The hydrodynamics of coastal water masses have different impacts on oceanographic processes occurring on continental margins (Huthnance 1995). Particularly in continental shelf and slope areas, shelf-edge physical processes and interactions between flow dynamics and bathymetry have a strong influence on upwelling/downwelling mechanisms and/or cross-margin water and particulate matter exchanges (Hickey 1997, Spurgin and Allen 2014). Submarine canyons can modify the water mass behaviour and enhance cross-margin particle fluxes, and can consequently have a major impact on sedimentary and biological processes (Gili et al. 1998, Palanques et al. 2005, Allen and Durrieu de Madron 2009, Puig et al. 2014). The hydrodynamics in submarine canyons depends upon several forcing conditions, such as general circulation, tidal regime, bottom morphology and atmospheric patterns. However, forcing conditions differ among canyons and can give different responses. Therefore, a detailed monitoring of oceanographic features that may be present in canyon hydrodynamics is needed to properly understand canyon water mass behaviour.

In this work, we will focus on the detailed hydrodynamics of a submarine canyon (the Besòs Canyon) that is deeply incised in the continental slope of the NW Mediterranean continental margin (Canals et al. 2013). The thermohaline structure and dynamics of water masses in this particular area is composed of a three-layer system (Hopkins 1978, Salat and Cruzado 1981, Salat et al. 2002). In the first layer, from the surface to 150-300 m, modified Atlantic Water (AW) is generally found. This water mass comes from the Gibraltar Strait and is transformed and modified as it spreads and circulates cyclonically around the western Mediterranean basin. The second layer is formed by Western Intermediate Water (WIW) (Lacombe and Tchernia 1972, Salat and Font 1987, Pinot and Ganachaud 1999) and is located between 300-600 m. The WIW forms by winter convection in the region of the Gulf of Lions and shows potential temperatures of 12.51°C to 12.81°C and salinities of 38.1 to 38.3, respectively. Below, a third layer between 600 and 800 m is occupied by the Levantine Intermediate Water (LIW), with potential temperatures of 13.0°C to 13.41°C and salinities of 38.48-38.54, respectively (Ovchinnikov et al. 1976, Font 1987, Millot 1999).

The oceanographic conditions at the surface are characterized by the presence of a quasi-permanent frontal current: the Liguro-Provençal-Catalan Current or Northern Current along the shelf-slope (e.g. Font et al. 1988, Masó and Tintoré 1991, Pinot et al. 1995, Millot 1999, Pinot et al. 2002). This current is associated with a baroclinic front, which separates fresh coastal waters, mainly from the Rhône and Ebro Rivers, from saltier open sea water in the deeper areas of the basin (Font et al. 1988). Analysis of altimetry, infrared satellite images and in situ observations has revealed a significant annual and seasonal variability of this frontal current (LaViolette et al 1990, López García et al. 1994, Font et al 1995, Mason and Pascual 2013) with instabilities, meanders and mesoscale eddies and filaments (e.g. Tintoré et al. 1990, García et al. 1994, Pascual et al. 2002). A major source of this frontal current variability is their interaction with bottom morphology (Arnau 2000, Pascual et al. 2004, Rubio et al. 2005, 2009). The tendency of the current to follow isobaths along the shelf break causes an increase in variability when it interacts with the submarine canyons. This has been documented in several sites: the Cap de Creus, Palamós and Blanes Canyons (e.g. Masó et al. 1990, Palanques et al. 2005, Flexas et al. 2008).

