Climate dynamics

Abstract:

The Quasi-Biennial Oscillation (QBO), the dominant mode of interannual variability of monthly-mean zonal winds and temperatures in the equatorial stratosphere, has a significant influence on the circulation in other regions of the stratosphere and troposphere in boreal winter. To better understand these teleconnections, results from twelve models are analysed for three complementary experiments in which the QBO is either internally generated by the models (free-running), nudged towards the observed QBO, or nudged towards the observed climatology. For the multi-model ensemble mean we assess how well the free-running experiment captures the observed teleconnections, and whether the simulated teleconnections improve when the models are nudged to have a more realistic QBO. The observed relationship between the QBO phase and the strength of the boreal winter Polar Night Jet (PNJ) is weakly captured by the free-running experiment. The PNJ response is larger when the models are nudged towards the observed QBO, primarily because nudging reduces the bias in the QBO amplitude, resulting in a larger proportion of the models exhibiting a stronger QBO-PNJ relationship. Both free-running and nudged-QBO experiments capture a small and marginally significant increase in Sudden Stratospheric Warming (SSW) frequency during the QBO-easterly phase relative to the QBO-westerly phase. Neither experiment captures the observed QBO influence on the timings of SSWs during winter nor the QBO teleconnection to the North Atlantic Oscillation. Nudging the QBO does, however, improve its influence on the latitudinal profile of the North Pacific sub-tropical jet and strength of tropical precipitation. We find that a more realistic representation of the QBO is beneficial for certain teleconnections, but their effective simulation likely depends on how well the underlying model processes respond to the QBO.

Abstract:

Abstract The Earth’s middle atmosphere spans the deep region from the upper troposphere/lower stratosphere at around 10 km altitude to the mesosphere/lower thermosphere at around 100 km altitude. It is being increasingly recognized for its role in driving extreme surface weather and regional climate change. Climate models predict large ongoing and future changes in the middle atmosphere composition and circulation. However, the observations needed to detect, attribute and understand these changes and their impacts, to test predictions, and thereby to improve our models, are lacking. Here we show the capacity of infrared limb-imaging tomography to provide the needed observations. This evaluation is based on studies performed within a recent satellite mission concept – the Changing-Atmosphere Infrared Tomography Explorer, CAIRT. Observing thermal infrared emissions simultaneously from the middle troposphere at about 4 km up to the lower thermosphere at about 115 km altitude this technique provides observations of temperature and an extensive range of trace gases with unprecedented spatial resolution of about 50 by 50 km horizontally and about 1 km vertically. We show how these observations would (a) help to quantify the changing atmospheric circulation, (b) allow characterization and quantification of the gravity waves that are critical in driving this circulation, (c) reveal how variability in solar radiation and energetic particles propagate downward to affect regional climate at the surface, (d) detect how volcanic eruptions and wildfires impact the middle atmosphere and climate, and (e) resolve how stratosphere-troposphere exchange affects ozone and water vapor in the crucial and climate-relevant tropopause region.

Abstract:

Winter wildfires in the southwestern United States (SWUS), especially Southern California, are less frequent than summer and autumn fires but can be highly destructive, as demonstrated by the January 2025 events. How winter anthropogenic warming and internal climate variability shape winter fire weather risk in the SWUS remains unclear. Using historical observations, reanalysis, and model simulations, we show that winter fire weather risk in the SWUS has nearly doubled during 1984-2024. An attribution analysis associates approximately 33% of this increase with anthropogenic ocean warming and 53% with internal Pacific Decadal Oscillation (PDO) variability. Winter North Pacific midlatitude warming associated with anthropogenic forcing and the negative PDO strengthens westerly winds and favors an eastward shift and southward extension of winter blocking anticyclones. While warming and prolonged drought from autumn into winter precondition fuels, winter midlatitude North Pacific blocking anticyclones extending into the western United States intensify winter surface air warming and vapor pressure deficit, and reduce precipitation and soil moisture in the SWUS, thus exacerbating winter fire weather risks in the SWUS. CMIP6 projections support the escalation of winter fire weather risks under continued warming.

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