Investigating Mechanisms Driving Spatiotemporal Variability of Barrier Layers in the Western Tropical Pacific

Abstract

This study investigates the physical mechanisms driving spatiotemporal variability of barrier layers in the Western Tropical Pacific (WTP) along 149°E, with a specific focus on the La Niña phase of the El Niño-Southern Oscillation (ENSO). Barrier layers, which separate the surface mixed layer from the thermocline, regulate ocean-atmosphere interactions and influence climate dynamics. This research assesses the relative contributions of freshwater input from precipitation, and wind stress on barrier layer formation and thickness. Data were collected during a research cruise in January 2025 aboard the R/V Thomas G. Thompson from an Underway Conductivity Temperature and Density (UCTD) sensor for temperature profiles, and public-source meteorological data for atmospheric conditions (ERA5). Seven stations, spaced two degrees apart in latitude, were sampled along a transect from 4°N to 15°N. Each station provided data to analyze barrier layer thickness, with spatiotemporal variability determined by comparing different formation mechanisms across stations. Spearman Correlation analyses were used to determine dominant factors influencing barrier layer thickness and variability. We found that barrier layer thickness in the WTP shows a general positive but statistically insignificant relationship with freshwater (ρ 0.32 and p-value 0.48), and a general negative but statistically insignificant relationship with wind stress (ρ 0.18 and p-value 0.70). During La Niña conditions, these effects are expected to drive variability, with thicker layers forming in regions of high precipitation and weak wind stress. Increased freshwater input enhances stratification, while strong wind stress likely promotes surface and subsurface mixing, leading to barrier layer thinning. Understanding these dynamics has implications for improving ocean-atmospheric interaction climate models in the tropical Pacific.