Trends in Southern Hemisphere wind-driven circulation in CMIP5 models over the 21st century: Ozone recovery versus greenhouse forcing

Abstract

During the late 20th century, Antarctic ozone depletion and increasing greenhouse gases (GHGs) conspired to generate conspicuous atmospheric circulation trends in the Southern Hemisphere (SH), contributing to a poleward intensification of the oceanic supergyre circulation. Forcing of Antarctic ozone depletion dominated the observed trends during the depletion period , but Antarctic ozone is projected to recover by the middle of the 21st century. The recovery provides a mechanism for offsetting the impact from increasing GHG emissions. To what extent will the recovery of ozone mitigate SH atmosphere and ocean circulation trends expected from increasing GHGs? We examine climate model output from the Representative Concentration Pathway 4.5 and 8.5 (RCP4.5 and RCP8.5, respectively) emission scenario experiments, submitted to the Coupled Model Intercomparison Project phase 5. Both scenarios are subject to the effect of ozone recovery. We show that during the recovery period , there is little poleward shift of the supergyre circulation under either RCP scenario in austral summer, due to the dominance of ozone recovery. Further, under RCP8.5 the trend in winter, a season in which ozone recovery has little impact, is greater (more poleward) than in summer, opposite to the seasonality of trends during the depletion period. Under RCP4.5, with the contribution from ozone recovery, the summer poleward shift is projected to stabilize into the postrecovery decades, whereas under RCP8.5, the summer poleward shift accelerates in the postrecovery period, presenting vastly different ocean circulation futures.

Fast and slow responses of Southern Ocean sea surface temperature to SAM in coupled climate models

“…Under scenarios with extended greenhouse gas emissions and gradual ozone recovery, CMIP5 models predict a significant and lasting poleward intensification of the westerlies throughout the twenty-first century (Wang et al 2014). Based on our analysis, we suggest that those models which have smaller biases in their climatological stratification provide better estimates of future SST anomalies in the SO.…”

“…In some models the step response function never changes sign and cooling persists, while in other GCMs the SST anomaly crosses over from negative to positive values only 3 years after a step increase in the SAM. This intermodel diversity can be related to the positive phase of the Southern Annular Mode (SAM) of natural variability, but they may in fact be a forced response (Thomas et al 2015), the result of ozone depletion (Thompson and Solomon 2002;Gillett and Thompson 2003;Sigmond et al 2011;Thompson et al 2011;Wang et al 2014). Figure 1 illustrates the synchronous evolution of observed SST and SAM anomalies over the SO.…”

Pacific western boundary currents and their roles in climate

“…Antarctic ozone is projected to recover by around the middle of this century, providing a mechanism to offset the impacts of a projected continuous increase in CO 2 . As such, climate models under the business-as-usual emission scenario, which currently matches the observed emission rates, simulate no further trend in the Southern Hemisphere westerly winds before 2045, but an acceleration in the post-2045 period 94 . In the following two paragraphs, we discuss projected changes averaged over the 2050-2100 period referenced to the mean conditions over the twentieth century, under this emission scenario.…”

“…Climate models suggest that the tropical trade winds are likely to weaken 53 , and that the southern mid-latitude westerlies are likely to undergo a poleward shift and intensification [91][92][93][94] , and contribute to the intensification of the WBCs, although changes in North Pacific winds are less clear 99 . These changes, together with a projected weakening in the global thermohaline circulation 98 , may simultaneously affect the Pacific WBCs.…”

The impact of the Southern Annular Mode on future changes in Southern Hemisphere rainfall

“…We base the analysis on the SAM index that is linearly independent of ENSO (hereafter, SAM |enso ). The smallest positive trend in the SAM in summer is likely to result from the competing effects between the recovery of the Antarctic stratospheric ozone and the increasing GHGs in the first half of the 21st century as suggested by Barnes et al [2014] and Wang et al [2014]. An upward trend in the SAM |enso is robustly projected to result from increasing GHGs [e.g., Kushner et al, 2001;Cai et al, 2003;Miller et al, 2006;Gillett and Fyfe, 2013;Purich et al, 2013;Zheng et al, 2014] ( Figure S2).…”