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

Abstract

he present study investigates the roles of El Niño?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 EAWMEN and an ENSO-unrelated part named EAWMres. Corresponding to a weak EAWMEN, 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 EAWMEN tends to be followed by a strong EASM and vice versa. As such, a link is established between the EAWMEN 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 EAWMres, the anomalous WNP anticyclone is only seen in winter, and there is no obvious relationship between the EAWMres 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 EAWMEN?EASM relationship is modulated by the PDO. There tends to be a much stronger EASM after a weak EAWMEN during the positive PDO phases than during the negative PDO phases.