Salinity-Driven Barrier Layer Dynamics in the Equatorial Pacific

Abstract

Barrier layers in the upper ocean suppress the upward entrainment of cold thermocline water, trapping heat and momentum near the surface and thereby influencing tropical air-sea interaction. However, the mechanisms governing their spatial and temporal variability are not fully understood. This study highlights the importance of salinity-induced vertical stratification in shaping the climatology and variability of the barrier layer in the Pacific Ocean on subannual and interannual timescales. Compared to observational and reanalysis data, coupled ocean-atmosphere models simulate a less eastward-extending warm pool, along with a thin barrier layer bias. This bias is linked to a saltier upper western Pacific and can be attributed to weaker precipitation and stronger easterly winds along the equator. Consistently, models with a more eastward-extending warm pool tend to exhibit a thicker barrier layer and lower salinity over the western Pacific. On interannual timescales, models agree with observational and reanalysis data that anomalous westerly winds and increased precipitation develop 10–13 months before the peak eastward shift of the warm pool eastern edge (WPEE), accompanied by an upper-ocean freshening. These anomalies peak with the WPEE shift and persist for another 9–10 months. Subannual variations exhibit more complex temporal patterns. Anomalous westerly winds and increased precipitation emerge 3–4 months prior to the WPEE peak extension and reverse rapidly one month after the peak. While models capture the timing and magnitude of wind and precipitation changes, they fail to reproduce subannual salinity variations. Improving salinity climatology and subannual variability in models remains essential for simulating barrier layers.