Summer Arctic Cold Anomaly Dynamically Linked to East Asian Heat Waves

During recent years, the rapidly warming Arctic and its impact on winter weather and climate variability in the mid- and low-latitudes have been the focus of many research efforts. In contrast, anomalous cool Arctic summers and their impacts on the large-scale circulation have received little attention. In this study, we use atmospheric re-analysis data to reveal a dominant pattern of summer 1000-500 hPa thickness variability north of 30°N and its association with East Asian heat waves. It is found that the second thickness pattern exhibits strong interannual variability but does not exhibit any trend. Spatially, the positive phase of the second thickness pattern corresponds with significant Arctic cold anomalies in the mid- and low-troposphere, which are surrounded by warm anomalies outside the Arctic. This pattern is the thermodynamic expression of the leading pattern of upper tropospheric westerly variability and significantly correlated with the frequency of East Asian heat waves.

The Arctic has experienced frequent summer cold anomalies since 2005, accompanied by strengthened tropospheric westerly winds over most of the Arctic and weakened westerlies over the mid- and low-latitudes of Asia. The former significantly enhances baroclinicity over the Arctic, which dynamically contributes to increased frequency of anomalous low surface-pressure during summer along with decreased frequency over high-latitudes of Eurasia and North America. The latter is exhibited by sustained high-pressure anomalies in the mid- and low-troposphere that dynamically facilitate the occurrence of East Asian heat waves. A systematic northward shift of Asian zonal winds dynamically links Arctic cold anomalies with East Asian heat waves and produces a seesaw structure in zonal wind anomalies over the Arctic and the Tibetan Plateau (the third pole). Evidence suggests that enhanced Arctic westerlies may provide a precursor to improve predictions of the East Asian winter monsoon, though the mechanism for this lag association is unclear.