This study uses 51 Conductivity, Temperature, and Depth (CTD) profiles collected in the Gulf of Oman, offshore Fujairah, between 9 and 18 July 2025 (Fig. 1). Profiles were obtained using a Sea-Bird SBE 19plus V2 SeaCAT CTD operating at 4 Hz and attached to a rosette water sampler to ensure stability during deployment. However, we did not collect water samples to adjust the CTD measurements. The profiling speed during downcasting was ∼0.35 m s⁻¹ (median value for all CTD profiles), resulting in a vertical sampling interval of ∼0.0875 m. During profiling, we stopped at 50 m intervals to collect continuous oxygen measurements for a few minutes for another purpose unrelated to the current work. However, as the data quality control (Fig. S1 in the Supplement) illustrates, these stops had no significant effect on salinity or density. The original CTD resolution was used for hydrographic parameters, Brunt-Väisälä frequency squared, Turner angle, and the subsequent Thope-scale analysis. High-resolution multibeam bathymetry was collected using a Kongsberg EM712 system with Seafloor Information System (SIS) software. This bathymetry dataset can be accessed from Abdelmaksoud et al. (2025a). ROV imagery was also used to assess relationships between benthic communities and the outflow.

Four distinct water masses were identified at the mouth of the Gulf of Oman during summer (Fig. 1b). The upper layer, termed Arabian Sea High-Salinity Water (ASHSW), is shallow and characterized by an absolute salinity of ∼37.2 g kg⁻¹ and temperature of ∼33 °C. Indian Ocean Central Water has a salinity of ∼36.6 g kg⁻¹ and temperature of ∼28 °C. The P/AG outflow exhibits very high salinity (37.0–39.8 g kg⁻¹) and moderate temperatures of 20–27 °C. Indian Ocean Deep Water, the deepest layer, has lower temperatures (∼13–20 °C) and salinities (∼36.0–37.0 g kg⁻¹). Oxygen concentrations are 4–5 mL L⁻¹ in the Arabian Sea High-Salinity Water and 5–6 mL L⁻¹ in IOCW. The P/AG outflow shows intermediate values (1.5–3.5 mL L⁻¹), whereas IODW exhibits the lowest oxygen (<1 mL L⁻¹).

The Turner-angle section (Fig. 2f) characterises the susceptibility of the water column to double diffusion between these water masses. Where the warm, salty P/AG core overlies cooler, fresher deep water (IODW), mainly where the plume detaches and lifts off the seabed, downstream conditions favour salt fingering at the base of the plume; the interior is largely doubly stable, with diffusive-convection-favourable conditions occurring where cooler, fresher water overlies the core (at depths of around 100–200 m, from station 32 to station 06). N2 distribution shows negative values, coinciding mainly with the Gulf outflow plume after the bathymetric step (at depths of around 150–300 m, from station 32 to station 06) (Sect. 3.3).

