The Leading Circulation Patterns of Meiyu Onset in the Yangtze–Huaihe River Valley and Its Relationship With the North Atlantic Sea Surface Temperature Anomalies

The Meiyu rainfall anomaly is closely associated with the early or late onset of Meiyu, and accurately predicting its onset is crucial for disaster prevention and mitigation. This study examines the diverse circulation patterns governing the Meiyu onset in the Yangtze–Huaihe River Valley and elucidates its underlying mechanism through statistical diagnosis and numerical simulation. The findings indicate that over the recent 31 years, three distinct circulation patterns of the Meiyu onset (referred to as MO-I, MO-II and MO-III) with different blocking situations in mid-high latitude are observed. The Rossby wave trains connect the weather systems in the mid-high latitude and subtropical regions, leading to the coevolution of the Western Pacific Subtropical High (WPSH), the South Asian High (SAH), the East Asian Subtropical Westerly Jet (EASWJ) and the Ural blocking pattern. Furtherly, the sea surface temperature (SST) anomalies in the North Atlantic prior to the Meiyu onset can stimulate the key Rossby wave trains that pass through the Ural Mountains and connect with the East Asian subtropical zone. In MO-I, a dipole SST pattern characterised by positive anomalies in southeast Greenland and negative anomalies in the mid-latitude West Atlantic induces negative geopotential anomalies over the Ural Mountains. Concurrently, the WPSH and the SAH exhibit weakened intensities compared to their climatological states, resulting in a delayed onset date relative to the other two types. Conversely, SST anomalies of MO-III exhibit an opposite pattern to MO-I, and the concurrent intensified Ural Blocking High, WPSH and SAH facilitate the earliest Meiyu onset. For MO-II, an antiphase distribution of SST anomalies in the mid-latitude West Atlantic and southern Iceland stimulates two wave trains along the polar front jet and the EASWJ, producing a circulation similar to the climatological state of the Meiyu onset. This research can serve as a foundation for Meiyu prediction and monitoring.