The Major Interannual Modes of Late-Summer Precipitation Over the Northern China Monsoon Region

ABSTRACT

Summer precipitation in the northern China monsoon region (NCMR; 35°–55°N, 108°–135°E) shows significant intraseasonal variability. The early-summer (June) and late-summer (July–August) precipitation patterns show clear differences in their formation mechanisms and the systems that affect them. We used empirical orthogonal function (EOF) analysis to investigate the two leading modes of July–August precipitation over the NCMR and their associated atmospheric circulation anomalies using linear regression. The results show that the first (EOF1) and second (EOF2) modes correspond to a pan-NCMR precipitation variation pattern and a precipitation oscillation pattern between North China (NC) and Northeast China (NEC), respectively. These two modes account for 22.1% and 10.1% of the total variance, respectively. The associated principal components (PCs) both have significant interannual variability with a period of 2–4 years. In addition, PC1 has significant interdecadal variability with a period of 20–30 years. Further analysis suggests that EOF1 and EOF2 clearly have a different relationship with the summer monsoon circulation system. In the positive phase of PC1, the East Asian subtropical westerly jet stream (EAWJS) shows a northward trend with higher intensity than normal the blocking high at mid- to high latitudes is inactive; and the western Pacific subtropical high (WPSH) is located to the north of its normal position. The NCMR is controlled by stronger southerly winds, which cause the convergence of water vapour, favouring more precipitation in this region and vice versa. In the positive phase of PC2, the EAWJS swings to the south of Lake Baikal. Significant positive height anomalies exist from western NC to NEC. Significant negative height anomalies occur to the subtropical northwestern Pacific. This indicates that the cold vortex in Northeast China is inactive, the WPSH tends to be weaker and located to the south of its normal position, and NEC (NC) is dominated by anomalous northeasterly (southeasterly) winds. The convergence (divergence) of water vapour in NC (NEC) favours more (less) precipitation in NC (NEC) and vice versa. Therefore, EOF1 is related to the large-scale circulation anomalies over East Asia and the northwest Pacific in July and August, whereas EOF2 is more closely related to the anomalies in the regional circulation over the NCMR and the subtropical northwestern Pacific.