The outflow forms a distinct layer characterized by elevated salinity, temperature, oxygen, and pH (Fig. 2a and b). As the outflow enters the Gulf of Oman, it separates into three lateral sectors. Across the mouth of the Gulf of Oman over relatively flat and shallow bathymetry (depths < 130 m) of the stable shelf, properties are highest (salinity ∼ 39.0 g kg⁻¹, temperature ∼26 °C, oxygen ∼3.0 mL L⁻¹, pH ∼ 8.51; Figs. 2a, c, e and 4a–c). Over the unstable shelf (depths of 130–370 m), which is characterized by pockmarks and erosional features, values decrease to ∼38.5 g kg⁻¹, ∼23.5 °C, ∼2.5 mL L⁻¹, and pH ∼ 8.42. Farther south over the lower shelf and upper slope (depths > 370 m), properties decline further (∼37.2 g kg⁻¹, ∼22 °C, ∼2.0 mL L⁻¹, pH ∼ 8.35). The western shallowest occurrence of the outflow is at ∼110 m depth. A high-turbidity layer occurs above the seafloor and thickens basinward from ∼20 m above the flat shelf to >400 m above the upper slope (Fig. 2a and b). P/AGw_f values reach ∼0.7 over the stable shelf and exhibit along-flow lateral variations, largely coincident with the rough, eroded bathymetry. The outflow core and plume are thin over the stable shelf and are disrupted by rough bathymetry (Fig. 3c). The core fades shortly after these flow disruptions. The roughness-to-plume-thickness ratio (hr / H) is used as a measure of the dynamical interaction between the P/AG outflow plume and the seabed. The hr / H increases to >1 immediately after the bathymetric step (Fig. 3d, near station 32 southwards). The top of the outflow occurs at depths of 50–100 m above the flat shelf, with a very thin layer that has a thickness of 10–30 m, exhibiting a southeastward deepening and thickness increase (Fig. 4e and f). The upper depth of the P/AG outflow increases up to 170 m in the center, then becomes shallower at the extreme southern portions, reaching ∼130–140 m (Fig. 4e). The thickness increases up to 200 m above the lower shelf and upper slope (Fig. 4f).

The critical velocity ci mirrors this structure (Fig. 3c). Over the stable shelf ci is low, falling to a minimum of ∼0.15–0.20 m s⁻¹ immediately upstream of the bathymetric step, where the plume is thinnest. At the step, ci increases sharply to ∼0.8 m s⁻¹ with subsequent fluctuations and the eventual convergence to ∼0.8–1.0 m s⁻¹ across the unstable shelf and downstream, following the thickening of the plume.

The Thorpe-scale section (Fig. 3a) shows that CTD-resolved density overturning is spatially heterogeneous along the P/AG outflow transect. Over the stable shelf, where the P/AG plume is thin and bottom-attached, LT values are generally low, indicating limited overturning within the compact plume. Toward the stable-to-unstable shelf transition (Fig. 3a, at station 32), LT increases coinciding with the zone where the plume thickness curve shows rapid thickening (Fig. 3c) and where the high-P/AGw_f core begins to weaken (Fig. 2h). Across the rough unstable shelf, elevated LT extend through a broader part of the plume and its upper interface, consistent with stronger vertical reordering of density in the same sector where salinity, temperature, oxygen, and pH decrease downstream.

The section of diapycnal diffusivity based on the Thorpe scales, Kρ(T) (Fig. 3b), shows a similar distribution along the transect. Low Kρ(T) over the stable shelf (stations 45 and 46) reflects the limited overturning in the thin attached plume. Higher Kρ(T) values (up to 0.03 m² s⁻¹ at station 32, depths of ∼150–200 m) occur mainly near the bathymetric transition and across the unstable shelf (e.g., ∼0.02 m² s⁻¹ at ∼250 m depth of station 29, ∼0.025 m² s⁻¹ at ∼300 m depth of station 19, and ∼0.025 m² s⁻¹ at ∼275–350 m depth of station 16), where the plume-thickness curve indicates rapid expansion of the P/AG layer (Fig. 3c) and the P/AGw_f section shows progressive dilution of the plume core (Fig. 2h). These enhanced diffusivity-proxy patches indicate locations where CTD fine structure records stronger overturning and vertical exchange within the transforming outflow.

Comparison with the along-transect hr / H curve (Fig. 3d) shows that the strongest Thorpe-derived signals occur where seabed roughness becomes large relative to the local P/AG plume thickness (Fig. 3a, b, and d). In the stable shelf sector, low hr / H, small plume thickness, and weak Thorpe-derived signals are consistent with a compact, bottom-attached plume. Immediately after the bathymetric transition, hr / H increases as roughness elements become comparable to the plume thickness, while the LT and Kρ(T) sections show enhanced patchiness within the plume. Farther across the unstable shelf, the combined pattern of high or variable hr / H, plume thickening, elevated LT, enhanced Kρ(T), P/AGw_f decay, reduced oxygen, and increased turbidity supports localized roughness-modulated overturning and dilution of the P/AG plume.

