Search

The Walker circulation is projected to slow down in response to greenhouse gas warming. However, detecting the impact of human activities on changes in the Walker circulation is challenging due to the significant influence of internal variability. Here, based on ensembles of multiple climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we show evidence that the emergence of the human-induced weakening of Walker circulation tends to occur earlier in the middle-upper troposphere than at the surface. This earlier emergence is attributed to a more pronounced initial weakening response of the middle-upper tropospheric Walker circulation to atmospheric CO2 radiative forcing. We further reveal that the emergence time of a weaker Walker circulation varies across models. This intermodel spread is governed by an ocean thermostat that operates by modulating the zonal sea surface temperature gradient over the tropical Indo-Pacific region. Our findings address the key question of whether and how to detect human-induced large-scale atmospheric circulation changes and provide valuable insights for assessing the associated risks. This study shows that human-induced weakening of the Walker Circulation tends to occur earlier in the middle-upper troposphere than at the surface, attributed to the stronger weakening response to CO2 radiative forcing in those heights. [ABSTRACT FROM AUTHOR]

The western North Pacific anomalous anticyclone (WNPAC) is a key bridge that links El Niño and East Asian climate variability. Future projections of ENSO-related WNPAC changes under global warming are highly uncertain across climate models. Based on a 40-member ensemble from the Community Earth System Model Large Ensemble (CESM-LE) project, we investigate the effects of internal variability on the El Niño–related WNPAC projection. Here, we first develop a decomposition method to separate the contributions of El Niño amplitude change and nonamplitude change from the leading uncertainty in the El Niño–related WNPAC projection. Based on the decomposition, approximately 23% of the uncertainty in the El Niño–related WNPAC projection is attributed to the El Niño amplitude change, while the remaining 77% is from the nonamplitude change, which is mainly related to the change in the El Niño decaying pace. A larger (smaller) El Niño amplitude can enhance (weaken) the WNPAC through a stronger (weaker) tropical Indian Ocean capacitor effect. For nonamplitude change, a faster (slower) El Niño decaying pace can also enhance (weaken) the WNPAC through descending Rossby waves in response to cold sea surface temperature anomalies over the tropical central-eastern Pacific. The decomposition method can be generalized to investigate the sources of uncertainty related to El Niño properties in climate projections and to help improve the understanding of changes in the interannual variability of East Asian–western Pacific climate under global warming. [ABSTRACT FROM AUTHOR]

Based on 34 coupled models from Phase 5 of the Coupled Model Intercomparison Project, we found that the projected changes in the western North Pacific anomalous anticyclone (WNPAC) during El Niño decaying summers show large spread among the models due to different decaying paces of El Niño. Models with a slower (faster) decaying El Niño are followed by weakened (intensified) tropical central Pacific cooling, which further drives a weaker (stronger) WNPAC through inducing weakened (intensified) descending Rossby waves to its west. The decaying pace of El Niño is dominated by the wintertime WNPAC because the easterly surface wind anomalies to its southern flank are critical to El Niño demise. Both the El Niño‐related sea surface temperature anomaly and the precipitation sensitivity to such kind of sea surface temperature anomaly under global warming could affect the wintertime WNPAC. Plain Language Summary: The western North Pacific anomalous anticyclone (WNPAC) is the key atmospheric circulation system that connects El Niño and East Asian climate variability. Understanding how the WNPAC will change in the future is of vital importance to people living in East Asia‐western Pacific region. There is extensive literature on the topic of changes in the WNPAC in response to global warming but how such kind of connection would change under global warming remains inconclusive. The causes of uncertainty in the WNPAC projection and the dominant mechanisms remain unknown. Based on 34 state‐of‐art climate coupled models, we show evidence that uncertainty in El Niño projection is a source of uncertainty in the WNPAC projection. Our results highlight the importance of a reliable projection of El Niño decaying pace to the projection of climate variability over the western Pacific and East Asia. Key Points: The projected changes in the WNPAC during El Niño decaying summers are uncertain among different climate modelsThe uncertainty of El Niño‐northwestern Pacific climate relationship projection is determined by the El Niño decaying pace under global warmingChanges in El Niño decaying pace depend on the strength of wintertime WNPAC [ABSTRACT FROM AUTHOR]