Symmetric and antisymmetric mass exchanges between the equatorial and off-equatorial Pacific associated with ENSO

Mass exchanges in the upper ocean between the equatorial and off-equatorial Pacific Ocean associated with the El Niño/Southern Oscillation (ENSO) are investigated using the National Centers for Environmental Prediction (NCEP) ocean assimilation data. The data show that ENSO-related meridional mass transport in the Northern Hemisphere (NH) is larger than that in the Southern Hemisphere (SH). We found that the antisymmetric characteristics are mainly due to a southward shift of themaximum zonal wind stress anomaly during the ENSO mature phase. The Ekman and geostrophic transport associated with ENSO are separated into symmetric and antisymmetric components. For the symmetric part, the mass divergence over the equatorial Pacific by the geostrophic transport is generally larger than the convergence by the Ekman transport during the El Niño mature phase. Therefore mass is transported from the equator to off the equator at this time. As for the antisymmetric part, the Ekman transport due to antisymmetric wind stress dominates the geostrophic transport so that the mass is transported from the SH to the NH during the El Niño mature phase. The net mass transport in the NH is larger than that in the SH. A theoretical interpretation and intermediate model experiments support these arguments.