Roles of ENSO and PDO in the link of the east asian winter monsoon to the following summer monsoon

Abstract

The present study investigates the roles of El Nin~ o-Southern Oscillation (ENSO) and the Pacific decadal oscillation (PDO) in the relationship between the EastAsian 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 EASMand vice versa. As such, a link is established between theEAWMEN 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 weakEAWMres, 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 thewinter 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 EASMafter a weakEAWMEN during the positivePDO phases than during the negative PDO phases. © 2013 American Meteorological Society.