The El Niño-Southern Oscillation (ENSO) is the strongest year-to-year climate variability in the planet, famous for prominent surface warming in the equatorial eastern Pacific known as an El Niño event. Recent decades have recognized weaker surface warming located more westward in some years, named as central Pacific El Niño (CPEN) 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.
Recently, the research team led by Prof. WANG Fan from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) has revealed dynamical mechanisms of ENSO diversity from the new perspective of ocean salinity.
The study was published in Geophysical Research Letters on Oct. 31.
Based on Argo observations and reanalysis products, it is found maximum salinity anomalies during the two types of El Niño show clear different zonal structure, which occur in the central equatorial Pacific (CEP) during the EPEN and in further western equatorial Pacific (WEP) during CPEN.
"Ocean salinity plays active roles in developing El Niño warming through determining ocean vertical stratification and modifying the cooling entrainment and mixing from the subsurface, but we are curious about whether these different salinity zonal structures will affect the El Niño diversity," said Dr. GUAN Cong, first author of the study.
Based on ocean model experiments, researchers found that 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 Pacific-located salinity anomalies during EPEN contributes to its stronger warming than those west-located salinity anomalies during CPEN, enhancing their sea surface temperature (SST) difference by 11%.
"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 predicting ENSO variations," said Prof. WANG.
Fig. 1 Interannual anomalies of sea surface temperature (left), salinity and freshwater flux (right) during EPEN and CPEN composites.
Fig. 2 El Niño warming in response to different longitudes of FWF-induced SSS anomalies (a) and its associated temperature budget terms during the EPEN and CPEN events (b, c).
Fig. 3 Schematic diagram illustrating different effects of zonal patterns of SSS anomalies on the CP El Niño and EP El Niño.
Guan, C., Tian, F., McPhaden, M. J., Hu, S., & Wang, F. (2023). Zonal structure of tropical Pacific surface salinity anomalies affects the eastern and central Pacific El Niños differently. Geophysical Research Letters, 50, e2023GL105554.
(Text by GUAN Cong)
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ZHANG Yiyi
Institute of Oceanology
E-mail: zhangyiyi@qdio.ac.cn
(Editor: ZHANG Yiyi)