Regulaciones ambientales para el manejo de la contaminación del suelo

The internal wave field has been analysed (Chapter 2) using current time series and local stratification for a depth range of 700m to 1000m over the region through which the MOW spreads, over the continental slope in the Gulf of Cadiz. The MOW undercurrent is influenced by tides, particularly M2, while never stopping. The MOW undercurrent continuously flows at deeper moorings (800m, 900m, 1000m approx) with high speed (0.2- 0.4 m/s). However, in one observation in the shallowest (700m approx) mooring, the MOW

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undercurrent flows southward during neap tides. Also the vertical shear structure is influenced by tides, mostly over the shelf. At the same location of the time series measurements, the MOW stratification is found to be highly variable.

Mixing diagnostics found diffusivity estimates to be larger (5x10-4 -1x10-3 m2 s-1) at interfaces. This enhanced mixing occurs mostly over the mid-slope of the continental slope (where there is broad agreement between the estimates from the different mixing analyses). Over the outer slope and shelf, mixing can be enhanced but not as much as mid-slope. Internal wave characteristics analysis, for the M2 frequency (at mid-slope) with the observed background stratification, found critical conditions over the mid slope, which were confirmed by the presence of strong currents at the M2 tide frequency.

Diapycnal mixing over the outer slope and shelf was found to be enhanced (1x10-4 - 5x10-4 m2 s-1) but significantly less than over the mid slope. The internal tide characteristics show critical slope conditions over the mid slope, which were confirmed by the time series records of tidal current speed which was found to be the strongest over this domain (0.05-0.1 m2 s-1, at 700m depth approx). On the outer slope, conditions for supercritical slopes were found in deeper regions (>1500m).

Either over the mid slope, the outer slope or shelf, the mixing parameterization estimates suggested significant mixing at the MOW-NACW interfaces. This diagnostic is supported by using only CTD profiles. Historical data reveal the presence of these interfaces defined mostly by the salinity contrast between layers, rather than their density contrast. Based on salinity profiles, the interfaces between layers have the strongest gradients O(10-2 - 10-3 salinity per metre). The strength of the gradients can vary as is revealed by the historical data and the sampling from the GO experiment. MOW interfaces can be detected from inferred salinity using temperature profiles (XBTs). In chapter 4, estimates of mixing were obtained only using temperature profiles with inferred salinity with an error factor 3-4.

A regional view of mixing is made using the XBTs cross sections taken over the Gulf of Cadiz (Figure 5.1, summarised in Chapter 2 and 3), including four sections: one main section over the seismic line and three cross sections forming a star shape, mostly over the shelf. Temperature profiles were used with inferred salinity calculated with the optimal

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choice (weighted average for δS/δTD, chapter 3), providing a stable profile at MOW

interfaces. The strain method is used to calculate the regional mixing along the sections.

Figure 5.1: XBTs data location map during the GO experiment in Spring 2007. TOP plot: XBTs deployed over the main seismic line over the continental slope (one cross section eastwards and another westwards). BOTTOM: left hand side, data source location of CTD casts and moorings; right hand side, star shape cross sections where XBTs were taken.

Figure 5.2 shows the sections of TKE dissipation rate and diapycnal diffusivity diagnosed in the GO experiment (1-5 May 2007). Both calculations are directly related (summarised in chapter 4). Salinity contours highlight the location of the MOW; the TKE dissipation rate at MOW interfaces is large O(10-7 W kg-1), also at fronts at mid-depths (600m-1500m). Along the outer slope also, enhanced TKE dissipation reaches O(10-7 W kg-1) associated with MOW detached from the slope. Enhanced diapycnal diffusivity is found at water mass interfaces [O(5x10-4 m2s-1)], along the front at mid-depths [O(6x10-4 m2s-1)], and also within the MOW detached from the slope [O(3x10-4 m2s-1)]. Mixing parameterizations reveal increased mixing always in the mid-slope and shelf, where the water-mass interfaces are related to the strong salinity gradients extending from the mid-slope to the shelf. Over the shelf, the salinity gradient is less strong O(10-4 salinity per metre) but estimates of enhanced mixing can be found.

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Figure 5.2: Estimates of mixing using finescale parameterizations with XBTs profiles and inferred salinity, with

salinity contoured in the background. TOP panel: TKE dissipation rate [Log10 (W kg

-1

)] cross section over the main seismic line during the GO experiment. BOTTOM panel: diapycnal diffusivity [Log10 (m2s-1)] along the

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Figure 5.3: TKE dissipation rate [ ε W kg-1_{] cross sections calculated using data from XBTs over the STAR}

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Figure 5.3: TKE dissipation rate [ ε W kg-1] cross sections calculated using data from XBTs over the STAR

shape lines during the GO experiment.

