Differences Between Lighter and Denser Portions of Subtropical Mode Water in Volume Variations and Thickness Anomaly Propagation

This study investigated the variability of volume, thickness, and potential vorticity (PV) in Subtropical Mode Water (STMW) using Argo data from 2004 to 2018. We revealed a dipole structure of thickness anomalies occurring primarily within the STMW layer itself, with a division at γ ≈ 25.3. The analysis also highlighted distinct timescales and propagation pathways between the lighter (L‐STMW) and denser (D‐STMW) components of STMW, extending the current understanding of STMW variability. D‐STMW volume was governed by decadal variability, whereas L‐STMW exhibited stronger interannual variations and a more distinct seasonal cycle because of its rapid erosion after formation. During periods of unstable Kuroshio Extension (KE), D‐STMW volume decreased, whereas L‐STMW maintained a higher proportion of the total STMW than in other years, leading to overall thinner and lighter STMW. Unlike D‐STMW, L‐STMW was primarily formed and distributed on the western side of the gyre. Thickness and PV anomalies in D‐STMW propagated continuously downstream from the southern edge of the outcrop area to the northern region of the southwestern corner of the gyre. In contrast, L‐STMW experienced signal intrusions during certain years near the Izu Ridge, where the original upstream signal was disrupted and replaced by a new signal that propagated downstream. These signal intrusions in L‐STMW may result from cross‐streamline southward transport driven by mesoscale eddies, emphasizing the need to examine potential temporal and vertical variations in eddy activity using higher‐resolution data sets.