[1] Climate change in the Southern Hemisphere (SH) has been robustly documented in the last several years. It has altered the atmospheric circulation in a surprising number of ways: a rising global tropopause, a poleward intensification of the westerly jet, a poleward shift in storm tracks, a poleward expansion of the Hadley cell, and many others. While these changes have been extensively related with anthropogenic warming resulting from the increase in greenhouse gases, their potential link to stratospheric cooling resulting from ozone depletion has only recently been examined and a comprehensive picture is still lacking. Examining model output from the coupled climate models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment (AR4), and grouping them depending on the stratospheric ozone forcing used, we here show that stratospheric ozone affects the entire atmospheric circulation in the SH, from the polar regions to the subtropics, and from the stratosphere to the surface. Furthermore, model projections suggest that the anticipated ozone recovery, resulting from the implementation of the Montreal Protocol, will likely decelerate future climate change resulting from increased greenhouse gases, although it might accelerate surface warming over Antarctica. [2] Recent studies have shown that, in the late 20th century, depletion of stratospheric ozone by anthropogenic halogen compounds has affected the SH surface climate by forcing sea-level-pressure (SLP) to decrease in high latitudes and to increase in mid latitudes [Thompson and Solomon, 2002;Marshall, 2003]. This dipolar SLP change, which is robustly found in climate model integrations [Gillett and Thompson, 2003;Shindell and Schmidt, 2004;Miller et al., 2006;Arblaster and Meehl, 2006;Cai and Cowan, 2007], is qualitatively similar to the so-called Southern Annular Mode (SAM) [Thompson et al., 2000], and intimately related to the latitudinal position of the midlatitude jet [Thompson et al., 2000;Son et al., 2008]. While this suggests that stratospheric ozone plays an important role in controlling tropospheric circulation [e.g., Gillett and Thompson, 2003], its detailed impact on the various components of the SH climate system, from the polar regions to the subtropics, remains unclear at present. This question is especially pertinent in view of the predicted recovery of stratospheric ozone [e.g., Eyring et al., 2007] which, as a result of the implementation of the Montreal Protocol, is expected to occur in the next 50-60 years.[3] In this study, we demonstrate the pervasive impacts of stratospheric ozone on SH climate change by examining output from the IPCC/AR4 models [Meehl et al., 2007]. Both past (20C3M) and future scenario integrations with moderate greenhouse-gas forcing (A1B) are analyzed, ozone forcing being the key discriminating factor among the models. As indicated in Table 1, not all IPCC/AR4 models included ozone depletion and recovery [Miller et al., 2006;Cai and Cowan, 2007;Karpechko et al., 2008;Son et al., 2008...
[1] The spatiotemporal structure of the lapse-rate tropopause is examined by using state-of-the-art Global Positioning System radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Formosa Satellite Mission 3 mission. The high temporal and spatial resolutions of the data reveal the detailed structure of tropopause properties such as pressure (p t ), temperature (T t ), and sharpness (N t 2 ) and their relationships to upper tropospheric and lower stratospheric processes. The overall results are generally in good agreement with previous studies. The climatology of all three tropopause properties shows largely homogeneous structure in the zonal direction: noticeable asymmetries are found only in the tropics and the Northern Hemisphere extratropics during boreal winter owing to localized tropospheric processes. This contrasts with the seasonal cycles of tropopause properties which are significantly influenced by stratospheric processes such as the Brewer-Dobson circulation, the polar vortex, and the radiative processes near the tropopause. On intraseasonal time scales, p t and T t exhibit significant variability over the Asian summer monsoon and the subtropics where double tropopauses frequently occur. In contrast, N t 2 shows maximum variability in the tropics where p t and T t have minimum variability, possibly a consequence of vertically propagating waves. The tropopause properties derived from COSMIC observations are further applied to evaluate tropopause data directly available from the NCEP-NCAR Reanalysis (NNR). Although the NNR tropopause data have been widely used in climate studies, they are found to have significant and systematic biases, especially in the subtropics. This suggests that the NNR tropopause data should be treated with great caution in any quantitative studies.
This study seeks a deeper understanding of the causes of Hadley Cell (HC) expansion, as projected under global warming, and HC contraction, as observed under El Niño. Using an idealized general circulation model, the authors show that a thermal forcing applied to a narrow region around the equator produces "El Nino-like" HC contraction, while a forcing with wider meridional extent produces "global warming-like" HC expansion. These circulation responses are sensitive primarily to the thermal forcing's meridional structure and are less sensitive to its vertical structure. If the thermal forcing is confined to the midlatitudes, the amount of HC expansion is more than three times that of a forcing of comparable amplitude that is spread over the tropics. This finding may be relevant to recently observed trends of rapid tropical widening.The shift of the HC edge is explained using a very simple model in which the transformed Eulerian mean (TEM) circulation acts to diffuse heat meridionally. In this context, the HC edge is defined as the downward maximum of residual vertical velocity in the upper troposphere iiJ*^^; this corresponds well with the conventional Eulerian definition of the HC edge. In response to a positive thermal forcing, there is anomalous diabatic cooling, and hence anomalous TEM descent, on the poleward flank of the thermal forcing. This causes the HC edge (í5j,gx) to shift toward the descending anomaly, so that a narrow forcing causes HC contraction and a wide forcing causes HC expansion.
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