Sea Surface Salinity and Barrier Layer Thickness Signatures of the North Pacific Meridional Mode

Abstract

This study uses reanalysis data sets (1958–2022) to investigate how sea surface salinity (SSS) and barrier layer thickness (BLT) vary with the North Pacific meridional mode (NPMM), with mechanisms elucidated through mixed layer (ML) heat and salinity budget analyses. Spring regression patterns of SSS and BLT onto the NPMM Index largely coincide with their second Rotated Empirical Orthogonal Function principal modes respectively, suggesting NPMM influence on interannual variability of SSS and BLT. Within the tropical Pacific (20°S–30°N, 175°E–95°W), NPMM influences on SSS/BLT are patchy and region‐dependent. During positive NPMM phases, negative SSS anomalies dominate most of the selected area, while positive anomalies occur off western Mexico and in the eastern southern tropical Pacific. This pattern results from a dipole in freshwater flux modifications, as NPMM‐driven northward displacement of the Intertropical Convergence Zone to 5°N–12°N creates a zonal precipitation band. Horizontal advection also significantly contributes to positive salinity anomalies in the southern Pacific. NPMM‐related BLT patterns, driven by salinity stratification, exhibit distinct spatial characteristics. In the northwestern (10°N–20°N, 175°E–170°W) and southwestern (10°S–20°S, 175°E–170°W) tropical Pacific, positive NPMM phases in the tropical Pacific produce negative BLT anomalies primarily from greater shallowing of isothermal layer depth via advection and air‐sea heat fluxes relative to ML depth decreases. Within 2°N–8°N, positive BLT anomalies arise from precipitation‐induced ML depth variations. Therefore, SSS only controls NPMM‐related BLT variability in the 2°N–8°N belt and the southwestern tropical Pacific (10°S–15°S, 170°W–155°W) where positive BLT anomalies occur; in the remaining regions, BLT variability relies on both salinity and temperature vertical structure.