Change of the Relationship Between the Spring NAO and East Asian Summer Monsoon and Its Possible Mechanism

Abstract

AbstractThe relationship between the East Asian summer monsoon (EASM) and North Atlantic Oscillation (NAO) was examined by using correlation analysis for the period from 1948 to 2009. The result shows that the EASM was well correlated with the preceding spring (April‐May) NAO on the annual timescale. But their relationship was characterized by large decadal variations, with change of sign from positive to negative in the 1970s. Composite analysis revealed that this change was closely related to the spring and pre‐winter (December‐March) ocean‐scale atmosphere‐ocean coupling mode, i.e. the NAO‐SSTA (sea surface temperature anomaly) tripole pattern, over the North Atlantic Ocean. The effect of spring NAO on the interannual variability of EASM depends on the persistence of the SSTA tripole pattern induced by the spring NAO itself. However, this pattern was not only controlled by the spring NAO, but also influenced by the pre‐winter NAO‐SSTA tripole coupling mode, the effect of which was decadal unstationary. Before the 1970s, the effect of pre‐winter NAO‐SSTA tripole coupling mode on the spring SSTA tripole pattern is asymmetric, i.e. the former only has a significant weakening effect on the positive phase of the latter. After the 1970s, the effect of the former on both the positive and negative phases of the latter was negligible. Therefore, the symmetric effect of spring NAO combined with the asymmetric effect of pre‐winter NAO‐SSTA tripole coupling mode caused the decadal unstationary relationship between the spring NAO and SSTA tripole pattern, resulting in change of the NAO‐EASM relationship in the 1970s.