Fig. 1.11 Annual mean (NCEP-NCAR) rate of evaporation minus precipitation (cm/yr). See color plate section.

1.1.3 Other external forcing

Tidal flows are one of the major components of the large-scale motions in the oceans. According to the traditional view, tides are not considered as part of oceanic general circulation; and in the classical theoretical framework, the oceanic general circulation is mostly driven by surface forcing. It is, however, now commonly accepted that tidal dissipation is also one of the key factors in regulating the thermohaline circulation in the world's oceans. In fact, tidal dissipation is a major contributor to diapycnal mixing in the deep ocean; therefore energy sources due to tidal dissipation will be discussed in connection with the mechanical energy balance in the world's oceans.

In addition, geothermal heat flux also contributes to the thermal circulation, although it is believed that its influence is mostly limited to the deep ocean and a few sites associated with the hot plumes above the "hot spots" on the seafloor of the world's oceans. In general, new seafloor is continuously formed at the top of the mid-ocean ridge, so it is relatively shallow and is associated with very active geothermal heat flux release. As the newly created seafloor spreads outward from the mid-ocean ridge, it moves away from the active geological processes in the Earth's mantle. As a result, the geothermal heat released from the seafloor also gradually diminishes.

Owing to the technical difficulty of making heat flux measurements over the seafloor of the world's oceans, there is no reliable global dataset for the geothermal heat flux. However, to estimate the global impact of geothermal heat flux, the following empirical formula can be used (DeLaughter et al., 2005; this formula is a modification from earlier studies by Stein and Stein (1992, 1994):

where heat flux is in mW/m2, and t is the age of the seafloor, in Myr (million years). The age of seafloor can be estimated from the following empirical formulae, which relate the age of seafloor to its depth (m):

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