Isolating ENSO influence on eastern China summer rainfall variability with month-dependent EOF method: Observation-model synthesis

Abstract East Asian summer rainfall is influenced by tropical sea surface temperature (SST) and the Asian subtropical westerly jet. Using a month-dependent empirical orthogonal function (EOF) method, two leading modes of interannual rainfall variability over eastern China (EC) are identified in observations. The first mode is related to antecedent winter El Niño/Southern Oscillation (ENSO), exhibiting a northward migration of increased rainfall across the Yangtze River from June to August. The attendant large-scale anomalous anticyclone in the lower troposphere indicates the Indo-western Pacific ocean capacitor effect. The second mode represents effects of concurrent developing El Niño, with a band of decreased rainfall along the Yangtze River in June that migrates northward to the Yellow River in July and August. A 30-member ensemble simulation with an atmospheric general circulation model forced by observed SST is used to interpret the precipitation modes. The ensemble mean captures the observed patterns of EC rainfall variability, the associated circulation anomalies in the lower troposphere, and to a lesser degree, at upper levels. The first two modes together explain 48.5% and 21.7% of the total rainfall variance in the model ensemble mean and observations, respectively. Atmospheric internal variability, as represented by the model ensemble spread, features circulation patterns at low levels that broadly resemble those forced by SST. The spatial similarity between the SST-forced and internal modes is due to the summer mean flow that supports barotropic energy conversion, rendering it difficult isolating SST-forced variability in observations of limited durations. The month-dependent EOFs successfully distinguish concurrent and post ENSO summers by considering subseasonal evolution of spatial patterns.