Detailed measurements inside these submarine canyons have shown a complex structure of currents and thermohaline structure variability, as the flow adjusts to the canyon shape (e.g. Puig et al. 2000, Palanques et al. 2005, Flexas et al. 2008). These measurements were obtained mainly through an extensive deployment of moorings, accompanied by hydrographic sampling during deployment, maintenance and recovery periods. However, the tracking of water mass time evolution was not possible in previous measurements, except when additional sensors were installed with the mooring deployment. With this purpose in mind, in 2012 a continuous monitoring of the full water column was designed with the help of an Aqualog profiler. The objective was to explore the temporal and vertical characteristics of the water masses inside the narrow Besòs Canyon, complementing the previous measurements made in other wide canyons of this region. This paper reports the observations carried out during this experiment, focusing on the temporal evolution of the water column properties and particularly fine-scale affects on the water mass variability at the canyon head. In the following sections we first describe the methods used to process the data, and then present the main results and discussion.

The Besòs Canyon is located on the northwestern Mediterranean continental margin, ~20 km offshore of Barcelona (2.52°E, 41.31°N, Fig. 1). The canyon is relatively narrow, with a mean width of around 5 km and steep sidewalls, and its head barely incises the shelf at the ~100 m isobath. Compared with other submarine canyons in the area, it has a very rectilinear and uniform signature almost perpendicular to the southwestward direction of the shelf. The data analysed here were obtained in an experiment consisting in deploying, close to the canyon head at a depth of 808 m, a mooring line equipped with an Aqualog profiling carrier (Carlson et al. 2013, Ostrovskii et al. 2013), a device that goes up and down along the mooring line carrying several probes (Fig. 2). For this experiment, the Aqualog was equipped with a SBE 52-MP CTD probe, a Nortek Aquadopp acoustic current meter and a Seapoint turbidity sensor. The mooring was deployed on 23 March 2012 and recovered on 22 May 2012. Unfortunately, due to a technical failure with the internal memory card, the Aqualog profiler stopped after 11 days of operation, providing data only until 3 April.

The mooring arrangement was designed to scan the vertical range of 62-792 m with six up and down casts per day. The upper shallow range varied slightly, finally stabilizing at 75 dbar, and the maximum attained depth was 801 dbar. The CTD was configured to sample the water column at 0.2 m resolution, whereas the Doppler current meter was set up to sample at 1 m resolution. To proceed with a homogeneous methodology for all the parameters, we first pre-processed the full set of CTD profiles to place temperature and salinity values at the same level as velocity measurements. Then, all the data were cut at the upper part at 78 dbar to have the longest possible time series of the same upper depth profiles. The Aqualog moves along the line at a relative constant speed of 0.17-0.18 m s^–1, taking 2 hours to complete a full up-down cycle; then it sleeps at the bottom until the next up-down cycle, which starts again after 2 hours. The sampled profiles were interpolated to obtain a regular spatial-temporal sampling, which gives a matrix for the whole sampling period composed of 131 profiles of 723 vertical points (from 23 March to 3 April and from 78 dbar to 801 dbar). Finally, the velocity field was rotated according to the canyon axis orientation to separate along- and across-canyon velocity components. A rotation of 40° anticlockwise was applied to the velocity field components.

To identify the water mass interfaces we will consider the typical water mass temperature and salinity values presented in the Introduction. This criterion will allow us to identify the interface between AW-WIW and WIW-LIW.

Double-diffusive mixing is one of the principal small-scale mixing processes that adjusts the density field, diminishing the existing salt and heat excess in adjacent water masses. In order to explore the intensity and presence of small-scale mixing processes, we computed the Turner angle, Tu (Ruddick 1983), which describes the likelihood of contacting layers in a stratified water column developing double-diffusive mixing. Tu can be defined as a polar angle in the ($\alpha {\overline{T}}_Z$, $\beta {\overline{S}}_Z$ ) plane measured relative to the $\alpha {\overline{T}}_Z$ = $\beta {\overline{S}}_Z$ >0 line (Radko 2013), where ${\overline{T}}_Z$ and ${\overline{S}}_Z$ are the mean vertical gradients of temperature and salinity, respectively and

are the coefficients of thermal expansion and salinity contraction, respectively, of the mean profile. In a more explicit form

where the first of the two arguments of the arctangent function is the "y”-argument and the second one the "x”-argument.

