When Mid–High Latitude Systems Meet the Monsoon:How the Northeast China Cold Vortex Regulates Summer Rain-Belt Timing and Meridional Shifts

Summer rainfall over eastern China is shaped by interactions between the East Asian monsoon and mid–high latitude circulation regimes. The Northeast China Cold Vortex (NCCV), a cut-off low over East Asia and the western Pacific, plays a central yet poorly understood role in modulating large-scale rainfall patterns and the timing and meridional position of summer rain belts. Here, we investigate the summer manifestation of NCCV activity using long-term reanalysis and gridded precipitation datasets from a circulation-regime perspective.

NCCVs are identified from 500-hPa geopotential height and temperature minima using a set of simplified, consistently applied detection schemes formulated under different constraint conditions across ERA5, NCEP/NCAR, and CRA reanalyses, yielding an ensemble NCCV dataset. Summer precipitation characteristics of the three major rain belts—Meiyu, North China, and Northeast China—are objectively quantified using five independent precipitation datasets (MSWEP, ERA5, NCEP/NCAR, CRA, and CPC), including onset, withdrawal, duration, accumulated precipitation, and a composite precipitation index.

Composite differences between years of exceptionally high and low NCCV activity, selected using strict criteria based on NCCV frequency and rain-belt precipitation indices, reveal a robust, recurring three-dimensional circulation regime. A pronounced dry–wet boundary emerges between 30°–40°N, accompanied by a meridional dipole in 500-hPa geopotential height and temperature, with positive anomalies to the south and negative anomalies to the north. This pattern persists throughout June–August but exhibits systematic seasonal migration, with the latitude of maximum upper-tropospheric westerly anomalies shifting northward from ~30°N in June to ~40°N in August.

Vertical cross-sections of the same composite differences further reveal pronounced meridional asymmetry, characterized by upper-tropospheric westerly wind anomalies near 40°N and deep-tropospheric easterly wind anomalies near 55°N. These anomalies are collocated with sharply tilted extrema in potential temperature and geopotential height, with a sign reversal in potential temperature across ~200 hPa and a coincident geopotential height anomaly maximum, indicating the dominance of meridional dynamical processes rather than purely zonal adjustments. Convergent meridional flow emerges as a preferred environment for NCCV development and precipitation enhancement, while thermal anomalies in the tropical upper atmosphere (10–100 hPa) may play a role in modulating the background westerly strength and the excitation of NCCV-related precipitation over eastern China.

Across datasets, NCCV activity primarily regulates summer rainfall over eastern China by shifting the timing and meridional position of regional rain belts rather than uniformly intensifying precipitation. Significant linkages are identified with Meiyu rainfall amount, the onset and withdrawal of North China rainfall, and the duration of Northeast China rainfall. Together, these results establish a physically interpretable circulation regime through which mid–high latitude systems interact with the monsoon to shape East Asian summer rainfall, offering robust observational constraints for future dynamical studies.