The Sun and the Moon
F ollow the tides
have a date with the sea
Tides account for some of the most significant variations in sea level. The combined attraction of the
Moon and the Sun is behind this phenomenon and its variations.
The Moon, like the Sun, attracts the Earth and
its oceans, which then change their shape. Water
accumulates at the place of maximum attraction, in
other words at that point on the globe which is the
closest to the celestial body. In addition, due to the
speed of the movement, an opposing centrifugal force
keeps the Earth on its orbit. This centrifugal force
pushes the water back and it then accumulates in the
opposite direction to the celestial body.
The respective positions of the Moon and Sun
in relation to the Earth cause the cycle of
monthly variations. When aligned, their influences
are reinforced, when at a right angle, they are reduced,
which means that tides are or more less high.
Changing currents
64°N
The tide move water masses, which causes currents.
These currents, sometimes violent in straits, are
particular in that they change in intensity and
direction according to the tides. It is vital for coastal
navigation, for preventing pollution and protecting
the shoreline, that we understand them and are able
to predict them. Project of undersea turbine —like
an immersed windmill— to generate electricity from
these currents are currently under study.
63°N
62°N
61°N
59°N
Moon or Sun
7 DAYS LATER
First quarter: neap tides
30°N
0
30°S
60°S
90°S
180°W
58°N
H: High tide amplitude
Tides are not only semi-diurnal
60°N
60°N
New Moon:
spring tides
90°N
150°W
0,1
12°30’W
10°W
7°30’W
5°W
2°30’W
0
2°30’E
5
10
15
20
25
30
35
90°W
0,5
60°W
30°W
0
1
40
Speed of currents due to the semi-diurnal, lunar tides close to the
Faroes and Shetland islands (North of Scotland).
If the celestial body (Moon or Sun) is in the
Earth's equatorial plane, then there are two
high and two low tides per day.
180°E
10
62°N
Variations in tides with the lunar cycle
cm
0
In order to be able to calculate tides, it is thus
necessary to know the attractive force of the Moon
and Sun and their respective positions. The relief of
the bottom of the seas and the shape of the coastline
also affect tides and can amplify the phenomenon up
to fifteen metres, as is the case for the bays of Mont
St Michel (France) or Fundy (Canada).
5
10
20
30
40
50
60
70
80
90
100 150
Amplitude of the main semi-diurnal, lunar component of the tide.
90°N
Around
North Atlantic
in 12h25’
60°N
30°N
30°S
Moon or Sun
60°S
90°S
180°W
150°W
120°W
90°W
60°W
30°W
0
30°E
60°E
90°E
120°E
150°E
180°E
Degrees
Tide levels in the port of Brest (France) in December 2000. The illustration shows the
alternating high and low tides and, over a month, the variations in amplitude due to
the respective positions of the Moon and Sun.
10
60°N
7,5
58°N
56°N
5
2,5
50°N
0
12 hours later
150°E
48°N
15°W
H: High tide amplitude
If the celestial body is not in
the equatorial plane, then one
of the high or the low tides
will be greater than the other
high or low tide respectively,
every day, at middle latitudes.
2
120°E
52°N
12 hours later
Moon or Sun
90°E
54°N
21 DAYS LATER
Third quarter:
neap tides
However the Moon is rarely in the Earth’s equatorial
plane. The two bulges are along the Earth-Moon or
Earth-Sun axis and hence, for a given latitude, the
amplitude will not be the same for the two daily tides.
Sometimes there is only one tide per day.
60°E
Amplitude of lunar wave with a period of six hours
(wave M4), in the particularly complex areas of
the North Sea and the English Channel. A long
series of altimetry data, from TOPEX/POSEIDON
and then Jason-1, should make it possible to
significantly improve forecasting.
