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Since the early 1990s the Pacific Walker circulation shows a multi‐decadal strengthening, which contradicts future model projections. Whether this trend, evident in many climate indices especially before the 2015 El Niño, reflects the coupled ocean‐atmosphere response to global warming or the negative phase of the Pacific Decadal Oscillation (PDO) remains debated. Here we show that sea surface temperature trends during 1980–2020 are dominated by three signals: a spatially uniform warming trend, a negative PDO pattern, and a Northern Hemisphere‐Indo‐West Pacific warming pattern. The latter pattern, which closely resembles the transient ocean thermostat‐like response to global warming emerging in a subset of CMIP6 models, shows cooling in the central‐eastern equatorial Pacific but warming in the western Pacific and tropical Indian Ocean. Together with the PDO, this pattern drives the Walker circulation strengthening in the equatorial band. Historical simulations appear to underestimate this pattern, contributing to the models' inability to replicate the Walker cell strengthening. Plain Language Summary: This paper investigates the observed changes in the tropical Pacific during the satellite era, including the recent decadal strengthening of the atmospheric zonal circulation—the Walker cell. We aim to understand the extent to which these changes represent a forced response to rising CO2 concentrations versus natural variability. We apply an approach in which we decompose the observed sea surface temperature trends into three components—a pattern associated with the Pacific Decadal Oscillation, which is part of natural variability, a uniform warming pattern, and a residual pattern. This residual pattern shows a remarkable resemblance to a forced ocean thermostat‐like transient response generated in some of the climate models, characterized by equatorial Pacific (EP) cooling, and a broad warming of the Northern Hemisphere, and the Indian Ocean and West Pacific. These results challenge studies arguing that the recent strengthening of the Pacific Walker cell can be explained simply by multi‐decadal natural variability in the tropics. Furthermore, the inability of climate models at large to fully capture this forced pattern with historical forcing puts into focus the reliability of future projections of climate change in the tropical Pacific, specifically the timing of emergence of the eastern EP warming. Key Points: A multi‐decadal strengthening of the Pacific Walker cell is observed in a wide range of indices, especially after 1990A Northern Hemisphere ‐ Indo West Pacific warming sea surface temperature pattern, which differs from the Pacific Decadal Oscillation, is evident since 1980This pattern resembles a forced response to abrupt CO2 forcing, emerging in a subset of climate models, and contributes to the Walker circulation strenthening [ABSTRACT FROM AUTHOR]

Previous studies have found inconsistent responses of the North Atlantic jet to Arctic sea‐ice loss. The response of wintertime atmospheric circulation and surface climate over the North Atlantic‐European region to future Arctic sea‐ice loss under 2°C global warming is analyzed, using model output from the Polar Amplification Model Intercomparison Project. The models agree that the North Atlantic jet shifts slightly southward in response to sea‐ice loss, but they disagree on the sign of the jet speed response. The jet response induces a dipole anomaly of precipitation and storm track activity over the North Atlantic‐European region. The changes in jet latitude and speed induce distinct regional surface climate responses, and together they strongly shape the North Atlantic‐European response to future Arctic sea‐ice loss. Constraining the North Atlantic jet response is important for reducing uncertainty in the North Atlantic‐European precipitation response to future Arctic sea‐ice loss. Plain Language Summary: Variations in the North Atlantic jet affect temperature, precipitation, and storminess in Europe. It is not well understood how the jet and European climate will respond to future sea‐ice loss in the Arctic under human‐caused increases in global temperature. We study how atmospheric circulation and surface climate in winter over the North Atlantic‐European region would respond to future Arctic sea‐ice loss under 2°C global warming, using an unprecedented number of coordinated simulations from different climate models. In many models the North Atlantic jet stream in winter responds by shifting southward, but the response of its speed is less clear. These jet stream changes are found to strongly shape the responses in precipitation and storm track activity. More precise estimates of the jet stream response are key to reducing uncertainty in the North Atlantic‐European precipitation response to future Arctic sea‐ice loss. Key Points: Projected Arctic sea‐ice loss causes a robust equatorward shift of the North Atlantic jet across models, but no robust change in jet speedChanges in jet position and speed strongly shape the European winter surface climate response to projected Arctic sea‐ice lossConstraining the jet response is important for reducing uncertainty in the European precipitation response to future Arctic sea‐ice loss [ABSTRACT FROM AUTHOR]

Mesoscale convective systems (MCSs) are frequent over the US Great Plains during spring. The link between large‐scale environments and spring MCS initiation were well established. Here, historical and future changes of spring large‐scale environments favorable for MCS initiation are investigated using an MCS tracking data set, ERA5 reanalysis, and 20 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. The frequency of Great Plains low‐level jet (GPLLJ)‐related MCS environments is found to have increased by ∼41% from 1979 to 2019, consistent with the enhanced GPLLJ and more frequent MCSs. Comparing CMIP6 AMIP and historical experiments, we find that the observed GPLLJ strengthening and more frequent MCS environments are mainly due to the decadal sea‐surface temperature variations rather than external forcings. Under a high emission scenario, the frequency of GPLLJ‐related environments favorable for MCS initiation will increase by ～65% during 2015–2100, along with a stronger GPLLJ, suggesting more frequent MCSs over the US Great Plains in a warming world. Plain Language Summary: Convective storm systems that extend over several hundred kilometers are frequently observed and contribute to a majority of the warm season rainfall over the US Great Plains. It is a major challenge for climate models to simulate these storms because the processes governing their initiation and evolution are not well resolved. However, large‐scale atmospheric circulations such as the Great Plains low‐level jet (GPLLJ) that provide favorable environments for these storms in spring are well observed and simulated by climate models. Therefore, the GPLLJ may be used to understand past and future changes of these storms. Here, we found GPLLJ‐related environments become more frequent from 1979 to 2019, related to the observed strengthening of the GPLLJ. To determine whether these changes are due to global warming, we compare simulations driven by the observed sea‐surface temperature variation with simulations driven only by anthropogenic forcing. The results suggest that the observed GPLLJ strengthening is mainly due to the natural decadal variation of sea‐surface temperature. However, with continued warming under a high greenhouse gas emission scenario, the GPLLJ is greatly enhanced during 2015–2100. Accompanied by more frequent occurrence of the GPLLJ‐related environments, the frequency of large convective systems may also increase in the future. Key Points: Low‐level jet‐related mesoscale convective system environments in spring over the US Great Plains became more frequent from 1979 to 2019The more frequent convection environments are a response to decadal sea‐surface temperature variation rather than to external forcingAlong with the enhanced low‐level jet under a high emission scenario, the low‐level jet‐related storm environment will become more frequent [ABSTRACT FROM AUTHOR]