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Stratospheric ozone, and its response to anthropogenic forcings, provides an important pathway for the coupling between atmospheric composition and climate. In addition to stratospheric ozone’s radiative impacts, recent studies have shown that changes in the ozone layer due to 4xCO₂have a considerable impact on the Northern Hemisphere (NH) tropospheric circulation, inducing an equatorward shift of the North Atlantic jet during boreal winter. Using simulations produced with the NASA Goddard Institute for Space Studies (GISS) high-top climate model (E2.2), we show that this equatorward shift of the Atlantic jet can induce a more rapid weakening of the Atlantic meridional overturning circulation (AMOC). The weaker AMOC, in turn, results in an eastward acceleration and poleward shift of the Atlantic and Pacific jets, respectively, on longer time scales. As such, coupled feedbacks from both stratospheric ozone and the AMOC result in a two-time-scale response of the NH midlatitude jet to abrupt 4xCO₂forcing: a “fast” response (5–20 years) during which it shifts equatorward and a “total” response (∼100–150 years) during which the jet accelerates and shifts poleward. The latter is driven by a weakening of the AMOC that develops in response to weaker surface zonal winds that result in reduced heat fluxes out of the subpolar gyre and reduced North Atlantic Deep Water formation. Our results suggest that stratospheric ozone changes in the lower stratosphere can have a surprisingly powerful effect on the AMOC, independent of other aspects of climate change.

 

Arctic amplification (AA), defined as the enhanced warming of the Arctic compared to the global average, is a robust feature of historical observations and simulations of future climate. Despite many studies investigating AA mechanisms, their relative importance remains contested. In this study, we examine the different timescales of these mechanisms to improve our understanding of AA’s fundamental causes. We use the Community Earth System Model v1, Large Ensemble configuration (CESM-LE), to generate large ensembles of 2 years simulations subjected to an instantaneous quadrupling of CO₂. We show that AA emerges almost immediately (within days) following CO₂increase and before any significant loss of Arctic sea ice has occurred. Through a detailed energy budget analysis of the atmospheric column, we determine the time-varying contributions of AA mechanisms over the simulation period. Additionally, we examine the dependence of these mechanisms on the season of CO₂quadrupling. We find that the surface heat uptake resulting from the different latent heat flux anomalies between the Arctic and global average, driven by the CO₂forcing, is the most important AA contributor on short (<1 month) timescales when CO₂is increased in January, followed by the lapse rate feedback. The latent heat flux anomaly remains the dominant AA mechanism when CO₂is increased in July and is joined by the surface albedo feedback, although AA takes longer to develop. Other feedbacks and energy transports become relevant on longer (>1 month) timescales. Our results confirm that AA is an inherently fast atmospheric response to radiative forcing and reveal a new AA mechanism.

 

Abstract. The Geoengineering Model Intercomparison Project (GeoMIP) is a coordinating framework, started in 2010, that includes a series of standardized climate model experiments aimed at understanding the physical processes and projected impacts of solar geoengineering. Numerous experiments have been conducted, and numerous more have been proposed as “test-bed” experiments, spanning a variety of geoengineering techniques aimed at modifying the planetary radiation budget: stratospheric aerosol injection, marine cloud brightening, surface albedo modification, cirrus cloud thinning, and sunshade mirrors. To date, more than 100 studies have been published that used results from GeoMIP simulations. Here we provide a critical assessment of GeoMIP and its experiments. We discuss its successes and missed opportunities, for instance in terms of which experiments elicited more interest from the scientific community and which did not, and the potential reasons why that happened. We also discuss the knowledge that GeoMIP has contributed to the field of geoengineering research and climate science as a whole: what have we learned in terms of intermodel differences, robustness of the projected outcomes for specific geoengineering methods, and future areas of model development that would be necessary in the future? We also offer multiple examples of cases where GeoMIP experiments were fundamental for international assessments of climate change. Finally, we provide a series of recommendations, regarding both future experiments and more general activities, with the goal of continuously deepening our understanding of the effects of potential geoengineering approaches and reducing uncertainties in climate outcomes, important for assessing wider impacts on societies and ecosystems. In doing so, we refine the purpose of GeoMIP and outline a series of criteria whereby GeoMIP can best serve its participants, stakeholders, and the broader science community. 

Sudden stratospheric warmings (SSWs) significantly influence Eurasian wintertime climate. The El Niño phase of the El Niño–Southern Oscillation (ENSO) also affects climate in that region through tropospheric and stratospheric pathways, including increased SSW frequency. However, most SSWs are unrelated to El Niño, and their importance compared to other El Niño pathways remains to be quantified. We here contrast these two sources of variability using two 200‐member ensembles of 1‐year integrations of the Whole Atmosphere Community Climate Model, one ensemble with prescribed El Niño sea surface temperatures (SSTs) and one with neutral‐ENSO SSTs. We form composites of wintertime climate anomalies, with and without SSWs, in each ensemble and contrast them to a basic state represented by neutral‐ENSO winters without SSWs. We find that El Niño and SSWs both result in negative North Atlantic Oscillation anomalies and have comparable impacts on European precipitation, but SSWs cause larger Eurasian cooling. Our results have implications for predictability of wintertime Eurasian climate.

 

Arctic amplification (AA) of surface warming is a prominent feature of anthropogenic climate change with important implications for human and natural systems. Despite its importance, the underlying causes of AA are not fully understood. Here, analyzing coupled climate model simulations, we show that AA develops rapidly (within the first few months) following an instantaneous quadrupling of atmospheric CO₂. This rapid AA response—which occurs before any significant loss of Arctic sea ice—is produced by a positive lapse rate feedback over the Arctic. Sea ice loss is therefore not needed to produce polar‐amplified warming, although it contributes significantly to this warming after the first few months. Our results provide new and compelling evidence that AA owes its existence, fundamentally, to fast atmospheric processes.