Interference of the East Asian Winter Monsoon in the Impact of ENSO on the East Asian Summer Monsoon in Decaying Phases

The variability of the East Asian winter monsoon (EAWM) can be divided into an ENSO-related part (EAWM(EN)) and an ENSO-unrelated part (EAWM(res)). The influence of EAWM(res) on the ENSO-East Asian summer monsoon (EASM) relationship in the decaying stages of ENSO is investigated in the present study. To achieve this, ENSO is divided into four groups based on the EAWM(res): (1) weak EAWM(res)-El Nino (WEAWM(res)-EN); (2) strong EAWM(res)-El Nino (SEAWM(res)-EN); (3) weak EAWM(res)-La Nina (WEAWM(res)-LN); (4) strong EAWM(res)-La Nina (SEAWM(res)-LN). Composite results demonstrate that the EAWM(res) may enhance the atmospheric responses over East Asia to ENSO for WEAWM(res)-EN and SEAWM(res)-LN. The corresponding low-level anticyclonic (cyclonic) anomalies over the western North Pacific (WNP) associated with El Nino (La Nina) tend to be strong. Importantly, this feature may persist into the following summer, causing abundant rainfall in northern China for WEAWM(res)-EN cases and in southwestern China for SEAWM(res)-LN cases. In contrast, for the SEAWM(res)-EN and WEAWM(res)-LN groups, the EAWM(res) tends to weaken the atmospheric circulation anomalies associated with El Nino or La Nina. In these cases, the anomalous WNP anticyclone or cyclone tend to be reduced and confined to lower latitudes, which results in deficient summer rainfall in northern China for SEAWM(res)-EN and in southwestern China for WEAWM(res)-LN. Further study suggests that anomalous EAWM(res) may have an effect on the extra-tropical sea surface temperature anomaly, which persists into the ensuing summer and may interfere with the influences of ENSO.