The Turner angle is quoted in degrees of rotation in such a way that angles between 45° and 90° represent the "salt-finger” regime of double-diffusive convection, with the strongest activity near 90°. Turner angles between –45° and –90° represent the "diffusive” regime of double-diffusive convection, with the strongest activity near –90°. Turner angles between –45° and 45° represent regions where the stratification is stably stratified in both temperature and salinity fields and Turner angles greater than 90° or less than –90° characterize a statically unstable water column (IOC 2010).

Finally, to analyse whether the mixing was diapycnal or isopycnal, we used the (βS’, αT’) plane, where αT’ and βS’ are the temperature and salinity anomalies normalized by the thermal expansion (α) and salinity contraction (β) coefficients (Zhurbas et al. 1987). It was shown by Pingree (1972) that for fine structure anomalies resulting from isopycnal advection, αT’= βS’, and for fine structure inhomogeneities resulting from vertical mixing, T’/S’ = T_Z/S_Z (where T_Z and S_Z are the mean vertical gradients in the investigated layer). The work of Pingree can be used to relate the complementary views of Turner angle and Zhurbas’ analysis.

After calculating Tu to determine the spatial scales at which small-scale mixing can occur, a spatial-temporal decomposition through empirical orthogonal functions (EOFs) of the anomaly of temperature and salinity was performed to see the main modes of variability in the sampled data. For each profile we removed the mean and normalized the anomalies with their standard deviation. EOFs were then computed using a singular value decomposition algorithm of the covariance matrix, retaining modes with associated eigenvectors significantly different from zero (Navarra and Simoncini 2010).

Progress in scientific and technological aspects during the past years has revealed many natural phenomena inside submarine canyons (Xu 2011). High-resolution sampling in both time and space is needed to resolve and to analyse the scales of the processes inside canyons. The dynamics of water circulation within the submarine canyons incised on the NW Mediterranean continental margin has been previously studied using moored current meters in various research projects mainly devoted to the quantification of sedimentary fluxes. These include observations at the Grand-Rhône Canyon (Durrieu de Madron 1994), the Foix Canyon (Puig et al. 2000), the Palamós Canyon (Palanques et al. 2005, Martín et al. 2007), the Cap de Creus Canyon (Puig et al. 2008, Martín et al. 2013) and the Blanes Canyon (Flexas et al. 2008, Zúñiga et al. 2009, López-Fernandez et al. 2013). However, these current meter time series, which were generally accompanied by temperature, salinity and turbidity measurements, provided very localized and partial information on the hydrodynamics of the various water masses interacting with the canyon. Even in cases in which the horizontal spatial scale was quite well addressed (by deploying several moorings along the canyon axis and on its flanks), the variability in the vertical spatial scale (throughout the water column) was poorly sampled. This was because few current meters were installed on each mooring array, and in most cases only near-bottom measurements were reported.

To provide the missing vertical spatial scale information, in this work we have presented the data provided by an Aqualog moored profiling carrier deployed at the head of the Besòs Canyon (NW Mediterranean). The experiment was relatively short and does not allow us to make generalizations on all the observed characteristics relative to the interaction of the canyon and the regional flow configuration. For this reason we have focused on aspects related to the smaller scales associated with the variability of the water masses inside the canyon.

The vertical positions of the limits of the three water masses found inside the canyon show intense, quasi-oscillatory vertical displacements. The limit between the modified AW and the WIW shows a 4- to 6-day oscillation (Fig. 3), which appears sometimes related to intensifications of the geostrophic current in the along-slope (towards SW) direction (Fig. 4B) associated with a deepening of the AW-WIW interface. Not all these events are similar with respect to the vertical structure. One appears to be structured in a three-layer response, while the others are more confined to the upper and WIW layers.

