PDO modulation of the ENSO impact on the summer South Asian high

Abstract

This study investigates modulation effects of the Pacific decadal oscillation (PDO) on the impact of boreal winter El Nino–Southern Oscillation (ENSO) on the South Asian high (SAH) variability in the following summer. In the El Nino together with positive PDO (EL/+PDO) or the La Nina together with negative PDO (LA/−PDO) years, boreal winter ENSO can influence the following summer SAH activity significantly. The SAH tends to be obviously strengthened (weakened) and located further south (north) during EL/+PDO (LA/−PDO). However, in the El Nino together with negative PDO (EL/−PDO) or the La Nina together with positive PDO (LA/+PDO) years, the influence of ENSO on the SAH tends to be weak. The strength and location of SAH are close to those in the climatology of 1950–2011 during the EL/−PDO or the LA/+PDO. Further analysis indicates that the PDO could exert pronounced influence on the ENSO-SAH connection via modulating the anomalous Walker circulation and charge effect over the tropical Indian Ocean (TIO). During the EL/+PDO or LA/−PDO, the anomalous Walker circulation associated with El Nino or La Nina is stronger and lasts for a longer time than those during the EL/−PDO or LA/+PDO. This leads to stronger descending (ascending) motion over the Maritime Continent and easterly (westerly) wind anomalies over the eastern Indian Ocean in the EL/+PDO (LA/−PDO), which further exert larger effects on the surface heat fluxes and subsurface ocean dynamical heating process over the Indian Ocean. As such, the induced warm (cold) sea surface temperature anomalies over the Indian Ocean are more significant and larger in the EL/+PDO (LA/−PDO). These larger sea surface temperature anomalies over the TIO could exert a more significant influence on the tropospheric temperature via moisture adjustment, which subsequently results in stronger SAH variability in the EL/+PDO or the LA/−PDO.