National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2024

Zonal structure of tropical Pacific surface salinity anomalies affects the eastern and central Pacific El Niño differently

Guan, C., F. Tian, M.J. McPhaden, S. Hu, and F. Wang

Geophys. Res. Lett., 50(21), e2023GL105554, doi: 10.1029/2023GL105554, View open access article at AGU/Wiley (external link) (2023)

Maximum sea surface salinity (SSS) anomalies are found in the central Pacific during the eastern Pacific El Niño (EPEN) and located further westward during the central Pacific El Niño (CPEN), but whether these differences affect event strengths is unclear. By performing ocean general circulation model experiments via modifying freshwater flux anomalies, we find salinity effects on surface warming during both types are highly sensitive to zonal locations of SSS anomalies, with the strongest warming induced by the SSS anomalies near the international dateline. Further analysis reveals that vertical mixing and entrainment dominate this temperature sensitivity, with the strongest response to SSS anomalies occurring in the central Pacific. The central-Pacific SSS anomalies increase EPEN warming by 0.15°C while the westward-located SSS anomalies make little contribution to CPEN warming. Therefore, the distinct zonal structures of SSS anomalies facilitate stronger EPEN than the CPEN, increasing their difference in intensity by about 10%.

Plain Language Summary. The El Niño-Southern Oscillation (ENSO) is the strongest year-to-year climate variability in the planet. The central Pacific El Niño (CPEN) have been recognized in recent decades, with weaker surface warming located more westward, in contrast to the traditional eastern Pacific El Niño (EPEN). Previous studies have underscored various air-sea processes in shaping these differences, however, the impact of salinity remains unknown. Here based on ocean model experiments, we found the salinity effect on El Niño warming is very sensitive to zonal locations of salinity anomalies. The salinity anomalies located in the central Pacific are more effective in modulating local ocean vertical stratification, weakening the colder subsurface water into the mixed layer and further enhancing the surface warming. Therefore, the central equatorial Pacific-located salinity anomalies during EPEN contributes to its stronger warming than those west-located salinity anomalies during CPEN, enhancing the sea surface temperature difference between the two events by about 10%. Our results provide new insight in understanding the ENSO diversity and also its low-frequency variability, which may be helpful for interpreting more complex model simulations and for and predicting ENSO variations.

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