Near-seafloor parameters (Fig. 4a–c) show sharp lateral transitions (salinity 38–36 g kg⁻¹, conservative temperature 25–15 °C, oxygen 3 to <0.5 mL L⁻¹) across the depth range where the outflow shifts from bottom-attached to detached flow. Where the outflow remains attached (Fig. 5c), the seabed consists of exposed or semi-consolidated substrate with little fine-sediment drape, consistent with persistent near-bottom shear and resuspension. At deeper sites where the near-bottom hydrographic signature of the outflow weakens (Fig. 5d), the seafloor is dominated by fine-sediment accumulation beneath a detached outflow layer.

Superimposed on this downstream decline are two localized patches of reduced near-bottom P/AGw_f, accompanied by lower salinity, temperature and oxygen, one in the north (near station 36) and one to the southwest (Fig. 4a–d). In both, the outflow signal persists near the seabed but is locally weakened rather than absent.

ROV imagery from ∼284 m depth (Fig. 5c) shows a conspicuous epibenthic community on hard or firm ground, dominated by sessile suspension feeders. Branching azooxanthellate scleractinian colonies and octocorals are common, with additional scattered epifauna on exposed patches of substrate. This assemblage is consistent with a relatively energetic bottom boundary layer associated with the attached outflow. By contrast, imagery from ∼360 m depth (Fig. 5d) shows a flat, soft-sediment seabed with sparse epifauna; tube anemones, consistent with cerianthids, are present and locally abundant. This shift, from coral-bearing hard-substrate assemblages to soft-sediment, cerianthid-dominated assemblages, coincides with the hydrographic transition, providing an independent benthic indicator of attachment and separation of the outflow along the margin.

Hydrographic CTD measurements in the northwestern Arabian Sea reveal spatial variability in P/AG outflow characteristics. The continental shelf of the northeastern Arabian Plate margin is subdivided into two domains: a stable and an unstable shelf, based on the integration of multibeam bathymetry and seismic reflection data. The outflow waters exit the Strait of Hormuz, hugging the flat continental stable shelf bathymetry, with a very thin bottom-attached flow of 10–30 m (Figs. 2–4). This flow has high salinity, temperature, and density, resulting in negative buoyancy and a compact, coherent core with high P/AGw_f and comparatively little dilution. The flow has a sharp upper interface (high N2), indicating stable stratification (Fig. 2d). Outflows exiting dense marginal seas typically begin as thin bottom-attached plumes before being transformed by entrainment (e.g., Bower et al., 2000).

At the transition from the stable to the unstable shelf, the abrupt bathymetric shift serves as the initial trigger for flow transformation. This transition marks an abrupt increase in the seafloor depths delimited by a major gravitational collapse, i.e., northern extension of the Batinah Offshore Normal Fault. The outflow suffers from rapid hydrographic and geometric adjustment, increasing in thickness from ∼20 to ∼170 m, with localized patches of N2<0 occurring in the immediate vicinity of the bathymetric transition/step (Figs. 2d and 4f). This situation is consistent with locally statically unstable stratification and transient density overturning during hydraulic adjustment of the bottom-attached plume across the bathymetric step (Alford et al., 2013; Jalali et al., 2017). This indicates the bathymetric step is dynamically active and likely promotes enhanced turbulent entrainment/mixing at the onset of plume transformation.

