So 32 34 36 38

Salinity (psu)

FIGURE 9.12. (a) Annual mean T and S profile at 50° W, 30° N in the Atlantic Ocean. The thermocline is clearly evident. The T scale is at the top, the S scale at the bottom. (b) The cycle of T at 50° W, 30° N in the Atlantic Ocean in the Levitus monthly mean climatology. The strong seasonal cycle at the surface has vanished at a depth of 500 m. Data from the Levitus World Ocean Atlas (1994).

numbers, 5 x 10-3 s-1. This implies a period for internal gravity waves in the ocean of 2n/N ~ 20 min5, roughly twice that of our estimate in Section 4.4 for atmospheric gravity waves. Indeed T, S surfaces in the interior of the ocean undulate constantly with just this kind of period, excited by winds, flow over topography, tides, and myriad other processes. These internal gravity waves are not merely oceanographic noise, in the sense that averaged over many oscillation cycles they disappear. If

5More detailed study shows that 2n/N is the minimum period for internal waves. More energy is at frequencies closer to f rather than N.

the waves break, mixing occurs. However, the mixing due to internal waves appears to be rather weak in the thermocline (typically eddy diffusivities are 10-5 m2 s-1, albeit two orders of magnitude larger than the molecular diffusivity of water k = 1.4 x 10-7m2s-1; see Table 9.3), but it can become much larger in the abyss (where N2 is small), near boundaries, and over topography.

An important distinction between the mixed layer and the interior of the ocean is that the former is able to respond rapidly to changes in meteorological forcing. However, the interior, by virtue of its very slow circulation and enormous heat capacity, can only respond very slowly to changing boundary conditions. Thus, for example, the surface mixed layer exhibits diurnal, seasonal and inter-annual variations, whereas the interior ocean evolves on interannual, decadal to centennial (and longer) time scales, the time scale typically increasing with depth. For example, Fig. 9.12(b) shows the seasonal cycle of SST and T at 500 m at 50° W, 30° N in the Atlantic Ocean. There is no seasonal cycle detectable at 500 m, although trends on decadal timescales are observed (not shown).

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