S 50s 30s 10s 0 10n 30n 50n

Fig. 3.34 Meridional section of MS_1 AGPE density distribution in the Atlantic Ocean (J/m3) (Feng et al, 2006).

seafloor. Over long geological time scales, the shapes of the basins in the world's oceans change and sea level can also change, owing to glacial-interglacial cycles. However, for the time scales relevant to dynamical oceanography, most of the GPE is dynamically inert.

Stratified GPE represents part of GPE which is associated with the density deviation from the global mean. Changes of SGPE are related to variations of stratification, induced by the departures of the mean temperature and salinity, on very long time scales.

The more relevant term is the APE. The definition of APE depends on the choice of reference state. The commonly used definition of APE is now redefined as the MS_1 APE, with typical length scale on the order of 100 km, which applies to meso-scale problems in the oceans. This term can be used for the study of the energetics of meso-scale motions in the oceans.

Thermohaline circulation has a length scale equal to the basin scale, to which the traditional definition of APE is not applicable. Thus, the original definition of available potential energy is used. For problems with centennial time scales, the combination of APE and MS_1 APE may be useful.

Comparison with kinetic energy The total amount of kinetic energy in the world's oceans remains poorly known. It is argued that most of the kinetic energy is associated with meso-scale eddies; however, there is no reliable estimate, owing to the technical difficulty in measuring velocity in the ocean with the high resolution relevant to meso-scale eddies. On the other hand, numerical model simulations are far from being able to resolve all the energetic eddies in the ocean. Thus, our discussion here is based on some rough estimates. The currently available estimates

Table 3.6. Different forms of gravitational potential energy (GPE)

Energy form Global sum Length scale

Type of motion


Time scale

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