Comparison of the Multi-Scale Features in Two Persistent Heavy Rainfall Events in the Middle and Lower Reaches of Yangtze River

Two persistent heavy rainfall (PHR) events in the middle and lower reaches of Yangtze River (MLYR) occurring in June 1982 and 1998 are studied in this paper. Though both events happened in the Meiyu season, their large-scale background and developing processes were quite different. During the PHR event in 1982, the Lake Baikal area was occupied by a strong westerly trough and the western Pacific subtropical high (WPSH) was stronger and more westward-extending than the normal years. Under such a condition, the cold-dry air and warm-moist air were continuously transported to the MLYR and favored the PHR there. For the event in 1998, the WPSH was similar to that in 1982, while the westerly trough in the Lake Baikal area was comparatively weak and a shortwave trough situating in East China contributed to advect cold-dry air to the MLYR. It is found that the high-latitude trough was closely related to the 10-30-day low-frequency oscillation while the anomaly of WPSH was linked with the combined effect of both 30-60- and 10-30-day low-frequency oscillations in the PHR event in 1982. By contrast, the 60-day low-pass perturbation demonstrated positive impact on the westward extension of WPSH and development of the Baikal trough while the 30-60-day oscillation played a role in strengthening the shortwave trough in East China and the WPSH in the case of 1998. Though the low-latitude 30-60-day oscillations contributed to the intensification and westward extension of the WPSH in both PHR events, their evolution exhibited evident differences. In the 1982 case, the 30-60-day anomalies originated from the western Indian Ocean were much more like the Madden-Julian Oscillation, while its counterpart in the 1998 case was much more similar to the first mode of the boreal summer intraseasonal oscillation.