[1] The role of stratospheric ozone recovery in the Southern Hemisphere climate system, in the coming decades, is examined by contrasting two 10‐member ensembles of Community Atmospheric Model (CAM3) integrations, over the period 2000–2060. Model integrations in the first ensemble are conducted with a complete set of forcings: greenhouse gas concentrations from the A1B scenario, SSTs from corresponding ocean‐atmosphere coupled model integrations, and ozone starting with severe depletion over the South Pole and recovering by mid‐century. The integrations in the second ensemble are very similar to the first, except that only the transient ozone forcing is specified, and all other forcings are kept at year 2000 levels. Specifying ozone recovery in isolation allows us to determine unambig-uously how it impacts the atmospheric circulation. We find that, in DJF, most key indices of atmospheric circulation show significant trends in the second ensemble, due to the closing of the ozone hole. In the first ensemble, however, trends are found to be statistically insignificant for nearly all key circu-lation indices. This suggests that ozone recovery will result in a nearly complete cancellation (and possible reversal) of the atmospheric circulation effects associated with increasing greenhouse gases, in Southern Hemisphere summer, over the coming half century. Citation: Polvani, L. M., M. Previdi, and

One pronounced feature in observed latitudinal dependence of lower-stratospheric temperature trends is the enhanced cooling near 308 latitude in both hemispheres. The observed phenomenon has not, to date, been explained in the literature. This study shows that the enhanced cooling is a direct response of the lower-strato-spheric temperature to the poleward shift of subtropical jets. Furthermore, this enhanced lower-stratospheric cooling can be used to quantify the poleward shift of subtropical jets. Using the lower-stratospheric tem-peratures observed by satellite-borne microwave sounding units, it is shown that the subtropical jets have shifted poleward by 0.68 6 0.18 and 1.08 6 0.38 latitude in the Southern and Northern Hemispheres, re-spectively, in last 30 years since 1979, indicating a widening of tropical belt by 1.68 6 0.48 latitude. 1.

[1] Historical radiosonde data are analyzed using the tropopause height frequency method to investigate the variation of the Southern Hemisphere tropical edge from 1979/80–2010/11, independently of reanalysis-derived data. Averaged across the hemisphere we identify a tropical expansion trend of 0.41 0.37 deg dec1, significant at the 90 % level. A comparison with four reanalyses shows generally consistent results between radiosondes and reanalyses. Estimated rates of tropical expansion in the SH are broadly similar, as is the interannual variability. However, notable differences remain. Some of these differences are related to the methodology used to identify the height of the tropopause in the reanalyses, which produces inconsistent results in the subtropics. Differences between radiosondes and reanalyses are also more manifest in data-poor regions. In these regions, the reanalyses are not fully constrained, allowing the internal model dynamics to drive the variability. The performance of the reanalyses varies temporally compared to the radiosonde data. These differences are particularly apparent

We investigate the impact of stratospheric ozone deple-tion on Antarctic climate, paying particular attention to the question of whether eddy parameterizations in the ocean fundamentally alter the results. This is ac-complished by contrasting two versions of the Commu-nity Climate System Model (version 3.5), one at 0.1◦ ocean and sea ice resolution and the other at 1 ◦ with parameterized ocean eddies. At both resolutions, pairs of integrations are performed: one with high (1960) and one with low (2000) ozone levels. We find that the ef-fect of ozone depletion is to warm the surface and the ocean to a depth of 1000m and to significantly reduce the sea ice extent. While the ocean warming is some-what weaker when the eddies are resolved, the total loss of sea ice area is roughly the same in the fine and coarse resolution cases. 1.

Analysis of the annual cycle of intensity, extent, and width of the Hadley circulation across a 31-yr period (1979–2009) from all existent reanalyses reveals a good agreement among the datasets. All datasets show that intensity is at a maximum in the winter hemisphere and at a minimum in the summer hemisphere. Maximum and minimum values of meridional extent are reached in the respective autumn and spring hemispheres. While considering the horizontal momentum balance, where a weakening of theHadley cell (HC) is expected in associationwith a widening, it is shown here that there is no direct relationship between intensity and extent on a monthly time scale. All reanalyses show an expansion in both hemispheres, most pronounced and statistically significant during summer and autumn at an average rate of expansion of 0.558 decade21 in each hemisphere. In contrast, intensity trends are inconsistent among the datasets, although there is a tendency toward intensification, particularly in winter and spring. Correlations between the HC and tropical and extratropical large-scale modes of variability suggest in-teractions where the extent of the HC is influenced by El Niño–SouthernOscillation (ENSO) and the annular modes. The cells tend to shrink (expand) during the warm (cold) phase of ENSO and during the low (high) phase of the annular modes. Intensity appears to be influenced only by ENSO and only during spring for the southern cell and during winter for the northern cell. 1.

