Asymmetric Connection of ENSO to Asian–Pacific–American Synoptic Temperature Variability in Boreal Winter

Abstract In this study, the asymmetric relationship between El Niño–Southern Oscillation (ENSO) and winter synoptic temperature variability (STV) over the Asian–Pacific–American region during boreal winter is examined based on reanalysis data and numerical experiments. On the one hand, significant STV anomalies are predominantly observed over eastern China and North America only during El Niño events, while the ENSO–STV relationship is relatively weak during La Niña years. On the other hand, when categorized by intensity, stronger ENSO events tend to have a more pronounced impact on winter STV across the Asian–Pacific–American region, whereas weaker ENSO events exert a more limited influence primarily around the Pacific. The mean-flow conditions associated with extratropical eddy intensification during different ENSO events also reflect this asymmetric feature. Furthermore, ENSO-related sea surface temperature anomalies (SSTAs) display an asymmetric pattern. While significant SSTAs are observed in the tropical Pacific during strong ENSO events, distinct significant SSTA patterns are found in the western North Pacific during strong El Niño events and in the eastern North Pacific during strong La Niña events, respectively. Additionally, during La Niña events, the significant tropical SSTAs extend farther westward compared to strong El Niño events. Conversely, during weak ENSO events, the SSTAs are more confined to the tropical Pacific. Moreover, the response of STV-related low-level temperature gradients to SSTA forcing is verified by atmospheric general circulation model (AGCM) experiments, which highlights the major role of significant Pacific SSTAs, suggesting that the asymmetry of the ENSO–STV connection is attributed to asymmetric SSTA distributions.