Dipole Structure of Mixed Layer Salinity in Response to El Niño‐La Niña Asymmetry in the Tropical Pacific.

The asymmetry of mixed layer salinity (MLS) anomalies in response to El Niño and La Niña events in the tropical Pacific is examined for the first time based on Argo observations and Estimating the Circulation and Climate of the Ocean simulation. The difference of MLS anomalies between El Niño and La Niña shows a dipole structure, with El Niño featuring strong negative salinity anomalies east of 160°E while La Niña shows remarkable positive salinity anomalies west of 160°E. A salinity budget analysis suggests that nonlinear zonal advection plays a dominant role in generating the asymmetric MLS structure. This dipole MLS structure acts to generate a dipole structure of barrier layer thickness and thus likely plays an important role in the development of El Niño–Southern Oscillation asymmetries in sea surface temperature. Plain Language Summary: The El Niño–Southern Oscillation (ENSO) is well known for its great impacts on global climate. Previous studies on the differences between El Niño and La Niña, defined as ENSO asymmetry, focused primarily on temperature, but the differences in salinity between the two ENSO phases and their potential effects on the ENSO asymmetry are relatively unexplored. In this paper, we have for the first time documented salinity differences associated with El Niño‐La Niña asymmetry and diagnosed the air‐sea processes in controlling these differences based on observations and model simulations. We find a dipole structure exists in mixed layer salinity (MLS) variations between El Niño and La Niña in the western‐central equatorial Pacific, with stronger negative MLS anomalies to the east of 160°E during El Niño and stronger positive MLS anomalies to the west of 160°E during La Niña. Based on a salinity budget analysis, the MLS dipole is mostly attributed to the nonlinear zonal advection. This dipole MLS structure in relation to a dipole structure in barrier layer thickness, and their potential to feedback on El Niño and La Niña sea surface temperature asymmetry, is also discussed. Key Points: A dipole structure of mixed layer salinity in the western‐central equatorial Pacific is a characteristic of El Niño‐La Niña asymmetryNonlinear zonal advection plays a dominant role in generating the dipole salinity structureThe dipole salinity structure has potential effects on the asymmetry of ENSO SST [ABSTRACT FROM AUTHOR]