A comparison of model‐simulated trends in stratospheric temperatures

Abstract

SUMMARYEstimates of annual-mean stratospheric temperature trends over the past twenty years, from a wide variety of models, are compared both with each other and with the observed cooling seen in trend analyses using radiosonde and satellite observations. The modelled temperature trends are driven by changes in ozone (either imposed from observations or calculated by the model), carbon dioxide and other relatively well-mixed greenhouse gases, and stratospheric water vapour.The comparison shows that whilst models generally simulate similar patterns in the vertical pro le of annualand global-mean temperature trends, there is a signi cant divergence in the size of the modelled trends, even when similar trace gas perturbations are imposed. Coupled-chemistry models are in as good agreement as models using imposed observed ozone trends, despite the extra degree of freedom that the coupled models possess.The modelled annual-and global-mean cooling of the upper stratosphere (near 1 hPa) is dominated by ozone and carbon dioxide changes, and is in reasonable agreement with observations. At about 5 hPa, the mean cooling from the models is systematically greater than that seen in the satellite data; however, for some models, depending on the size of the temperature trend due to stratospheric water vapour changes, the uncertainty estimates of the model and observations just overlap. Near 10 hPa there is good agreement with observations. In the lower stratosphere (20-70 hPa), ozone appears to be the dominant contributor to the observed cooling, although it does not, on its own, seem to explain the entire cooling.Annual-and zonal-mean temperature trends at 100 hPa and 50 hPa are also examined. At 100 hPa, the modelled cooling due to ozone depletion alone is in reasonable agreement with the observed cooling at all latitudes. At 50 hPa, however, the observed cooling at midlatitudes of the northern hemisphere signi cantly exceeds the modelled cooling due to ozone depletion alone. There is an indication of a similar effect in high northern latitudes, but the greater variability in both models and observations precludes a rm conclusion.The discrepancies between modelled and observed temperature trends in the lower stratosphere are reduced if the cooling effects of increased stratospheric water vapour concentration are included, and could be largely removed if certain assumptions were made regarding the size and distribution of the water vapour increase. However, given the uncertainties in the geographical extent of water vapour changes in the lower stratosphere, and the time period over which such changes have been sustained, other reasons for the discrepancy between modelled and observed temperature trends cannot be ruled out.

Quantifying the contributions to stratospheric ozone changes from ozone depleting substances and greenhouse gases

“…These results are broadly in agreement with previous estimates of the relative contribution of ozone depletion and CO 2 cooling to the upper stratospheric temperature trends (Shine et al, 2003;Jonsson et al, 2009). The exact contribution assigned to ODSs will be sensitive to the magnitude of ozone depletion in the model, though the decrease of ozone in these CMAM REF-B2 experiments is roughly in agreement with observations in the mid to upper stratosphere (Jonsson et al, 2009).…”

“…Cooling in the upper stratosphere is largely a result of decreases in ozone, decreasing the shortwave heating by ozone, and increases in longwave cooling from increasing CO 2 (Shine et al, 2003). Comparing temperature trends of the REF-B2 and GHG simulations, we find that for the 1979-2000 period CO 2 , with a small additional contribution from increasing H 2 O, contributes a minimum of 60% of the total cooling found in REF-B2 at the stratopause, with larger contributions above and below.…”

“…Foremost among these is the effect of climate change, driven largely by increasing concentrations of CO 2 . Increased concentrations of CO 2 cool the stratosphere, with the effect maximizing near the stratopause, and are believed to be responsible for approximately one half the observed cooling trend in the upper stratosphere (Shine et al, 2003;WMO, 2007). The cooling leads to an increase in the concentration of ozone through…”

Robust Hadley Circulation changes and increasing global dryness due to CO 2 warming from CMIP5 model projections

“…This is reflected in the expanding region of reduced RH from the midtroposphere to the surface in the poleward flank of the climatological dry zones (regions with RH < 40), where the RH deficit is at a maximum. The increased RH near the tropopause and lower stratosphere is associated with the cooling from increased longwave radiative loss to space under global warming (35,36). Even a small increase in moisture due to enhanced vertical transport will result in a large increase in RH in these regions.…”

Implementing growth and sedimentation of NAT particles in a global Eulerian model

“…the uptake and subsequent sedimentation of HNO 3 within Polar Stratospheric Clouds, PSC's) both increases and prolongs stratospheric ozone loss (Crutzen and Arnold, 1986;Fahey et al, 1989;Fahey et al, 1990). Although this process is more prevalent and intense in the Antarctic winter stratosphere, as a consequence of lower temperatures, Shine et al (2003) have conducted a recent comparison of stratospheric temperature changes and found that one possible outcome is that a future cooling could occur in the Arctic which would result in an increase in the importance of denitrification in the Arctic winter stratosphere. Moreover,during the last decade, observational evidence of this vertical redistribution of HNO 3 in the Arctic stratosphere has also been found (e.g.…”