The critical-velocity estimate provides an independent, if indirect, line of support for this interpretation. Independent values for the P/AG outflow speed in this region are ∼0.2–0.55 m s⁻¹ from moored-ADCP, lowered-ADCP, and model studies (Bower et al., 2000; Ezam et al., 2010; Pous et al., 2004a, b). The ci based on our CTD data falls to ∼0.15–0.20 m s⁻¹ upstream of the step (Fig. 3e) which is below the estimates from the literature, implying near-critical to supercritical conditions (Fr≳1) over the thin, bottom-attached plume. Downstream of the step, ci rises to ∼0.8–1.0 m s⁻¹, exceeding those same speeds and implying subcritical conditions (Fr<1). A transition from supercritical to subcritical flow across a topographic step is the signature of a hydraulic jump and is consistent with the abrupt thickening, the transient N2<0, and the enhanced overturn proxies at the same location (Figs. 2d and 3a–c). We stress that the historical current data are non-synoptic and that ci increases partly because the plume thickens, so this comparison is indicative rather than a direct measurement of hydraulic state. Consistent with this, model results place the maximum outflow velocity where the bottom slope steepens (Ezam et al., 2010).

The unstable shelf has extensive crescent-shaped and elongated pockmarks and irregularities on the seafloor (Abdelmaksoud et al., 2025b). Within this domain, the outflow is characterized by lower salinity, temperature, and oxygen content than the undisrupted flow above the stable shelf, with continued plume thickening and weakening of P/AGw_f. The outflow in this domain exhibits disruption, and its compact core eventually fades. Its ambient interface is consistent with sustained interfacial entrainment/mixing, dilution of the dense core, and the development of a thicker, more diffuse interfacial mixing layer. Mechanistically, the roughness effect is expressed in terms of roughness height relative to plume thickness, hr / H. The hr / H exceeds unity immediately after the bathymetric step. Where the roughness elements become a substantial fraction of plume thickness, they enhance form drag, wake generation, and near-bottom turbulence, thereby sustaining entrainment and dilution across the pockmarked shelf sector (Cenedese and Adduce, 2010; Cenedese et al., 2018; Maggi et al., 2022; Ottolenghi et al., 2017; Özgökmen and Fischer, 2008). This situation is consistent with roughness-enhanced boundary-layer turbulence and hydraulic transitions documented for other marginal-sea overflows (Peters and Johns, 2005; Price et al., 1993).

Once the P/AG outflow water gains enough buoyancy, it begins to detach from the seafloor above the unstable shelf and to flow at an intermediate level (Fig. 5). The detached flow from the seafloor has lower salinity, temperature, and oxygen content, with its top occurring higher in the water column, i.e., towards shallower depths (Figs. 2–4). The intensity of the mixing and dilution of the P/AG outflow changes across the unstable shelf domain. This intensity is broadly consistent with the density of seafloor irregularities and with the plume thickening downstream of the bathymetric step, with higher-mixing zones tending to occur where irregularities are denser (Fig. 5a).

Over the lower shelf to the upper slope, the near-bottom P/AG outflow signatures weaken strongly, and the P/AGw_f near the seabed becomes small or disappears (Fig. 2h), indicating that the bottom attachment is lost and the plume persists higher in the water column. The outflow transitions into a detached intermediate-depth plume, following from the preceding thickening and dilution across the unstable shelf. The disappearance of the pronounced N2 maximum at the upper plume interface (Fig. 2d, at station 46 southward to station 32 and at 100–150 m depths) shortly after the stable-unstable shelf transition or bathymetric step (Fig. 2d, at station 29 and around 150 m depth) is consistent with erosion of the initially sharp plume to the ambient density interface by entrainment and dilution until the bottom-attached gravity current can no longer maintain a clear near-bottom density excess. Farther offshore, the gradual re-establishment of moderate N2 suggests formation of a new, weaker and broader interface around the detached plume as it adjusts toward its intermediate intrusion/neutral-buoyancy depth (Fig. 2d, at stations 16, 07, and 06 at 100–150 m depths). The detached intermediate-depth plume/intrusion exhibits continued along-slope propagation, and the maps (Fig. 4) suggest that, throughout the study area, the plume remains organized as a margin following, bathymetrically steered boundary plume/current, with a clear shallow-side/upslope limit imposed by the shelf morphology. Along-path dilution of the intermediate-depth plume within the Gulf of Oman is also evident, with the plume exhibiting P/AGw_f of around 0.4 near the mouth of the Gulf (this study, Fig. 2h), and the value is reduced to less than 0.3 to the south across the Omani Margin (Font et al., 2024).

