B. Steil

Multimodel projections of stratospheric ozone in the 21st century

[1] Simulations from eleven coupled chemistry-climate models (CCMs) employing nearly identical forcings have been used to project the evolution of stratospheric ozone throughout the 21st century. The model-to-model agreement in projected temperature trends is good, and all CCMs predict continued, global mean cooling of the stratosphere over the next 5 decades, increasing from around 0.25 K/decade at 50 hPa to around 1 K/ decade at 1 hPa under the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) A1B scenario. In general, the simulated ozone evolution is mainly determined by decreases in halogen concentrations and continued cooling of the global stratosphere due to increases in greenhouse gases (GHGs). Column ozone is projected to increase as stratospheric halogen concentrations return to 1980s levels. Because of ozone increases in the middle and upper stratosphere due to GHGinduced cooling, total ozone averaged over midlatitudes, outside the polar regions, and globally, is projected to increase to 1980 values between 2035 and 2050 and before lowerstratospheric halogen amounts decrease to 1980 values. In the polar regions the CCMs simulate small temperature trends in the first and second half of the 21st century in midwinter. Differences in stratospheric inorganic chlorine (Cly) among the CCMs are key to diagnosing the intermodel differences in simulated ozone recovery, in particular in the Antarctic. It is found that there are substantial quantitative differences in the simulated Cly, with the October mean Antarctic Cly peak value varying from less than 2 ppb to over 3.5 ppb in the CCMs, and the date at which the Cly returns to 1980 values varying from before 2030 to after 2050. There is a similar variation in the timing of recovery of Antarctic springtime column ozone back to 1980 values. As most models underestimate peak Clynear 2000, ozone recovery in the Antarctic could occur even later, between 2060 and 2070. In the Arctic the column ozone increase in spring does not follow halogen decreases as closely as in the Antarctic, reaching 1980 values before Arctic halogen amounts decrease

Chemistry-climate model simulations of twenty-first century stratospheric climate and circulation changes

The response of stratospheric climate and circulation to increasing amounts of greenhouse gases (GHGs) and ozone recovery in the twenty-first century is analyzed in simulations of 11 chemistry–climate models using near-identical forcings and experimental setup. In addition to an overall global cooling of the stratosphere in the simulations (0.59 6 0.07 K decade 21 at 10 hPa), ozone recovery causes a warming of the Southern Hemisphere polar lower stratosphere in summer with enhanced cooling above. The rate of warming correlates with the rate of ozone recovery projected by the models and, on average, changes from 0.8 to 0.48 K decade 21 at 100 hPa as the rate of recovery declines from the first to the second half of the century. In the winter northern polar lower stratosphere the increased radiative cooling from the growing abundance of GHGs is, in most models, balanced by adiabatic warming from stronger polar downwelling. In the Antarctic lower stratosphere the models simulate an increase in low temperature extremes required for polar stratospheric cloud (PSC) formation, but the positive trend is decreasing over the twenty-first century in all models. In the Arctic, none of the models simulates a statistically significant increase in Arctic PSCs throughout the twentyfirst century. The subtropical jets accelerate in response to climate change and the ozone recovery produces a westward acceleration of the lower-stratospheric wind over the Antarctic during summer, though this response is sensitive to the rate of recovery projected by the models. There is a strengthening of the Brewer–Dobson

Assessment of the future development of the ozone layer

ECHAM3/CHEM is used to estimate the future development of the ozone layer. The general circulation model ECHAM3 and the chemistry module CHEM are coupled in a CTM-like mode, i.e. no feedback of simulated chemical species on radiation is considered. Currently CHEM does not include bromine chemistry. Two time-slice experiments representing 1991 and 2015 conditions are carried out. Chemical species are transported by winds calculated with different CO 2 mixing ratios as a proxy for other greenhouse-gases. For 2015, the adopted increase of CO 2 and the corresponding modification of the sea-surface temperature lead to a warming of the troposphere and a cooling of the stratosphere. The assessment for 2015 indicates that the ozone layer will not homogeneously recover, despite the employed decrease of chlorine in the model. Whereas in low and mid-latitudes an ascent of stratospheric O 3 is obvious, no significant increase of O 3 is found in the polar regions during spring time.

Impact of stratospheric dynamics and chemistry on northern hemisphere midlatitude ozone loss

The importance of dynamics for stratospheric ozone distribution in the northern hemisphere is investigated by using multiannual simulations of the coupled dynamic-chemical general circulation model ECHAM3/CHEM. This model includes a parameterization for heterogeneous reactions on the surfaces of polar stratospheric clouds (PSCs) and on sulfate aerosols. A warm and a cold stratospheric winter are examined to estimate the range of chemical ozone loss in the model due to heterogeneous reactions on PSCs. Ozone depletion in the model mainly occurs inside the polar vortex. An additional ozone reduction due to heterogeneous reactions on PSCs is found outside the polar vortex. Secondary vortex formation and vortex contraction after an elongation lead to a transport of air masses with chemically reduced ozone values out of the vortex. Except for such events the edge of the modeled polar vortex acts as a barrier to transport. During the formation of secondary vortices no additional heterogeneous reactions occur therein. Other dynamic events, such as the elongation of the polar vortex and its displacement to lower latitudes, lead to an intense ozone depletion. A minor stratospheric warming in the model causes a total deactivation of chlorine compounds and prevents further ozone depletion. In midlatitudes, the amplitude of short-term variations of total ozone is amplified by PSC heterogeneous chemical ozone reduction.

The summer circulation over the Eastern Mediterranean and the Middle East: Influence of the South Asian monsoon and mid-latitude dynamics

The summer circulation in the Eastern Mediterranean and the Middle East (EMME) is dominated by persistent northerly winds (Etesians) whose ventilating effect counteracts the adiabatic warming induced by subsidence prevailing over the eastern Mediterranean. The ERA40 dataset is used to investigate the South Asian Monsoon and mid-latitude influences on the EMME circulation. Consistent with past modeling studies, in late spring an upper level warm structure and subsidence area expanding towards the EMME are identified, attributed to Rossby waves excited by monsoon convection. Steep sloping isentropes develop over the EMME with subsidence mainly over the eastern Mediterranean and Iran, where orographically induced circulation patterns enhance the mid-latitude northwesterly flow and the air mass subsidence along isentropes. These phenomena have a maximum in July and are strikingly synchronous to the convection over northern India where the background state favors a stronger Rossby wave response. The monsoon induced large-scale background state over the EMME is modified by synoptic activity originating in the Atlantic that introduces high frequency variability over the EMME. During ‘etesian outbreaks’ a ridge develops over the Balkans and sharp tropopause folds appear over the Aegean.