Roles of ENSO and PDO in the Link of the East Asian Winter Monsoon to the following Summer Monsoon

The present study investigates the roles of El Nino-Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) in the relationship between the East Asian winter monsoon (EAWM) and the following East Asian summer monsoon (EASM). The variability of the EAWM is divided into an ENSO-related part named EAWM(EN) and an ENSO-unrelated part named EAWM(res). Corresponding to a weak EAWM(EN), an anomalous low-level anticyclone forms over the western North Pacific (WNP) and persists from winter to the following summer. This anticyclone enhances southerlies over the coast of East Asia in summer. Hence, a weak EAWM(EN) tends to be followed by a strong EASM and vice versa. As such, a link is established between the EAWM(EN) and the EASM. The persistence of this WNP anticyclone may be mainly attributed to the sea surface temperature anomalies associated with the ENSO-related EAWM part in the tropical Indian Ocean and the extratropical North Pacific. In contrast, corresponding to a weak EAWM(res), the anomalous WNP anticyclone is only seen in winter, and there is no obvious relationship between the EAWM(res) and the following EASM. Therefore, the observed EAWM-EASM relationship is dominated by the winter monsoon variability associated with ENSO. It is found that the EAWM(EN)-EASM relationship is modulated by the PDO. There tends to be a much stronger EASM after a weak EAWM(EN) during the positive PDO phases than during the negative PDO phases.