three reanalysis datasets and results from a stratosphere-resolving chemistry-climate model indicate no statistically significant trends in PSC volume, nor any change in their probability density functions, during the period 1979-2011. 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.

Drivers of hemispheric differences in return dates of mid-latitude stratospheric ozone

In a new ACP paper, H. Garny and co-authors investigate the main factors driving the hemispherical asymmetry in ozone return dates. They find that the hemispherical return date differences, which range between 0-30 years across the CCM projections analysed, are affected by both the sensitivity of ozone to Cly (ozone trends have a larger effect on return dates when sensitivity is lower) and the difference in ozone trends between hemispheres. An attribution analysis performed with two CCMs shows that chemically-induced changes in ozone are the major driver of the earlier return of ozone to 1980 levels in northern mid-latitudes. The causes for chemically-induced asymmetric ozone trends relevant for the total column ozone return date differences are found to be (i) stronger increases in ozone production due to enhanced NOx concentrations in the Northern Hemisphere lowermost stratosphere and troposphere, (ii) stronger decreases in the destruction rates of ozone by the NOx cycle in the Northern Hemisphere lower stratosphere linked to effects of dynamics and temperature on NOx concentrations, and (iii) an increasing efficiency of heterogeneous ozone destruction by Cly in the Southern Hemisphere mid-latitudes as a result of decreasing lower stratospheric temperatures. The full abstract can be found here.