Synoptic-scale precursors of the East Asia/Pacific teleconnection pattern responsible for persistent extreme precipitation in the Yangtze River Valley

Synoptic-scale precursors of the typical East Asia/Pacific (EAP) teleconnection pattern responsible for persistent extreme precipitation events (PEPEs) in the Yangtze River Valley (YRV) are investigated, based on a composite analysis. The results reveal that, about one week prior to PEPEs, a blocking high develops near the Sea of Okhotsk owing to an eastward energy dispersion and further strengthens markedly due to poleward energy dispersion from low latitudes. Subsequently, a meridional tripole structure of the typical EAP pattern becomes well established by this blocking, along with a westward-migrated strong negative anomaly at midlatitudes/positive anomaly at lower latitudes. In the lower troposphere, a westward-progressive anomalous anticyclone-cyclone pair can be identified up to about a week prior to PEPEs, contributing to greatly enhanced moisture transport towards the YRV with a magnitude anomaly over three standard deviations above normal. A midlatitude anomalous cyclone associated with the EAP pattern evolution and the eastward-extended South Asia High combine to provide favourable upper-level divergence. Correspondingly, strong ascent of low-level warm/moist air along a quasi-stationary front leads to PEPEs in the YRV. A contrastive analysis between evolution of typical wet and dry EAP regimes indicates that EAP-induced PEPEs are more likely to occur in the YRV, with the ridge of the western Pacific subtropical high typically staying around the northeastern quadrant of the South China Sea. This contrasting analysis also highlights the importance of the upstream pre-existing ridge to early strengthening of the Okhotsk blocking.