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Horizontal ranges calculated using the zonal direction only (Across-shelf for R14, and Along-current for B16) tended to be larger than those calculated using the meridional

direction only (Along-shelf for R14, and Across-current for B16) (Fig. 4.6 d-e, Fig.

4.7d-e). Average differences are clearer between R14 along- and across-shelf ranges

(Table 4.3), than with the B16 across- and along-current ranges (Table 4.4). This suggests that there was anisotropy at these regions during the deployments. However, many R14 along-shelf ranges could not be calculated due to the poor fits (low r2), and as discussed in Section. 4.3.1, this apparent anisotropy could just be a result of the glider meridional sampling. R14 and B16 domain-wide ranges tended to be larger than the ranges calculated using single directional axes. Average along-shelf and across-shelf R14 c(O2) ranges were

often smaller than temperature and salinity ranges, whereas average domain-wide c(O2)

ranges were often larger than temperature and salinity ranges. This suggests that biological processes may have been the driving force in c(O2) variability. Average R14 temperature,

salinity, and c(O2) ranges were between 5 km and 36 km, 6 km and 60 km, and 2 km and

43 km, respectively, irrespective of direction. Small temperature and c(O2) ranges were

found within the top 10 m of the water column during R14. Average B16 temperature,

salinity, c(O2), and backscatter ranges were typically between 23 km and 72 km

irrespective of direction, when excluding the anomalous domain-wide surface average backscatter range of 1 km. Large ranges were quite often found at the surface, such as in

the across-current direction. B16 pHT ranges were typically lower (between 2 km and

21 km) than the other measured parameters, irrespective of direction.

Temperature and salinity on spatial scales of variability found in this study (5 km to 72 km) are a product of large-scale meteorological conditions affecting vertical mixing and surface temperatures and salinity, ocean circulation, coastal upwelling, riverine output, eddies and fronts, and internal tides (Ruhl et al., 2011; Schaeffer et al., 2016; Mahadevan and Campbell, 2002). The water column was stratified during R14 due to calm meteorological conditions (average wind speed of 2 m s−1 at meteorological buoy M1 - see Fig. 2.1 for location). In contrast, the water column during B16 was mostly well mixed, with later periods of stratification (see Section 3.4). During R14, the surface circulation at depths corresponding to MAW was northward off the shelf, and southward across the shelf slope,

and mostly northward at depths corresponding to LIW during R14. Transport varied

considerably in the across-shelf direction, with strong eastward or westward currents

measured depending on location (Knoll et al., 2017). Surface circulation was mostly

west/ south-westward during B16 (Fig. 3.1) . Therefore, differences in vertical density, buoyancy gradients, and circulation likely contributed to the differences in horizontal ranges between R14 and B16. The anisotropy found in both R14 and B16 deployments was the same; the horizontal ranges were larger in the zonal direction than in the meridional direction. This could either be related to oceanographic processes in the region,

4.4 Results and Discussion 89

or glider sampling. For R14, this orientation crossed the shelf, whereas for B16, this orientation was off-shelf including a westward-flowing geostrophic current (Millot, 1999; Niewiadomska et al., 2008). Large physical and biogeochemical horizontal ranges can arise as a result of physical flows, such as boundary currents (Schaeffer et al., 2016; Doney et al., 2003). The larger along-current B16 horizontal ranges may be explained by the

predominant west/ south-westward flow. During R14, the across-shelf flows were

considerable (-0.26 Sv to 0.32 Sv) at the glider transect locations (Knoll et al., 2017). However, the larger across-shelf R14 ranges are likely related to there being more overall measurements in the open ocean (158,582 measurements west of 8◦E), than on the shelf (84,878 measurements east of 8◦E , Fig. 4.6k). Open ocean environments are dominated

by large scale processes (Schaeffer et al., 2016). Furthermore, the R14 data gaps

along-shelf may have contributed to the smaller spatial scales of variability (Fig. A.6 in the Appendix, Table. 4.2).

c(O2) and pHT variability is related (directly or indirectly) to physical processes such as

mixing, ocean circulation, and air-sea gas exchange, and biological processes, such as photosynthesis, respiration, grazing, nutrient-cycling, and remineralisation (Ruhl et al., 2011; Schaeffer et al., 2016; Mahadevan and Campbell, 2002). c(O2) close to the surface is

predominantly affected by air-sea gas exchange and therefore should vary on similar scales to surface temperature, as both are influenced by meteorological forcing (Schaeffer et al., 2016). This was the case for R14 across-shelf ranges. A DCM situated at depths of between 30 m and 80 m was present during R14. Phytoplankton distributions are generally fine-scale and patchy because the time for phytoplankton to grow in the presence of nutrients is less than the time for surface temperature to equilibrate with atmospheric forcing (Mahadevan and Campbell, 2002). As this layer is biologically-driven, it could be expected that c(O2) ranges here would be small. Smaller R14 domain-wide c(O2) ranges

were calculated at these depths when compared with layers directly above and below, but there was no clear reduction at these depths when analysing R14 across- and along-shelf ranges. pHT is a function of hydrogen ion activity, which is related to the dissociation of

carbonate species that when combined form c(DIC) (Zeebe and Wolf-Gladrow, 2001). pHT variability is mostly affected by photosynthesis and respiration at the surface, altering

the carbonate equilibria (Cornwall et al., 2013; Copin-Mont´egut and B´egovic, 2002), and

the effect of CO2 air-sea gas exchange is small (Fig. 3.17, Mahadevan and Campbell

(2002)). pHT B16 ranges were generally smaller than ranges calculated using other

parameters, suggesting that variability was mainly biologically-driven.

The first baroclinic Rossby radius of deformation (Rd) is useful for characterising

mesoscale processes on horizontal spatial scales of variability , which depends on the density of the water, latitude, and depth (Chelton et al., 1998). In the Mediterranean Sea, Rdis much smaller than in other ocean regions (generally < 15 km) (Brasseur et al., 1996).

parameter used, were mostly larger than Rd. When comparing both R14 and B16

irrespective of direction, some variable (e.g. c(O2), pHT) ranges were lower than Rd at

some depths (e.g. at the surface). The horizontal spatial scales of variability found in the northwestern Mediterranean Sea suggests that physical variability at certain depth levels (e.g. R14 temperature in the top 10 m of the water column, Table. 4.3), were related to local buoyancy effects, but the majority of physical variability at most depths were a result of planetary rotation processes, such as geostrophic currents and eddies.