FY 2025

Barrier layer variability in the central equatorial Pacific conducive to ENSO development

Guan, C., M.J. McPhaden, S. Hu, F. Wang, Y. Li, and M. Cui

Geophys. Res. Lett., 52(7), e2024GL113396, doi: 10.1029/2024GL113396, View open access article at AGU/Wiley (external link) (2025)

Previous studies suggest that a thick barrier layer can strengthen surface warming during El Niño and vice versa for La Niña. Here we find barrier layer changes of up to 6 m in the central equatorial Pacific (CEP, 170°E−160°W, 5°S–5°N) in response to a 1°C change in the Niño 3.4 index using 19 years of Argo observations. Our analysis reveals that mixed layer variability due to strong interannual salinity anomalies determines the changes in CEP barrier layer thickness (BLT) during El Niño Southern Oscillation events since isothermal layer variations are minimal there. During El Niño, enhanced rainfall leads to anomalous surface freshening in the CEP especially west of the dateline, while anomalous vertical-sheared zonal current results in stronger vertical salinity stratification and thicker barrier layer east of the dateline in the CEP. Surface-layer zonal current advection in general contributes to a thicker barrier layer in the entire CEP.

Plain Language Summary. Salinity has positive effects on El Niño Southern Oscillation (ENSO) development by changing vertical stratification. During El Niño, the western Pacific fresh pool moves eastward, leading to a thicker barrier layer that prevents mixing with subsurface cold water to promote surface warming. Here based on 19 years of Argo observations, we highlight the central equatorial Pacific (CEP) as the location of maximum barrier layer variability that positively affects ENSO. It is found in the CEP with a mean BLT of about 14.5 m, the ENSO-related BLT varies with 3.8 m per 1°C change in the Niño 3.4 index. The prominent variability in CEP barrier layer is attributed to the large change in mixed layer salinity anomalies. Additionally, as is well-known of the zonal seesaw of the air-sea interactions (including the zonal gradient of thermocline) along the equatorial Pacific during ENSO cycle, the CEP happens to be its fulcrum. So that the CEP also experiences weak changes in isothermal layer depth during ENSO events, further contributing to the strong change in BLT. This study emphasizes the importance of salinity effects in the CEP during the ENSO cycle, proper representation of which may help with improving ENSO modeling and forecasting skill.