Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 48

Bay of Fundy which is host to the world's largest tides, and in the Cook Strait of
New Zealand which alone dissipates more than 1% of the global total.

2.5.3 Source of energy dissipated by the tides

The energy dissipated by tidal friction in the ocean comes ultimately from the kinetic
energy of the Earth's rotation. The frictional stresses acting in the ocean combine to
exert a restraining torque on the Earth, tending to slow its motion and reduce its
angular momentum. Figure 2.14 illustrates how friction with the seabed allows the
daily spinning of the Earth to drag the tidal ellipsoid forward of the Moon's overhead
position, which acts as a brake on the Earth's spin. Since the Earth-Moon system is not
subject to external torques, the Earth and Moon must jointly conserve angular
momentum. 1 The consequence is that as the Earth's rotation slows, the Moon must
accelerate in its orbit. As it does so, it moves farther away from the Earth at a rate
of 2 cm per year, an adjustment that has been directly measured by laser ranging on a

mirror left on theMoon by theApollo astronauts (Dickey et al., 1994 ). The acceleration
of the Moon in its orbit can thus be determined with high precision and is the basis of
our best estimates of the total tidal energy input to the ocean. It is interesting to note
that tidal friction works both ways. Tides set up within the solid body of the Moon
also generate frictional stresses, which have acted to slow down the Moon's spin to the
extent that it now has one face tidally locked to the Earth (Gladman et al., 1996 ).

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Summary

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The motions of the shelf seas are driven by a combination of buoyancy inputs from
surface heat exchange and freshwater runoff and direct mechanical forcing by the
atmosphere and by the tides.
In terms of energy, surface heat exchange in the seasonal cycle is the predominant
buoyancy source for much of the shelf. Heat input is largely due to short wave radiation
from the Sun, most of which is absorbed close to the surface. Heat loss occurs from a
surface microlayer ( 1 mm thick) and is due to a mixture of long wave back radiation,

latent heat loss due to evaporation and sensible heat transfer from ocean to atmosphere
by conduction. These surface exchange processes generally dominate over horizontal
heat fluxes and combine to control the seasonal cycle of heat storage in the water
column. Buoyancy inputs from freshwater discharge may outcompete heat exchange
as a buoyancy source, but usually only in estuaries and adjacent regions of freshwater
influence (ROFIs). Particularly large inputs of freshwater occur in tropical shelf seas
with, in monsoonal areas, a strong seasonal variation. The large shelf seas of the Arctic
Ocean are also in receipt of freshwater inputs which greatly exceed those inmid latitudes.

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1 A fundamental law of physics requires that, in the absence of external forces, the angular momentum
of a mechanical system must be conserved. A spinning top would maintain its spin forever were it
not for weak damping due to air resistance and friction with the ground.