Asymmetry of Atmospheric Responses to Two-Type El Niño and La Niña over Northwest Pacific

Abstract

The mechanism for asymmetric atmospheric responses to the central Pacific (CP) El Nino and La Nina over the western North Pacific (WNP) is studied in this paper. The negative anomalies of rainfall over the key region of WNP are explained by diagnosing the column-integrated equations of moisture and moist static energy (MSE). It is revealed that the nonlinear advection of moist enthalpy is critical to introduce negative rainfall anomalies over the region. The anomalous easterly (westerly) in La Nina (CP El Nino) causes negative advection of anomalous moist enthalpy, inducing negative heating anomaly and an anticyclone anomaly in the WNP, which weakens (strengthens) the cyclone (anticyclone) in La Nina (CP El Nino). The MSE budget analysis shows a larger nonlinear term in CP El Nino than in eastern Pacific (EP) El Nino, inconsistent with the amplitudes of sea surface temperature anomalies. The reason is that the nonlinear term transforms to positive above 700 hPa in EP El Nino, offsetting the negative advection below 700 hPa and thus making the nonlinear term smaller. The nonlinear term is negative at low levels in CP El Nino, resulting in a larger nonlinear term. The stronger precipitation anomalies in the WNP during EP El Nino can be attributed to the linear moist enthalpy advection. The mean easterly wind at mid levels causes a larger (smaller) positive moist enthalpy advection in CP (EP) El Nino, due to a larger (smaller) moist enthalpy gradient, resulting in a positive (negative) linear moist enthalpy advection, which weakens (strengthens) the negative precipitation anomalies in the key region.