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.

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.

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.

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.

A new ACP article by N.A. Kramarova and co-authors presents a validation of ozone profiles from the Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments that were recently reprocessed using an updated (version 8.6) algorithm. The SBUV data record covers the period 1970-2011, with a 5 year gap in the 1970s. Validation with MLS (on board the UARS and Aura satellites) and SAGE-II satellite observations, as well as ground-based observations from microwave spectrometers, lidars, Umkehr instruments and balloon-borne ozonesondes. They find that in the stratosphere, between 25 and 1 hPa, the mean biases and standard deviations are mostly within 5% for monthly zonal mean ozone profiles. Above and below this layer the vertical resolution of the SBUV algorithm decreases. In order to account for the lower resolution in the troposphere/lower stratosphere, they combine several layers of data. The drift of the SBUV instruments is also estimated, as well as its potential effect on the long-term stability of the combined data record. The features of individual SBUV(/2) instruments are discussed and recommendations for creating a merged SBUV data set are provided. The full abstract can be found here.

A recent JGR article by J. Hsu and co-authors uses the NCAR Community Atmosphere Model to evaluate the sensitivity of stratospheric dynamics to the uncertainty in ozone production related to uncertainty in O2 cross-sections in the Hertzberg Continuum. Reducing the O2 cross-sections by 30% is found to increase ozone abundances in the lower stratosphere, which as a result warms between 60°S -60°N (2K maximum at the equator) and lowers the tropopause height by 100-200m between 30°S -30°N. The study points to the important role of ozone in the lower tropical stratosphere in determining the physical characteristics of the tropical tropopause layer. The full abstract can be found here.

The effects of imposed stratospheric cooling on the maximum intensity of tropical cyclones

Using a cloud-resolving model, H. Ramsay presents results that elucidate the effect of stratospheric cooling and sea surface warming on the potential intensity (PI) of tropical cyclones in a recent article in the Journal of Climate. With fixed sea surface temperatures, cooling near and above the tropopause (~90hPa) is shown to increase PI at a rate of 1m/s per degree cooling. With fixed stratospheric temperatures, sea surface warming increases the PI by approximately twice as much, as a rate of about 2m/s per degree warming. These results have significant implications in terms of global tropical cyclone PI trends in response to climate change. Tropical sea surface temperatures have warmed by about 0.15K/decade since the 1970s, while the stratosphere has cooled anywhere from 0.3K/decade to over 1K/decade, depending on the data set used. Therefore, global PI trends in recent decades appear to have been driven more by stratospheric cooling than by surface warming. Find the full abstract here.

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.