Quasi-Biweekly Oscillation of Northeast China Cold Vortex Frequency in the Warm Season

The Northeast China cold vortex (NCCV) is the most important midlatitude synoptic system for weather and climate anomalies in Northeast China in the warm season. Many previous studies have focused on its synoptic and climatic variability. However, little is known about the variability of the NCCV on subseasonal timescales. In this study, we investigate the subseasonal variability of the NCCV in the warm season (May to August) and its impact based on the NCEP–NCAR reanalysis dataset and observational climate data from 1981 to 2020. Results show that the NCCV frequency exhibits a significant quasi-biweekly oscillation (QBWO, 10–25 days). In 32 out of 40 years, there is at least one significant period between 10 and 25 days. Our result provides the first direct evidence for a significant QBWO signal in the NCCV frequency. The QBWO circulation on NCCV days features a cold low-pressure anomaly surrounded by warm high-pressure anomalies from northwest to southeast in a clockwise direction, which is related to an upstream wave train propagating southeastward from the Ural Mountains into Northeast China and a downstream blocking high to the northeast. The NCCV QBWO causes more rainfall, with a quadrature phase shift as rainfall leading the NCCV for approximately three days, and synchronized reduced surface air temperature in Northeast China.