FY 2022

Zonal structure of tropical Pacific surface salinity anomalies affects ENSO intensity and asymmetry

Guan, C., F. Tian, M.J. McPhaden, F. Wang, S. Hu, and R.-H. Zhang

Geophys. Res. Lett., 49(1), e2021GL096197, doi: 10.1029/2021GL096197, View online (open access) (2022)

Previous studies have revealed the potential importance of salinity in El Niño-Southern Oscillation (ENSO) asymmetry, but the responsible mechanisms remain unclear. Here, we investigate how the zonal structure of surface salinity anomalies affects ENSO intensity and asymmetry by modifying freshwater flux in ocean general circulation model simulations. Results show that the amplitudes of El Niño and La Niña are both highly sensitive to zonal patterns of salinity anomalies, with the strongest sea surface temperature change occurring when maximum salinity anomalies are located near 170°W. Furthermore, the effect of salinity anomalies in the central Pacific on El Niño warming is larger than those in the western Pacific on La Niña cooling. Thus, the asymmetric salinity anomalies strengthen the temperature asymmetry between El Niño and La Niña. Temperature budget analysis shows that vertical mixing and entrainment due to salinity-induced changes in stratification are a major response to different zonal patterns of freshwater flux forcing.

Plain Language Summary. Salinity has been suggested to play active roles in the development of ENSO events by affecting vertical stratification. Recent studies found that maximum salinity anomalies are located in the central equatorial Pacific (CEP) during El Niño and in the western equatorial Pacific (WEP) during La Niña. This pattern suggests a role for salinity in ENSO temperature asymmetry, but the underlying mechanisms are unclear. In this study, we conduct a series of sensitivity experiments to investigate salinity effects on ENSO intensity and asymmetry using an ocean general circulation model. Results show that intensities of both El Niño and La Niña are highly sensitive to the zonal patterns of salinity anomalies in the tropical Pacific. Salinity anomalies have the largest impacts on the development of ENSO events when they are located in the CEP near 170°W. Therefore, salinity anomalies in the CEP contribute to stronger warming during El Niño while those in the WEP lead to relatively weaker cooling during La Niña, which strengthens the sea surface temperature asymmetry between El Niño and La Niña. Our experimental results further show that changes in vertical mixing and entrainment are a major factor in the temperature response to salinity anomalies.