Temperature salinity and density

The sea surface temperature is altered through the heat loss or gain at the ocean surface and by mixing with water from below the surface or from advective horizontal mixing with water from another location. We have already encountered these processes. The high specific heat of water (Table 1.3) means that to change the temperature of 1 kg of water by 1°C requires over 4000 J of energy. Recalling that the global ocean absorbs 30 Wm-2 on average a typical summer mixed layer of 20 m depth would require over 33 days of energy absorption at this rate to warm by just 1°C. Changing the density of sea water through its temperature thus requires large exchanges of energy. However, for fresh water this is all that changes the density apart from pressure (ignoring the effect of suspended sediment, which can be significant in turbidity currents). Water gets slowly denser as it is cooled - until 4°C, below which it becomes less dense as the water molecules begin to move towards the more ordered structure of an ice lattice.

Fig. 2.13. A temperature-salinity diagram, showing the variation in potential density of sea water with varying temperature and salinity. This diagram implicitly assumes the water is at the surface, and ignores any pressure effect.

Fig. 2.13. A temperature-salinity diagram, showing the variation in potential density of sea water with varying temperature and salinity. This diagram implicitly assumes the water is at the surface, and ignores any pressure effect.

In sea water, however, salinity also affects density. In §1.3.1 we saw how salinity causes the freezing point of sea water to be lowered - by almost 1.9°C in the saltiest ocean waters - and increases the mean density by 2.6%. The equation of state for sea water means that salinity change has relatively more impact on the density of cold water, while warmer waters are more affected by temperature changes (Fig. 2.13). The ocean surface is subject to significant fluxes of both heat and water, which are of different signs in different locations and depend on the time of the year. Surface density is therefore rather variable in space and time.

The impact of pressure is a minor factor in the density equation. Sea water is almost incompressible: density is only 4% greater at a depth of 10 000 m than at the surface. Nevertheless, density gradients can be very small, especially in the deep ocean. Also, the slight compressibility means that the temperature of deep water can be c. 1.5°C warmer than it would otherwise be. It is thus usual to refer the density to a constant level (normally, but not always, the surface), considering the density differences within the fluid due to adiabatic processes (see §1.2.1) rather than the non-adiabatic effect of pressure. Such a density is known as potential density,4 as it is analogous to potential temperature (Appendix C).

The ocean, like the atmosphere, is normally vertically stratified with less dense water overlying denser water. We have seen how sharp gradients in density exist at the seasonal and permanent thermocline (Fig. 2.11). The T-S diagram in Fig. 2.13 demonstrates how deep convection can occur. Cooling of salty surface water increases the density more rapidly when the water is a few degrees above freezing, as is typical of the Labrador and Greenland Seas and the Southern

4 Potential density is normally expressed relative to 1000 kgm 3; seawater is typically 20-30 kgm-3 denser than this.

Fig. 2.14. Mixed layer depth across the Labrador Sea during a two week period in late winter of 1997. Note the convection exceeding 1000 m depth in two small areas only a few tens of kilometres across at most towards the southwestern part of the Labrador Sea. [Adapted from Fig. 12d of Pickart et al. (2002). Reproduced with permission from the American Meteorological Society.]

Fig. 2.14. Mixed layer depth across the Labrador Sea during a two week period in late winter of 1997. Note the convection exceeding 1000 m depth in two small areas only a few tens of kilometres across at most towards the southwestern part of the Labrador Sea. [Adapted from Fig. 12d of Pickart et al. (2002). Reproduced with permission from the American Meteorological Society.]

Ocean around 60° S, than near the freezing point. We can also see that such waters, with a potential density in excess of 28.0 kgm-3, may be lighter than slightly fresher water near freezing. The latter is typical of water conditions near Antarctica when sea-ice forms. In that case salt is added to water at a constant temperature, increasing density more rapidly than purely by cooling. The potential density of deep water is comparable with these extreme surface conditions, and in localized regions where the density becomes equivalent over some depth deep convection takes place (Fig. 2.14).

Note, however, that freshening makes the surface waters less dense, and so assists stratification. Melting sea-ice in sub-zero water (entering the ocean at a roughly constant temperature) has the most pronounced impact, along with warming at a constant salinity in sub-tropical waters. Sufficiently intense precipitation accompanying cooling could lead to a minimal impact on density, particularly in water a few degrees above 0°C.

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment