Subsurface and deep-water mass characteristics and variability in the Southwest Iberian margin from year-long observations

The Southwest Iberian margin is a critical oceanographic region, forming the boundary of an eastern boundary upwelling system and serving as the primary pathway for Mediterranean Outflow Water (MOW) into the Atlantic Ocean. An analysis of year-long (November 2023–December 2024) observations from moored instruments at two sites on the continental slope was conducted: a mid-slope location (PA-II; 1488 m depth) and a lower-slope site (PA-I; 2606 m depth), the latter located on the so-called Shackleton site. We investigated the natural variability of temperature, salinity, turbidity, and current velocity and direction at both subsurface and deep levels at each mooring.

Analysis of Temperature-Salinity (TS) diagrams revealed three distinct water masses. At 2606 m depth in PA-I, the North East Atlantic Deep Water (NEADW) was present, while more diluted NEADW and MOW were occasionally identified at ~ 1488 m depth in PA-II. Subsurface waters (~353 m depth in PA-I and ~418 m depth in PA-II) were characterized by the presence of the Eastern North Atlantic Central Water (ENACW) of subtropical and subpolar origins, respectively. The TS time series reveals that short-term fluctuations were more prominent than clear seasonal signals.

Current speeds were higher in subsurface waters (≥0.3 ms⁻¹) than in deep waters (0-0.3 ms⁻¹). After tidal removal, the dominant directions in PA-I were eastward in the subsurface level and north/northwestward at 2606 m depth. At PA-II, subsurface currents flow north/northeast, while deep waters move north/-northwest. Notably, currents at ~ 1488 m depth in PA-II were highly influenced by tidal components as indicated by a directional change from northeast/ southwest to north/northwest and maximum speed reduction from 0.4 ms⁻¹ to 0.3 ms⁻¹, a pattern not observed at other depths, after removing the tidal influence.

Persistent values ranging from 0.1–0.5 FTU (Formazin Turbidity Units) over extended periods were interpreted as long-lived increases in turbidity associated with upwelling, background sedimentation, and resuspension cycles. Short-lived turbidity peaks (≥0.5 FTU), lasting hours to days, are also recorded. Turbidity amplitudes were generally lower in deep waters compared to subsurface waters.

Based on surface winds, surface temperature, chlorophyll concentration, and the upwelling index, we interpret the subsurface, low-moderate turbidity signals at PA-I as offshore transport of particles along isopycnals during the peak upwelling phase (July- September). During this period, ENACW was upwelled, consistent with the subsurface current flow directions at both sites. The low-to-moderate deep-water turbidity variations, indicative of near-bottom resuspension events, coincided with the timing of local bottom trawling activities. A prominent short-lived event recorded in subsurface waters at PA-II is linked to a regional earthquake in August 2024 (~ 57 km to epicentre), while other short-lived events coincided with increased riverine sediment discharge driven by rainstorms in the west part of the peninsula.

Overall, these integrated hydrographic, currents, and turbidity observations underscore the strong coupling between water-mass structure, upwelling dynamics, and lateral transport pathways. They emphasize how both physical oceanographic processes and episodic natural and human-induced forcing are pivotal in shaping subsurface and deep-water environments in this dynamic boundary region.