The two near-bottom low-P/AGw_f patches around stations 36 and 11 (Fig. S5) are interpreted as localized lateral interleaving of ambient, lower-P/AG water into the near-bottom outflow layer. On the σ0=26 isopycnal, which removes the effect of plume thickness and depth, the P/AGw_f field shows a lateral structure (Fig. S5), indicating interleaving of P/AG-rich and P/AG-poor water across the shelf. Such interleaving is expected at the margins of the boundary plume, where mesoscale and submesoscale stirring can peel P/AG water from the boundary and draws ambient water inward (L'Hégaret et al., 2016; Vic et al., 2015). Because the survey is non-synoptic, a partly transient origin, such as plume meandering on sub-seasonal timescales (Pous et al., 2004a, b), cannot be excluded. Internal tides are well documented in the Gulf of Oman, with generation attributed to tidal flow across the shelf break off the Musandam Peninsula (Small and Martin, 2002) and to steep topography within the Strait of Hormuz (Subeesh et al., 2025); moored records further southeast on the Al-Batinah shelf show a predominantly diurnal, shoreward-propagating internal tide of largely remote origin (Bruss et al., 2026). Reported pycnocline displacements are of order 10–25 m, so internal tides can displace a density interface on diurnal to semidiurnal timescales. Because each station represents a single snapshot, aliasing of the internal-tide phase may contribute to the observed patchiness, although the present data cannot separate this from mesoscale and submesoscale stirring. The patches are therefore not attributed to the seabed erosional features, and there is no evidence that they represent local detachment of the outflow.

The Thorpe-scale analysis provides a quantitative hydrographic basis for interpreting the fine structure and step-like density profiles observed along the P/AG outflow pathway. Elevated LT and Kρ(T) occur preferentially near the stable-to-unstable shelf transition and across the rough bathymetry, where the P/AG plume thickens, the P/AGw_f core weakens, and the sharp upper-interface N2 maximum is eroded. LT and Kρ(T) estimates are not direct microstructure measurements, and their interpretation is constrained by standard CTD-overturn quality-control requirements and by the known uncertainty of overturn-based estimates near rough topography (Galbraith and Kelley, 1996; Gargett and Garner, 2008; Jalali et al., 2017). Nevertheless, their spatial coincidence with high hr / H, plume thickening, P/AGw_f decay, turbidity enhancement, oxygen reduction, and weakening of the upper plume interface supports the interpretation that bathymetric roughness modulates localized overturning and dilution of the P/AG plume. This relationship is strongest where the hydrographic overturn proxies coincide with the bathymetric transition and rough seabed sectors, whereas isolated overturns outside the plume or away from rough terrain are interpreted more cautiously because they may also reflect internal-wave activity, mesoscale intrusion, or double-diffusive layering.

Double diffusion is intrinsic to the P/AG outflow: its warm, salty core overlies cooler, fresher deep water, favouring salt fingering at the base of the plume, consistent with salt fingering reported beneath P/AG water further downstream in the Sea of Oman (Font et al., 2024) and in the Arabian Sea (L'Hégaret et al., 2021). Because double diffusion operates within statically stable stratification (N2>0) and transfers heat and salt across the plume's interfaces without producing density inversions, the salt-fingering and diffusive-convection regimes (Fig. 2f) are dynamically distinct from the overturning detected by the Thorpe scale (Fig. 3a and b). Double diffusion contributes to the slow, along-path erosion of the plume's salinity, temperature, and oxygen contrasts, whereas the abrupt transformation at the bathymetric step is driven by hydraulic adjustment and overturning. In the hierarchy of processes acting on the outflow it is therefore a secondary, background contributor to interface erosion, rather than the primary driver of the step-localised transformation. The Turner angle diagnoses only susceptibility to double diffusion, not its flux, which, having a lower effective diffusivity and a different mixing efficiency than shear-driven turbulence, is not captured by Kρ(T) and cannot be quantified without microstructure data.

