Mechanism for asymmetric atmospheric responses in the western North Pacific to El Nino and La Nina

The cause of asymmetric atmospheric circulation responses over the tropical western North Pacific (WNP) to El Nino and La Nina was investigated through observational analyses and idealized modeling experiments. Firstly, column integrated moisture and moist static energy budget analyses were carried out to reveal the cause of asymmetric precipitation anomalies over the WNP. The result indicates that negative nonlinear moist enthalpy advection anomalies occur in both El Nino and La Nina, and they tend to induce a negative precipitation anomaly in the key WNP region and thus an anomalous anticyclone during both El Nino and La Nina winters. This, together with linear moist enthalpy advection, results in an asymmetric atmospheric circulation response. Secondly, the relative roles of the nonlinear moist enthalpy advection and the zonal shift of longitudinal location of anomalous heating over the central-eastern Pacific between El Nino and La Nina were investigated through an anomaly general circulation model. It is found that both the nonlinear advection and the zonally asymmetric heating contribute equally to the observed zonal shift of the anomalous WNP anticyclonic and cyclonic circulation centers between El Nino and La Nina.