Water mass spiciness and thickness anomalies, and their propagation in the North Pacific Subtropical Mode Water

Water masses in the upper ocean play a critical role in modulating ocean stratification and circulation, serving as pathways for atmospheric signals to penetrate the ocean interior and influencing climate variability and marine ecosystems. The North Pacific Subtropical Mode Water (STMW) is a distinct water mass in the northwestern subtropical gyre of the North Pacific Ocean. Although previous studies have extensively highlighted the importance of investigating variations in STMW properties, they predominantly treated STMW as a homogeneous entity, overlooking differences across its different density ranges.

Building on this foundation, we examined spiciness anomalies (density-compensated salinity and temperature anomalies) and isopycnal thickness anomalies within STMW based on Argo observations from 2004 to 2018, with a focus on the comparisons between its lighter (L-STMW) and denser (D-STMW) portions.

Firstly, we investigated interannual to decadal variations in STMW properties across different density ranges. The isopycnal thickness anomalies exhibited a seesaw pattern between L-STMW and D-STMW, separated by a threshold at γ = 25.3 (Figure 1). The volume of D-STMW was primarily governed by decadal variability linked to the Kuroshio Extension, while L-STMW displayed weaker decadal variability with a different phase, as well as stronger interannual and seasonal variations. In contrast, STMW salinity and temperature showed consistent variations across different density ranges.

Secondly, the propagation patterns of thickness and potential vorticity (PV) anomalies differed markedly between L-STMW and D-STMW. For D-STMW, thickness and PV anomalies propagated steadily 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, likely caused by off-stream southward transport driven by mesoscale eddies, potentially influenced by topographic effects. However, spiciness anomalies in STMW displayed consistent downstream propagation on all the isopycnals, without significant difference between L-STMW and D-STMW.

These results provide insights into the seesaw structure of mode water variability and may offer broader implications for discovering similar processes in other ocean basins.

Fig. 1 Annual mean thickness anomalies of each 0.05 γ range, averaged in 20–30°N.