The outflow results in further erosion and deepening of the seafloor irregularities of the unstable shelf. The outflow water then carries the erosional products and deposits its load directly after detaching from the seafloor above the lower shelf and slope (Fig. 2b). Strong bottom flows over rough bathymetry are known to maintain thick bottom nepheloid layers through resuspension (McCave, 1986; Gardner et al., 2018). Marine nepheloid layers, particularly bottom nepheloid layers on continental margins, mark loci of active suspension and redeposition driven by energetic near-bed currents (Biscaye and Eittreim, 1977; Gardner et al., 2018).

Benthic zonation closely follows the attachment behaviour of P/AG outflow. Where the outflow remains bottom-attached, conditions consistent with an energetic near-bed boundary layer limit fine-sediment accumulation, maintaining exposed or semi-consolidated substrate and enhancing delivery of suspended particles and oxygenated water. Such conditions favor sessile suspension feeders, including corals and other epifauna, consistent with the importance of hydrodynamics and substrate availability in deep-water coral ecosystems (Flach et al., 1998; Roberts et al., 2006).

Below the detachment depth, fine sediments dominate and the hydrographic signature weakens, indicating lower-energy, lower-oxygen conditions. Cerianthid tube anemones commonly occur on such soft substrates and can form distinctive assemblages observable in video surveys (Davies et al., 2014; Flach, 2002). The thick nepheloid layer along the lower shelf and slope may further enhance food supply by retaining suspended organic matter, acting as a reservoir redistributed by mixing (Hanz et al., 2019). These patterns provide a biological imprint of boundary-layer dynamics and independently support the hydrographic interpretation of outflow attachment and separation.

P/AG outflow brings high-oxygen waters from the Arabian Gulf to the Arabian Sea-OMZ, thereby ventilating the OMZ. Glider observations off Oman directly demonstrate that P/AG water supplies oxygen to the OMZ at intermediate depths, quantifying its ventilation contribution along the slope (Font et al., 2024). Modeling further indicates that changes in Gulf properties can modulate OMZ intensity by altering the density and ventilation potential of exported P/AG water (Lachkar et al., 2019). Downstream changes in the oxygen content and pH of the outflow water show consistent dilution and oxygen loss along the continental shelf. Layer fine structure and step-like profiles along transects indicate active mixing above the seafloor. These results provide a high-resolution continuous mapping of P/AG outflow O₂ modification along the continental shelf, linking local mixing intensity to the outflow's potential to ventilate the upper oxycline of the Arabian Sea. The spatial correspondence between P/AG outflow persistence and elevated near-bottom oxygen levels suggests that the upper oxycline is effectively ventilated. Where the shelf-trapped P/AG water layer is strongest and most persistent, near-bottom oxygen is elevated relative to adjacent waters, indicating effective ventilation near the upper oxycline depths (Font et al., 2024; Swift and Bower, 2003). Downstream maps show progressive oxygen reduction along the outflow pathway basinwards, reflecting dilution and consumption (Fig. 5). As mixing intensity varies across the stable-unstable shelf boundary, depending on seafloor roughness, the magnitude and spatial footprint of P/AG outflow ventilation depend on local thickness, attachment state, and mixing intensity.

The attachment–detachment thresholds documented here imply that the P/AG outflow's pathway is sensitive to moderate shifts in source-water density, which depends strongly on the Gulf surface temperature and evaporation. Warming in the P/AG would reduce outflow density, alter lift-off depths, modify downstream oxygen delivery, and potentially reorganize benthic zonation along the margin, indicating a climatically sensitive mechanism. Interannual to decadal changes in evaporation and heat content modulate the density contrast across the Strait of Hormuz and, by extension, the strength and properties of the exported outflow (Campos et al., 2020).