Abrupt shift in the association between late-summer Northeast China cold vortex days and early-summer tropical Indian Ocean sea surface temperature

Based on the Northeast China cold vortex (NCCV) case calendar provided by the Liaoning Meteorological Bureau, ERSST V5 sea surface temperature data, and NCEP/NCAR atmospheric reanalysis dataset from 1979 to 2021, this study examines the association between late-summer NCCV days and early-summer tropical Indian Ocean (TIO) sea surface temperature and its temporal stability. Results show that the strongest positive correlation between late-summer NECV days and preceding TIO sea surface temperature occurs in early summer. That is warm early-summer TIO sea surface temperature anomalies tend to be followed by more late-summer NCCV days. However, this linkage is non-stationary and the significant positive correlation becomes statistically insignificant after 2004. The abrupt shift is attributed to an eastward displacement of the late-summer TIO warming and corresponding atmospheric response to the early-summer TIO warming. During 1989-2003, the early-summer TIO warming persists into late summer, exciting eastward-propagating Kelvin waves that reinforced anticyclonic anomalies over the western North Pacific. Low-level divergent flow anomalies associated with the anticyclonic anomalies over the western North Pacific converge markedly over Northeast Asia, inducing the mid–low-tropospheric cyclonic anomalies and negative geopotential height anomalies that increase NCCV days. By contrast, during 2004-2017 the eastward shift of the late-summer heat source precludes significant anticyclonic anomalies over the western North Pacific, thereby weakening the association between late-summer NCCV days and early-summer TIO sea surface temperature.