5 Tides
5.1 Introduction
Rhythmic variations of sea level, known as tides, have been observed by sailors
and coastal communities for thousands of years. From the earliest time it has
been realized that there is some connection between tides and the motion of
the Moon and Sun. However, it was only in the seventeenth century that
Isaac Newton (1642-1723) and Pierre-Simon Laplace (1749-1827) provided the
theoretical explanation of the nature of tides and opened the door for tidal
prediction.
The keystone for understanding tides is the law of gravitational attraction
between the Earth, the Moon and the Sun. The simplest explanation possible
is the hypothetical example when the continents on the Earth are neglected and
the Earth is assumed to be a perfectly smooth sphere, completely covered by
water. The water is acted upon by the same forces that act on the solid Earth.
The prediction model associated with this case is known as the equilibrium
model of tides, as tides in this model result from the equilibrium of gravitational
forces. However, the equilibrium model cannot explain many aspects of tides,
especially the varying tidal amplitudes in many locations on the Earth. A
substantial improvement in tidal prediction has been achieved by considering
tides, in a dynamic way, as waves. In fact, tides are the longest oceanic waves
with periods of the order of 12 hours. This wave type approach, expanded
and revised many times since Laplace's first formulation, is now known as the
dynamic model of tides.
Complexity of land contours on the Earth and complicated bathymetry cause
great difficulties in the prediction of tides at some points on the Earth. However, the tides can still be predicted using the harmonic analysis of a sufficiently long record of water level fluctuation. Harmonic analysis was already
mentioned in Chap. 4, as the technique for representing an arbitrary, irregular
time series in the form of a summation of many sinusoidal and co sinusoidal
curves with various amplitudes and phase lags. In this chapter we describe the
nature of tidal generating forces and the available prediction techniques. The
significance of tide variations and tidal induced currents for life in the ocean
will be discussed in Part III of the book.
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999
5.1 Introduction
Rhythmic variations of sea level, known as tides, have been observed by sailors
and coastal communities for thousands of years. From the earliest time it has
been realized that there is some connection between tides and the motion of
the Moon and Sun. However, it was only in the seventeenth century that
Isaac Newton (1642-1723) and Pierre-Simon Laplace (1749-1827) provided the
theoretical explanation of the nature of tides and opened the door for tidal
prediction.
The keystone for understanding tides is the law of gravitational attraction
between the Earth, the Moon and the Sun. The simplest explanation possible
is the hypothetical example when the continents on the Earth are neglected and
the Earth is assumed to be a perfectly smooth sphere, completely covered by
water. The water is acted upon by the same forces that act on the solid Earth.
The prediction model associated with this case is known as the equilibrium
model of tides, as tides in this model result from the equilibrium of gravitational
forces. However, the equilibrium model cannot explain many aspects of tides,
especially the varying tidal amplitudes in many locations on the Earth. A
substantial improvement in tidal prediction has been achieved by considering
tides, in a dynamic way, as waves. In fact, tides are the longest oceanic waves
with periods of the order of 12 hours. This wave type approach, expanded
and revised many times since Laplace's first formulation, is now known as the
dynamic model of tides.
Complexity of land contours on the Earth and complicated bathymetry cause
great difficulties in the prediction of tides at some points on the Earth. However, the tides can still be predicted using the harmonic analysis of a sufficiently long record of water level fluctuation. Harmonic analysis was already
mentioned in Chap. 4, as the technique for representing an arbitrary, irregular
time series in the form of a summation of many sinusoidal and co sinusoidal
curves with various amplitudes and phase lags. In this chapter we describe the
nature of tidal generating forces and the available prediction techniques. The
significance of tide variations and tidal induced currents for life in the ocean
will be discussed in Part III of the book.
S. R. Massel, Fluid Mechanics for Marine Ecologists
© Springer-Verlag Berlin Heidelberg 1999