Interdecadal modulation of ENSO-related spring rainfall over South China by the Pacific Decadal Oscillation

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The interdecadal modulation of the relationship between El Nino–Southern Oscillation (ENSO) and the South China spring rainfall (SCSR) by the Pacific Decadal Oscillation (PDO) is investigated using long-term observational datasets. When ENSO and PDO are in-phase [i.e., El Nino events during warm PDO (EN_WPDO) and La Nina events during cold PDO (LN_CPDO)], the positive correlations between ENSO and SCSR are enhanced significantly, with above-normal (below-normal) SCSR generally following EN_WPDO (LN_CPDO) events. In contrast, the ENSO–SCSR relationship becomes ambiguous when ENSO and PDO are out-of-phase [i.e., El Nino events during cold PDO (EN_CPDO) and La Nina events during warm PDO (LN_WPDO)]. The PDO modulates the ENSO–SCSR relationship through the impact of variations in the lower-tropospheric subtropical anticyclone over the western North Pacific (WNP) and upper-tropospheric westerly jets over East Asia and the midlatitude North Pacific. An EN_WPDO (LN_CPDO) event induces an enhanced subtropical anticyclone (cyclone) over the WNP and intensified (weakened) subtropical westerly jet around the southern Tibetan Plateau due to modification by the PDO-forced anomalous circulation. Thus, South China falls just under the influence of the anomalous lower-tropospheric southwesterlies (northeasterlies) and upper-tropospheric divergent (convergent) environment, leading to above-normal (below-normal) SCSR. In contrast, the SCSR anomalies exhibit no wet or dry preference following EN_CPDO (LN_CPDO) events, because ENSO-induced and PDO-forced circulation anomalies tend to cancel each other out. These modulating effects by the PDO on the ENSO–SCSR relationship and related physical processes are also examined with coupled model simulations.