Evolutions of the Circulation Anomalies and the Quasi-biweekly Oscillations Associated with Extreme Heat Events in South China

South China, located in the tropical-subtropical East Asian monsoonal region, presents a unique anticyclonic-cyclonic circulation pattern during extreme heat (EH), obviously different from the typical anticyclone responsible for EH in many other regions. Associated with the evolution of EH in South China, the anticyclonic-cyclonic anomalies propagate northwestwards over the Philippines and South China. Before the EH onsets, the anticyclonic anomaly dominates South China, and results in stronger subsidence over South China and stronger southerly/southwesterly over the western/northern margins of South China. The southerly/southwesterly transports more water vapor to the north of South China, and thus, together with the local stronger subsidence, result in drier air condition and accordingly favor the EH occurrence. Conversely, after the EH onsets, the cyclonic component approaches South China and offsets the high temperature.

The oscillations of temperature and circulation anomalies over South China exhibit a periodicity of ~10 days and indicate the influence of quasi-biweekly oscillation, which originates from the tropical western Pacific and propagates northwestwards. Therefore, the 5–25-day filtered data are extracted to further analyze the quasi-biweekly oscillation. It turns out that the evolutions of the filtered circulation remarkably resemble the original anomalies with comparable amplitudes, indicating the quasi-biweekly oscillation is critical for the EH occurrence in South China. The quasi-biweekly oscillation could explain for more than 50% of the intraseasonal variance of T_max and vorticity over South China, and 80% of the warming amplitude on EH onsets. The close relationship between the circulations of quasi-biweekly oscillation and the EH occurrence indicates the possibility of medium-range forecast for high temperature in South China.