Figure 5.4: Diapycnal diffusivity [ κz m2s-1] cross sections calculated using data from XBTs over the STAR

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Figure 5.4: Diapycnal diffusivity [ κz m2s-1] cross sections calculated using data from XBTs over the STAR

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Figures 5.3 and 5.4 shows the cross sections of TKE dissipation rate and diffusivity along the star shape lines over the mid-slope and shelf. Three cross sections reveal that over the interface highlighted by salinity contours, the TKE dissipation is relatively enhanced O(3x10-7 W kg-1). The present sections follow one another in less than a semidiurnal cycle. In relation to the previous plot the elapsed time is around three quarters of a day. In all the sections, the diapycnal diffusivity is found to be large over the mid slope and shelf [O(8x10-4 m2s-1)]. The salinity gradient over the interfaces is also relatively strong, O(10-3 salinity per metre). These mixing estimates can be compared directly with those obtained using CTD profiles because they were made for the same days, however the regional patterns of mixing estimates are less correlated because the MOW signal varies so fast, even in less than an hour at similar positions along the Continental Slope.

Figures 5.5 and 5.6 shows the estimates of diapycnal diffusivity calculated using finescale parameterization for XBT profiles with inferred salinity from the GO experiment during 19-20 April 2007. The MOW structure in figure 5.5 is found to be more extended towards the outer slope than in other cross sections. There is large diapycnal diffusivity at the upper interface [O(2x10-4 m2s-1)], lower interface [O(5x10-4 m2s-1)], at the front over the outer slope [O(3x10-4 m2s-1)], and considerable patchy mixing inside the MOW [O(8x10-4 m2s-1)]. This MOW structure differs in detail from that observed ten days later (c.f. Figure 5.2). Figure 5.5 also shows the mixing estimates for the repeated section; a MOW structure is highlighted by salinity contours. The MOW signal is expanded in the outer-slope lower interface and the upper interface is detected at around 700m as in previous cross sections. Diapycnal diffusivity is found to be large at the upper interface over the mid slope [O(3x10-4 m2s-1)] and outer slope [O(5x10-4 m2s-1)], at the front over the outer slope [O(3x10-4 m2s-1)], and there is some patchy mixing inside the MOW reaching values O(>7x10-4 m2s-1). In both plots of Figure 5.5, the diagnosed mixing decreases towards the shelf or shallower region, with the exception of a couple of profiles (Figure 5.5 upper). Three cross sections (Figure 5.6) were made in a star shape between the previous sections and results revealed that diapycnal diffusivity is enhanced O(2x10-4 m2s-1) at the MOW-NACW interface, while some enhanced mixing occurred towards the shelf as in Figures 5.2, 5.3. These cross sections were made during less than a diurnal cycle. All the sections (Figures 5.2 to 5.6) occurred during spring tides.

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Figure 5.5: Diapycnal diffusivity [ κz m2s-1] cross section calculated using XBTs over the main seismic line

100 Figure 5.6: Diapycnal diffusivity [ κz m

2

s-1] cross sections calculated using XBTs over the STAR shape lines during the GO experiment.

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Figure 5.6: Diapycnal diffusivity [ κz m2s-1] cross sections calculated using XBTs over the STAR shape lines

during the GO experiment.

Two more cross sections (Figure 5.7) providing mixing estimates were available over the main seismic line, revealing two different MOW structures. In both cases, diapycnal diffusivity was found to be large on the upper interface [O(3x10-4 m2s-1)], lower interface [O(5x10-4 m2s-1)], at the front over the outer slope [O(6x10-4 m2s-1)], and considerable [O(10-4 m2s-1)] inside the MOW across the slope.

In all the cross sections, it is clearly seen that there is less mixing in the NACW layer. There is though enhanced diapycnal diffusivity O(2x10-4 m2s-1) above the NACW in the surface mixed layer, as well as over the shelf, reaching O(4x10-4 m2s-1). Due to the presence of the NACW layer and the lack of mixing there, the interactions between the surface mixed layer and the MOW undercurrent are relatively weak.

102 Figure 5.7: Diapycnal diffusivity [ κz m

2

s-1] cross sections calculated using XBTs over the main seismic line. TOP was done on 29 April, between figures 5.5 and 5.2. BOTTOM was done on 10-11 May, 9 days after Figure 5.2.

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On the mid-slope and shelf, tidal processes can become more important and affect the strength of turbulence. The MOW current is expected to experience strong bottom friction, particularly at the early stage of travelling down the slope. There is also more turbulence at shallow depths over the shelf.