Low-frequency oscillation and circulation evolution of persistent heavy rainfall in the Haihe River Basin

To enhance extended-range forecasting of persistent heavy rainfall in the Haihe River Basin (HRB), this study investigates the low-frequency oscillation (LFO) characteristics of such events and the associated atmospheric circulation evolution utilizing Chinese station observed daily precipitation data and NCEP/NCAR daily reanalysis data (1961—2020). Results indicate that: (1) More than 80% of persistent heavy rainfall events occur from late July to early August, coinciding with the peak flood season of the North China rainy season. (2) The 10 ~ 30-day and 30 ~ 60-day LFO components frequently superimpose, significantly amplifying the intensity of LFO and promoting heavy rainfall. Notably, the 10 ~ 30-day component contributes predominantly, a characteristic distinct from rainfall patterns in southern China (e.g., South China and the Yangtze River Basin). (3) Prominent LFO circulations exhibit baroclinic vertical structure and cooperate spatially to form a precipitation-favorable pattern: The Bay of Bengal cyclone and the southwest jet, as well as the cyclones over the South China Sea and northerly flow to their north, establish two primary moisture channels at low level, promoting warm, moist air converges strongly under the influence of North China cyclone. Concurrently, at mid-upper levels, the upstream Central Asian Low favors the southward intrusion of cold mid-to-high latitudinal air, facilitating cold-warm air convergence. The downstream Northeast Asian high (NEAH), characterized by a deep vertical structure, blocks the eastward propagation of upstream LFO circulations, and prolongs the persistence of the circulation pattern. (4) The meridional propagation of prominent LFO circulations promotes the convergence of southern and northern airflows over the HRB, forming a phase-locked region: Northeasterly originating near 60°N and southwesterly near 10°N propagate toward and interact with each other 2 days before heavy rainfall onset, establishing a baroclinic instability region conducive to unstable energy release over the HRB. Furthermore, mid-level anticyclones propagating from near 60°N and low-latitudinal anticyclones converge and merge, significantly intensifying the NEAH. The subtropical westerly jet shifts northward while the high-latitudinal westerly jet shifts southward, merging and intensifying over the Sea of Japan. The entrance region of the jet corresponds aloft to the HRB, providing favorable conditions for upper-level divergence and dynamic uplift. These findings provide a theoretical foundation for prominent precursor circulation indicators selection and improve the extended-range forecasting of persistent heavy rainfall in the HRB.