Climate dynamics

Abstract:

Southern Hemisphere (SH) circulation and surface climate changes are emerging in recent decades. For example, in most seasons, the SH Hadley cell edge and the eddy‐driven jet stream have shifted polewards, and the Southern Annular Mode (SAM) has trended towards its positive phase. However, attributing these changes to specific external forcings, such as greenhouse gas (GHG) increases or stratospheric ozone depletion, has been hindered by small ensemble sizes and lack of coherent forcing methodology across multiple models. In this study, we analyzed simulations from 10 models within the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP) to isolate the long‐term (1850–2014) SH climate response to individual forcings (GHGs, aerosols, and ozone). We found that long‐term SH climate trends are dominated by GHGs across all seasons. Over this time period, stratospheric ozone depletion exerts an influence comparable to GHGs but is restricted to austral spring and summer. Although anthropogenic aerosols show weaker effects on climate trends that oppose those induced by GHG forcing and ozone depletion, they have a noticeable impact. Notably, ozone recovery following the Montreal Protocol is currently weakening or reversing spring and summer trends in key circulation metrics. Finally, we quantify the degree to which the observed sea surface cooling of the Southern Ocean and Antarctic sea ice increase from 1980 to 2014 fall outside the models' ensemble spread. Our results highlight the need to better understand the roles of different forcings, model differences, and discrepancies between models and observations to constrain projections of future Southern Hemisphere climate change. Atmospheric and oceanic circulation changes are emerging in the Southern Hemisphere (SH). To better understand the drivers of these changes, in this study we use a new multi‐model large ensemble data set to look at how specific factors (forcings) have individually influenced the long‐term SH climate trends from 1850 to 2014. We find that greenhouse gases are the main driver of long‐term changes in the SH since the preindustrial era. The impact of stratospheric ozone depletion is comparable to greenhouse gases, but only during austral spring and summer. On the other hand, anthropogenic aerosols partially counteract greenhouse gas effects on the circulation. The models are able to simulate the recent weakening or even reversal of some of the trends in key atmospheric circulation metrics due to the recovery of the ozone layer following the enactment of the Montreal Protocol and its amendments. However, the disagreement between model simulations and real‐world observations for recent trends in Southern Ocean sea surface temperatures and Antarctic sea ice highlights the need for more research to reliably project future climate change in this crucial region. Greenhouse gases drive Southern Hemisphere circulation changes year‐round, while ozone is a key driver during austral spring and summer Although weaker, anthropogenic aerosol effects are substantial and should be accounted for in attribution studies Persistent model‐observational discrepancies exist for recent trends in Southern Ocean sea surface temperature and sea ice extent Greenhouse gases drive Southern Hemisphere circulation changes year‐round, while ozone is a key driver during austral spring and summer Although weaker, anthropogenic aerosol effects are substantial and should be accounted for in attribution studies Persistent model‐observational discrepancies exist for recent trends in Southern Ocean sea surface temperature and sea ice extent

Abstract:

Abstract. The teleconnections of the Quasi-Biennial Oscillation are revisited using ∼65 000 years of model output contributed by four modeling centers to the Large Ensemble Single Forcing Model Intercomparison Project (LESFMIP). The large ensemble size (at least 10, and in many cases 50) allows isolation of weak signals that are usually hidden by internal variability, as well as better quantification of the role of internal variability in possible model–observation discrepancies in the magnitude of the signals. All four models simulate a Holton–Tan effect, and two of the models also simulate a subtropical downward arching wind horseshoe teleconnection that is most prominent in the Pacific sector. The magnitudes of these teleconnections are statistically indistinguishable from those observed in two of the models but not in the other two; this is a notable improvement from previous work that analyzed small ensembles. These large-scale teleconnections lead to surface temperature and precipitation anomalies over the mid-latitude continents, including an impact on western North America surface temperature which appears to have not been noted before. Furthermore, all models show impacts of the QBO on tropical surface temperature and precipitation, however the nature of these responses differs across the models due, in part, to qualitatively different interactions with El Niño. Remarkably, one of the models simulates a connection between the QBO and the Madden Julian Oscillation that mimics observations, although it remains too weak. Finally, the LESFMIP simulations allow an exploration of external forcings impacting the magnitude of teleconnections. Among these experiments, greenhouse gas forcing is seen to significantly strengthen the subtropical wind horseshoe of the QBO.