Mixing In The Ocean

As emphasised already, flow in the oceans is turbulent (Section 3.1.1). and mixing is predominantly the result of stirring' by turbulent eddies. Turbulent mixing is most pronounced along tgenily sloping) isopyenie (density I surfaces, where it may occur with the least expenditure of energy, but mixing together of w ater with different densities also occurs. This is referred to as 'diapycnal mixing' (i.e. mixing across isopycnals).

In Section 6.3. we discussed the spread of water mass characteristics as if water masses alw ays spread away from their source regions in an even manner. For example. Figure 6.14 suggests that Mediterranean Water spreads out gradually through the Atlantic in a continuous process, with the plume of Mediterranean water mixing at its edges with adjacent water masses and gradually losing its distinctive characteristics This may be true for much of the Mediterranean Water entering the Atlantic but some, at least, is carried into the body of the Atlantic in spinning lenses of Mediterranean waler known as 'meddies'. Meddies have been observed as close to the Snails of Gibraltar as the Canary Basin and as far away as the Bahamas, and it has even been suggested thai the 'tongue' of Mediterranean Water shown in Figure 6.14 consists partly or wholly of 'decayed' meddies.

Meddies form from the Mediterranean outflow off south-west Portugal. They are typically 4(1-150 km across and may be as much as 1 km thick, and they can last as coherent bodies of identifiable Mediterranean origin for a considerable lime. Figure 6.27 shows the tracks of three Sofar iloats (Section 4.3.3) that were dropped into meddies in 19X4 and 19S5. Together, lhe tracks of the floats travelling in meddies I and 3 indicate that meddies may last at least two-and-a-half years. More recent observations indicate that meddies can last for up to five years, and that most eventually 'die' as a result of colliding w ith seatnounls.

It is becoming clear that meddles and other mesoscale eddies play a significant part in the redistribution of heat and salt - exactly how much is not yet certain. On the one hand, their stir ring" motion acts to even oui in homogeneities within water masses; on the other, individual eddies transport hodics ot water with particular temperature and salinity characteristics from one pari of the ocean to another (Sections 3 5,2. 4 J.5 and 5.5.2).

It was long ihought thai bulk mixing by turbulence was the only significant mechanism whereby water mass characteristics Could be changed, According to ihis view, (he behaviour of sea water is entirely determined by its joint temperature and salinity Characteristics. However, if the discovery of mesoscale eddies shed new light on the processes contributing to oceanic mixing tin a large scale, laboratory experiments demonstrated the existence of some very small-scale mixing processes. Over the past few decades, it has become clear that variations in temperature and salinity of tlwinsfhrs may cause mixing. Even in the absence of turbulent mixing, heat and salt diffuse through sea water as a result of processes occurring at the molecular level. Heal diffuses faster than salL. and the relatively fast transfer of heat from a layer Of warmer, more saline water to a layer of colder, less saline water may be sufficient to cause small-scale instability. The best-known phenomenon associated w ith the double-diffusion of heat and sail is salt fingering (Figure 6.2Xi. w hich was first observed below the outflow of Mediterranean Water into the Atlantic, It is now recognized that salt fingering and relaled processes can make a significant contribution lo vertical mixing within the oceans generally, and that these very small-scale convective overturns affect the large-scale characteristics of water masses.

Figure 6.28 (a) When warm saline water overlies cooler and less saline water, the more rapid diffusion of heat (red arrows) than salt (dots), leads to instability and the development of 'salt fingers', as shown in (b) and (c).

Figure 6.28 (a) When warm saline water overlies cooler and less saline water, the more rapid diffusion of heat (red arrows) than salt (dots), leads to instability and the development of 'salt fingers', as shown in (b) and (c).

(a) (b) (c)

However, it we assume that mixing occurs through turbulent processes only, and ignore mixing through processes occurring at a molecular level, we may use joint temperature-salinity characteristics to study the global distribution of water masses and how different water masses mix together.

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