Intraseasonal Variability and Eddy-Induced Structural Modulation of the North Pacific Intermediate Water Revealed by Multi-Mooring Observations

Abstract. The North Pacific Intermediate Water (NPIW) plays a crucial role in modulating oceanic thermohaline circulation and biogeochemical processes. However, limited continuous observations have hindered the understanding of its short-term variability and structural response to mesoscale processes. This study investigates the intraseasonal structural variability of the NPIW and its modulation by mesoscale eddies, based on long-term mooring observations from three sites (M1–M3) across the western Pacific. The thickness of the NPIW displays substantial intraseasonal variability, dominated by an approximately 80-day period that is coherent among all mooring sites. Unlike previous studies that mainly focused on temperature and salinity anomalies, this work introduces NPIW thickness as a new structural diagnostic parameter to capture the vertical compression and expansion of the intermediate layer induced by eddy activity. The analysis identifies a strong inverse correlation between layer thickness and isopycnal-averaged salinity, demonstrating that anticyclonic (cyclonic) eddies correspond to thinner (thicker) and more saline (fresher) intermediate layers. Spatial composites further reveal that thickness variability is most pronounced near the western boundary, which may be associated with locally complex water mass exchange and mixing driven by eddies. These findings provide the first quantitative evidence of intraseasonal variability in NPIW thickness and highlight its role as a key indicator for diagnosing mesoscale–intermediate layer interactions in the North Pacific.