Because of the strong interactions between the ocean and the atmosphere, the sea surface temperature (SST) (Fig. 1.11) is very close to the temperature of the air above it (Fig. 1.3). One exception is the polar regions where sea ice (see section 1.4.1) insulates the ocean from the cold polar atmosphere.
The sea surface salinity is strongly influenced by the freshwater fluxes at the surface. The salinity reaches a maximum in subtropical areas because of the large evaporation and low rainfall there. The high precipitation rates induce lower salinity at the equator, while the weak evaporation is responsible for the lower salinity observed at mid and high latitudes. River input also has a large regional impact, as seen on Fig. 1.11, with low values close to the mouths of the Amazon and Mississippi rivers.
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As with the surface temperature over land, the first tenths of metres of the ocean at mid- and high latitudes show a clear seasonal cycle (Fig. 1.12). However, this cycle is shifted by 1 to 3 months compared to that of land surface temperature and its amplitude is weaker because of the large thermal inertia of the ocean (see section 2.1.5). In winter, the stirring by the winds and the cooling at the surface, which tend to destabilise the water column and generate shallow convection, induce strong mixing in the ocean. This homogenises a surface layer, called the oceanic mixed layer. Its depth is generally of 50 to 100 metres in winter but it can reach several hundred metres in some regions. When temperature rises in spring and summer, the density at the surface decreases. This tends to stabilise the water column. As the winds also tend to be weaker in spring and summer, generating less mixing, the mixed layer becomes shallower. The warming is thus concentrated in a shallow layer, whose depth is generally lower than 40m. Below this summer mixed layer, the temperature is insulated from the surface and thus still conserves the properties that it has acquired by contact with the atmosphere in winter. This seasonal process induces the formation of a region with strong vertical gradients at the base of the summer mixed layer referred to as the seasonal thermocline (Fig. 1.12).
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The mixed layer dynamics, and in particular the seasonal changes in its depth, have a considerable influence on the surface ocean properties and on the exchanges of heat, water and gases between the ocean and the atmosphere (see section 2.1.6). Its development also has a large impact on the growth of phytoplankton, which is the basis of the whole oceanic food web. In order for photosynthesis to occur, phytoplankton need light, which is only available close to the surface. If the mixed layer is deep, as in winter, the phytoplankton is mixed over a large depth range by surface turbulence and thus spends a large part of its time in the dark, deep levels. If the relatively low flux of solar radiation at the surface during winter is also taken into account, it is easy to see why photosynthesis cannot take place in winter. By contrast, the availability of light is high in summer because of the shallow mixed layer and the large amount of incoming solar radiation. However, the shallow mixed layer limits the exchanges between the surface and the deep water which is rich in the nutrients required by the phytoplankton (see section 2.3.2)). The concentration of those nutrients is thus generally too low to sustain a large production in summer. As a consequence, the phytoplankton growth generally reaches its maximum during spring blooms. The mixed layer is relatively shallow during this period but the nutrient concentration is high enough thanks to the exchanges with deeper layers that occurred during the previous winter.