The main surface currents observed are in the across-canyon direction consistent with the along-shelf/slope frontal current that characterized the regional circulation (Font et al. 1988). However, the predominance of currents along the canyon axis is a consequence of the narrowness of the canyon topography, as was observed in the similarly narrow Foix Canyon (Puig et al. 2000). The Besòs Canyon has a characteristic width of about 5 km, which is smaller than the typical Rossby radius of deformation in the area (around 12 km), preventing the adjustment of the frontal flow to the canyon morphology, as has been described in the literature (e.g. Klinck 1996, Jordi et al. 2005). In wider canyons, such as the Blanes or Palamós Canyons located several kilometres upstream, the main currents across the canyon axis adjust to the shape of the canyon walls (Palanques et al. 2005). In wider canyons, numerical simulations and observational measurements show that the flow adjustment for similar configurations (right-bounded flow) produces a downwelling flow in the upper layers on the upstream wall and an upwelling flow on the downstream wall (Klinck 1996, Jordi 2005).

The occurrence of flow intensification and deepening of the upper layer (2-4 days) contrasts with the shorter variability of the WIW-LIW interface. Without additional information it is difficult to decide which process is responsible for such variability. Meandering of the frontal current, eddies around the canyon, low-frequency meteorological forcing and propagation of topographic waves along the shelf have been proposed in this area (e.g. Puig et al. 2000, Palanques et al. 2005). Similar low-frequency fluctuations have also been described elsewhere in the northwestern Mediterranean, and have been identified as topographic waves (Crépon et al. 1982, Sammari et al. 1995, Durrieu de Madron et al. 1999).

At smaller scales, the analysis of the results indicate that this variability of the AW-WIW and WIW-LIW interface is characterized by Tu angles compatible with salt-fingering in the AW-WIW interface and thermal diffusion in the WIW-LIW interface. After the split of the first three anomaly modes, we also found that isopycnal mixing processes can occur. Although the relative contribution of the selected EOF second mode to the total variance is not so high, it shows a clear signature of three layers and then allows us to use a (βS’, αT’)-plane representation to find that isopycnal processes are dominant in it. Further investigations should be conducted to understand properly the scales implied in isopycnal processes at the studied scales, which are the ones that our analysis reveals as the most influential. However, the coupling along modes analysed (shown by the variance spread along the EOFs) indicates a join contribution of these processes to the measured phenomena, without a clear dominance of a single, driven process.

To summarize the experiments presented here, the observations of water masses and currents inside the Besòs Canyon have revealed an unexpected rich internal structure that could not be seen with traditional surveys with moorings or ship cruises, as has been done in nearby canyons in the same region. The interaction of the shelf frontal current and a narrow canyon is quite complex in terms of the structure and variability of water masses and currents. Major vertical excursions at short time scales were observed, associated with enhanced along- and across-canyon events near the semi-diurnal tidal, inertial and longer temporal scales. These events sometimes appeared in the velocity field, having a spotty character, which may also be the signature of quasi-inertial oscillations generated by the adjustment process of the frontal current over the canyon or by the meandering of the frontal current, combined with signal amplifications towards the canyon head region. The intermittent nature of these events, with different vertical responses, and the lack of a dominant mode of variability may be due to the mooring location close to the head of the canyon. The constrained shape of the Besòs Canyon head (narrow and steep) probably favours a response, which is the contribution of many short-term events of different natures that spread the variance among many modes. The similar absence of a clear spectral signature in near-bottom currents was also noticed at the head of the Foix Canyon (Puig et al. 2000), where current reversals were more frequent than at deeper canyon sites that have shorter along-canyon axis displacements. Finally, the small-scale mixing processes at the second EOF mode scale shows an isopycnal nature. However, further analysis should be carried out to quantify the role of each process (isopycnal or diffusive mixing) in small-scale mixing. Unfortunately, the short duration of the deployment and the experimental configuration with only one mooring did not allow us to provide robust and statistically significant analysis, and further intensive observations should be carried out in the near future.