Hydrographic variables

This section gives a broad outline of temperature, salinity and density during this study. The distributions of these characteristics during previous AMT transects were described by Aiken et al. (2000) and Robinson et al. (2006). The hydrography during AMT24 and AMT25 did not vary from the typical salinity and temperature climatology for September - November in the study area.

In-situ temperature and salinity were logged from the ship’s thermosalinograph (in-line with the non-toxic seawater supply) and the CTD probe to investigate vertical hydrographic structure of the water column.

6According to the Central Limit Theorem, the distribution of means will follow a Gaussian distribution (i.e.

normal bell shaped distribution) if the sample size is large enough (i.e. more than 30 data points).

Distributions of surface temperature, salinity and Sigma-T (σ^t), seawater density (derived from S and T) expressed as density (kg L^-1) – 1000 kg L^-1, were relatively similar on the two transects (Figure 2.19).

Surface water in the tropical and equatorial regions has low σ^t due to low salinity and the high temperatures. In the northern hemisphere equatorial region, the highest temperature is associated with a sharp decline in salinity, which may reflect some dilution of the salinity by precipitation (Aiken et al., 2000; Hooker et al., 2000).

Temperature reaches a plateau in the northern hemisphere tropics (~ 28° N) and remains above 20°C until it decreases towards the higher latitudes in the southern hemisphere. The coastal upwelling off the west coast of Africa is not clear from the surface temperature profile because the cruise tracks are too far from this region (Figure 2.19).

Salinity shows a similar pattern; increasing from higher to lower latitudes. Salinity maxima were observed in the tropics (37 - 37.5) in association with high surface temperature typical of the regions. Salinity minima of ~ 32 – 34 were observed around ~ 5°N on both cruises in association with salinity decreases in the underlying water (Figure 2.19), indicating some upwelling of low salinity upwelled water to the surface and/or the signature of excess precipitation at the Equator. However, this is not very clear from the temperature plot (Figure 2.19).

Highest σ^t occurs at high latitudes in both hemispheres and declines sharply to < 22 in the equatorial region (i.e. 10°N) due to the higher temperatures (Figure 2.19).

Depth profiles of S, T and σ^t are shown in Figure 2.20.

The main features of the North Atlantic Gyre (NAG) and the Southern gyre (SAG) between

~35°N and ~20°S are the high surface temperatures which declined through the water column associated with the tropical regions (Figure 2.20).

Clearly seen in the temperature profile is cool (and low salinity) water south of ~ 42°S. Low temperature and salinity around the equatorial regions may result from equatorial upwelling on both cruises (Figure 2.19). Upwelling African coastal water was especially discernible on AMT25 (see section 2.2) but it does not show clearly on the temperature and salinity plots (Figure 2.20).

Along the full extent of both transects σ^t increases with depth. The lower values in the Northern and Southern Gyres are observable to ~ 300 m depth. This is due to high solar radiation penetration in these surface optically clear waters and also high temperature at depth due to convergence. These less dense waters extend deeper in the Southern Hemisphere, crossing further into the central gyre (Figure 2.19). Minima in σ^t are also found between 10°N and the equator to a depth ~ 50 m associated with low salinity and high temperature. This may reflect the influence of Amazon outflow carried by the NECC (see section 2.4) (Aiken et al., 2000;

Hooker et al., 2000) and is more noticeable on AMT24 than on the more eastward AMT25 (see section 2.2).

In the high latitudes, north of 40°N and south of 40°S around the European and South-West American continental shelves there is less vertical contrast in σ^t. These variations in density down to 500 m allowed Hooker et al. (2000) to identify 17 distinct biogeochemical provinces along AMT transects.

2.8.1 Dissolved oxygen profile in the Atlantic Ocean

As for salinity (S) and temperature (T), the latitudinal distributions of dissolved oxygen to 500 m depth were similar during AMT24 and AMT25 (Figure 2.21).

Figure 2.21. Latitudinal cross section of dissolved oxygen (µg L^-1) on AMT24 (top) and AMT25 (bottom)

There was a distinct oxygen minimum (~ 100 µmol L^-1) between ~ 20°N and ~ 10°S. However, concentrations were slightly higher around the equator. Maximal oxygen was at high latitudes, especially south of 40°S where it is associated with the spring phytoplankton bloom.

2.8.2 Nutrients

Samples were analysed for nutrients on board using a Bran and Luebbe segmented flow colorimetric Auto-analyser after Woodward and Rees (2001) and Hydes et al. (2010).

2.8.3 Mixed Layer Depth (MLD)

Mixed layer depth (MLD) is a boundary between the mixed layer of the surface and stratified deeper layer (Longhurst et al., 1995; de Boyer Montégut et al., 2004).

Mixed layer determination was adapted from Hooker et al (2000), an approach valid for previous AMT cruises. According to their model, the pycnocline or thermocline starts where gradients of three out of four continuous depths are greater than 0.035 kgm^-3for σ^t or 0.1°C/m in temperature (Hooker et al., 2000). In areas with lower gradients the threshold values were increased to 0.1 kg m^-3 for σ^t or 0.5°C/m in temperature from surface water (i.e. the reference).

Hooker et al. (2000) however found strong compatibility between MLD derived from temperature and density. So, it was suggested that MLD determination based only on temperature is reliable enough (Hooker et al., 2000).

During this study, the gradients in temperature were thus derived to identify the MLD after Hooker et al. (2000). It was revealed that the mixed layer determination in this way is consistent with the depth of the mixed layer identified during sampling based on temperature profile by the ship’s system within the analytical error of ± 0.1. So, the MLD presented in this study were derived from on board temperature profiles during sampling.