Asymmetric Changes in Intraseasonal Oscillation Intensity over the Tropical Western North Pacific in El Nino and La Nina Developing Summers

The intraseasonal oscillations (ISOs) over the tropical western North Pacific (WNP) modulate atmospheric convection and heating and affect weather and climate in remote regions through atmospheric teleconnection. The present study unravels the fact that the ISO intensity increase over the tropical WNP in El Nino developing summers is larger, with the center located eastward, compared with the decrease in La Nina developing summers. The asymmetric ISO intensity changes are attributed to the eastward shift of regions of anomalous low-level westerly winds, ascent, easterly shear of zonal winds, and large moisture in El Nino developing summers and westward shift of regions of opposite anomalies in La Nina developing summers, respectively. The asymmetric atmospheric mean anomalies, in turn, are due to the westward shift of anomalous cooling in La Nina developing summers compared to anomalous warming in El Nino developing summers. The 10-20- and 30-60-day ISOs show different patterns of intensity variations due to their different source regions and propagation paths. Atmospheric model simulations confirm the asymmetric response of boreal summer ISO intensity over the tropical WNP to El Nino and La Nina events and the role of asymmetric atmospheric background field changes. Sensitivity experiments illustrate that asymmetric changes in the ISO intensity and atmospheric background fields over the tropical WNP are due to their asymmetric response to opposite tropical central-eastern Pacific SST anomalies. The asymmetry in tropical central-eastern Pacific SST anomalies in El Nino and La Nina events has a small impact on asymmetric ISO intensity changes over the tropical WNP.