Complexity of El Niño-Southern Oscillation (ENSO) is a key issue in improving forecast model skill. Salinity is suggested to exert active impacts on the development of ENSO events, but its role in ENSO asymmetry is poorly understood.

Recently, a research team led by Prof. WANG Fan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has revealed the mechanisms of ENSO asymmetry from the perspective of ocean salinity for the first time.

The study was published in Geophysical Research Letters.

Based on observations of Argo floats and satellite, the researchers found that the maximum salinity anomalies occurred in the central equatorial Pacific during El Niño and in the western equatorial Pacific during La Niña, with differences characterized as a dipole structure.

"Such differences in sea surface salinity anomalies in response to ENSO asymmetry produce a similar dipole of barrier layer via changing ocean density stratification, indicating its potential influence on ENSO sea surface temperature asymmetry in turn," said Dr. GUAN Cong, the first author of the study.

They further examined the role of salinity in affecting ENSO intensity and asymmetry in ocean general circulation model simulations by modifying freshwater flux.

Results showed that El Niño and La Niña amplitudes were both highly sensitive to zonal patterns of salinity anomalies, with the strongest sea surface temperature anomalies occurring when maximum salinity anomalies were located near 170°W in the central equatorial Pacific.

"The effect of salinity anomalies on El Niño warming in the central equatorial Pacific is larger than that on La Niña cooling in the western equatorial Pacific. Thus, the asymmetric salinity anomalies strengthen the asymmetry of temperature anomalies between El Niño and La Niña," said Prof. WANG.

The study emphasizes the importance of salinity in affecting ENSO asymmetry. It is expected to enrich the understanding of the complexity characteristics of ENSO.

Argo-based interannual anomalies of sea surface salinity (SSS), barrier layer thickness (BLT) and sea surface temperature (SST) during mature phases of El Niño, La Niña, and their differences in the tropical Pacific. (Image by IOCAS)