Physical forcing of the shelf seas: what drives the motion of ocean? - Introduction to the Physical and Biological Oceanography of Shelf Seas - page 25

2

Physical forcing of the shelf seas: what
drives the motion of ocean?

In this chapter we consider the powerful forces that drive the shelf seas, and
supply the large amounts of energy which are dissipated within them. We shall
see that these forces act mainly through the transfer of properties (momentum,
heat, freshwater, etc.) at the sea surface and through the lateral boundary where
the shelf seas meet the deep ocean. Together the various forcing mechanisms
produce an energetic regime which, in most shelf seas, maintains a high level
of energy dissipation far greater than that of the deep ocean. We begin by
identifying the principal energy and momentum sources and then consider, in
turn, the forcing mechanisms involved and the extent to which the resultant
inputs are known and can be related to measurable parameters.

2.1 Energy sources

......................................................................................................................
Perhaps the most obvious and striking form of mechanical energy input to the sea
arises from surface wind stresses and pressure gradients imposed by the atmosphere.
These forces drive ocean currents and generate surface waves whose impact at the
coast can be dramatic and is often seen as symbolic of the ocean's power. In many
shelf seas, however, energy input through tidal forcing is a more consistent and more
powerful source of mechanical energy. Most tidal energy is delivered to the shelf in
the form of energy fluxes in tidal waves which originate in the deep ocean, although
there is also a (usually small) contribution arising from tidal body forces acting
directly on the waters of the shelf seas. Both winds and tides inject very large amounts
of kinetic energy to the ocean as a whole; total inputs have been estimated recently as
∼ 3.5 TW for wind and tidal inputs respectively (Munk and Wunsch, 1998 ).

These large mechanical energy inputs are, however, small in relation to the very
large seasonal exchange of heat energy through the sea surface. The average rate of
mechanical energy input per unit area to the shelf seas is

1and ∼

0.1W m 2 . This is almost
negligible in comparison with seasonal heating and cooling rates which in temperate
latitudes have an amplitude of

∼

100 Wm 2 . Heating and cooling do not, of course,
inject momentum directly into the surface of the ocean. Their primary effect is to
modify the density of the seawater making it more, or less, buoyant. In changing the

∼