[1] We investigate the effect of stratospheric ozone recovery onAntarctic sea ice in the next half-century, by comparing two ensembles of integrations of the Whole Atmosphere Com-munity Climate Model, from 2001 to 2065. One ensemble is performed by specifying all forcings as per the Representative Concentration Pathway 4.5; the second ensemble is identical in all respects, except for the surface concentrations of ozone depleting substances, which are held fixed at year 2000 levels, thus preventing stratospheric ozone recovery. Sea ice extent declines in both ensembles, as a consequence of increasing greenhouse gas concentrations. However, we find that sea ice loss is 33 % greater for the ensemble in which stratospheric ozone recovery does not take place, and that this effect is sta-tistically significant. Our results, which confirm a previous study dealing with ozone depletion, suggest that ozone recovery will substantially mitigate Antarctic sea ice loss in the

The Antarctic continent contains the majority of the global ice volume and plays an important role in a changing climate. The nature and causes of Antarctic climate variability are, however, poorly understood beyond interannual time scales due to the paucity of long, reliable meteorological observations. This study analyzes decadal–interdecadal climate variability over Antarctica using a network of annually resolved ice core records and various instrumental and tropical proxy data for the nineteenth and twentieth centuries. During the twentieth century, Antarctic ice core records indicate strong linkages to sea surface temperature (SST) variations in the tropical Pacific and Atlantic on decadal–interdecadal time scales. Antarctic surface temperature anomalies inferred from the ice cores are consistent with the associated changes in atmospheric circulation and thermal advection. A set of atmospheric general circulation model experiments supports the idea that decadal SST variations in the tropics force atmospheric teleconnections that affect Antarctic surface temperatures. When coral and other proxies for tropical climate are used to extend the analysis back to 1799, a similar Antarctic–tropical Pacific linkage is found, although the relationship is weaker during the first half of the nineteenth century. Over the past 50 years, a change in the phase of Pacific and Atlantic interdecadal variability may have contributed to the rapid warming of the Antarctic Peninsula and West Antarctica and related changes in ice sheet dynamics. 1.

Recent changes in the summer climate of the Southern Hemi-sphere extra-tropics are primarily related to the dominance of the positive phase of the Southern Annular Mode1,2. This shift in the behaviour of the Southern Annular Mode—essentially a measure of the pressure gradient between Southern Hemi-sphere mid and high latitudes—has been predominantly in-duced by polar stratospheric ozone depletion2–4. The concomi-tant southward expansion of the dry subtropical belts5,6 could have consequences for forest growth. Here, we use tree-ring records from over 3,000 trees in South America, Tasmania and New Zealand to identify dominant patterns of tree growth in recent centuries. We show that the foremost patterns of growth between 1950 and 2000 differed significantly from those in the previous 250 years. Specifically, growth was higher than the long-term average in the subalpine forests of Tasmania and

Stratospheric ozone is expected to recover by the end of this century because of the regulation of ozone-depleting substances by the Montreal Protocol. Targeted modeling studies have suggested that the climate response to ozone recovery will greatly oppose the climate response to rising greenhouse gas (GHG) emis-sions. However, the extent of this cancellation remains unclear since only a few such studies are available. Here, a much larger set of simulations performed for phase 5 of the Coupled Model Intercomparison Project is analyzed, which includes ozone recovery. It is shown that the closing of the ozone hole will cause a delay in summertime [December–February (DJF)] Southern Hemisphere climate change between now and 2045. Specifically, it is found that the position of the jet stream, the width of the subtropical dry zones, the sea-sonality of surface temperatures, and sea ice concentrations all exhibit significantly reduced summertime trends over the first half of the twenty-first century as a consequence of ozone recovery. After 2045, forcing from GHG emissions begins to dominate the climate response. Finally, comparing the relative influences of future GHG emissions and historic ozone depletion, it is found that the simulated DJF tropospheric circu-lation changes between 1965 and 2005 (driven primarily by ozone depletion) are larger than the projected changes in any future scenario over the entire twenty-first century. 1.

Abstract not found