Mechanism For Asymmetric Atmospheric Responses In The Western North Pacific To El Ni?o And La Ni?a

The anomalous anticyclone over the western North Pacific(WNPAC)at low level links ENSO and the variability of East Asian winter and summer climate in El Ni?o.The WNPAC in El Ni?o and anomalous cyclone in La Ni?a(WNPC)are asymmetric in their center locations and intensities,and the precipitation anomalies also show a significant asymmetry between El Ni?o and La Ni?a over the western North Pacific(WNP).What causes asymmetric atmospheric circulation responses over the tropical WNP to El Ni?o and La Ni?a was investigated through observational analyses and modeling experiments.The results of column integrated moisture and moist static energy budget analyses indicate that the nonlinear moist enthalpy advection is important to cause negative precipitation anomalies in the key region during both El Ni?o and La Ni?a winters.The northwesterly anomalies in El Ni?o and southeasterly anomalies in La Ni?a both lead to negative moist enthalpy advection,suppressing the precipitation anomalies in situ.The negative heating induces an anomalous anticyclone,strengthening the anomalous anticyclone in El Ni?o,and weakening the anomalous cyclone in La Ni?a winter,which causes the asymmetric atmospheric circulation.In addition,besides the nonlinear moist enthalpy advection,zonal shifting of sea surface temperature anomaly(SSTA)over the central eastern Pacific(CEP)has an influence on circulation over the WNP between El Ni?o and La Ni?a.Relative roles of the two factors were investigated through an anomaly atmospheric general circulation model(AGCM).It is found that both the nonlinear moist enthalpy advection and the zonal shifting of heating anomaly contribute to the observed zonal shifting of center location between WNPAC and WNPC,and the contributions of the two factors are almost equal.For the intensities of WNPAC and WNPC,the effect of two factors is the opposite.Negative nonlinear moist enthalpy advection tends to strengthen WNPAC,and weaken WNPC.Zonal shifting of heating anomaly over CEP tends to push convection far from the key region,decreasing the effect of nonlinear moist enthalpy advection,which weakens WNPAC,and strengthens the WNPC.