14 DAYS LATER
Full Moon:
spring tides
Moon or Sun
30°E
Comparison of semi-diurnal and diurnal tides in the world. The blue/violet areas correspond to dominant,
diurnal tides, while the yellow/red areas are semi-diurnal.
cm/s
0
120°W
For most people living on the shores of the
Atlantic, tides are phenomena which occur
twice a day. But some places in the world
only have one tide per day (Gulf of Mexico,
China Sea, North Pacific, Antarctic Ocean,
etc.) Other rarer places sometimes have up
to three or four tides in the same day.
The breakdown of tides into sinusoidal
waves reveals that there are other different
periods than the day and half day, for
instance 6 hours. (wave M4). Depending on
the areas, some of these waves have to be
taken into account.
0
30
60
90
120
150
180
210
Phase of the main, semi-diurnal, lunar component of the tide.
240
270
300
330
360
The main semi-diurnal component of
the tide (wave M2), due to the Moon
is, in many regions, the dominant
component. It may be observed that
regions with an increase in tide
variations or tidal range (difference
in height between successive low and
high tides) have significantly high
values, in particular near the coasts on
the North Atlantic. We can also see
regions with very low amplitude, as
well as points where the tidal range
is nil, around which the tide “turns”.
The phase map is used to identify the
direction and propagation speed of the
wave phase which changes from 0° to
360° during its period (12h25’ in the
case of M2).
10°W
5°W
0
5°E
10°E
cm
0
F orecasts which are increasingly
T ides and the climate
more reliable
Forecasting of tides requires integrating observations into numerical models for greater precision.
Tide gauges and altimetry satellites are both useful and their combination enables tide forecasting to
within 2 to 3 centimetres in the middle of the ocean.
Breaking down tides to calculate
them better
In order to calculate tides, they have to be broken
down into sinusoidal waves of given periods, each
of which represents one component of the problem.
Thus, one of the waves, called M2, is due to the
attraction of an ideal Moon placed on a perfectly
circular orbit in the Earth’s equatorial plane. It has
two high and two low tides per day (semi-diurnal
wave). The K1 wave, with a diurnal period, reflects
variations in declination of the Moon and Sun.
The amplitude of the tide at a given moment and
given place is the sum of all these sinusoidal waves.
In certain areas, a hundred of these waves have to
be added to obtain a precise forecast.
51°N
50°N
50°N
49°N
49°N
48°N
48°N
47°N
47°N
46°N
46°N
45°N
45°N
44°N
44°N
43°N
43°N
42°N
42°N
7°30’W
100
5°W
125
150
2°30’W
175
200
225
0
250
cm
275
300
22°30’N
20°N
20°N
17°30’N
17°30’N
15°N
15°N
12°30’N
12°30’N
10°N
107°30’E
10
15
20
110°E
25
112°30’E
30
35
115°E
cm
40
45
Ocean waters are stratified according
to their density. The different layers
mix with difficulty. However the
tidal currents coming into contact
with the relief of the ocean bottom
(even if this is very deep) creates
waves which are propagated at the
interface between two layers of
different densities. It is currently
thought that this mechanism
contributes more than half of the
vertical mixing of water masses.
Now this mixing is fundamental
for large-scale ocean circulation
(thermohaline circulation) which
enables the redistribution of heat
from the equator to the poles.
Tide models are based on physical laws. Their precision can be improved by integrating observation
data. Tide gauges installed just off the coast provide
precise, local measurements. Altimetry satellites such
as TOPEX/POSEIDON, ERS, EnviSat or Jason-1,
cover the whole Earth, particularly in the middle
of the sea, at regular intervals. Using mathematical
procedures, this complementary information can be
integrated into tide models. In addition, tides have to
be filtered out of altimetry data in order to determine
other ocean variations (currents, eddies, seasonal
variations, etc.).
High tide, low tide… the rhythm of the
tides seems to rock the oceans, reflecting the
movements of the Moon and Sun. Precise
knowledge of them is vital for many maritime and
coastal activities. It is not easy to ignore
tides, given that variations in sea level can
reach 10 metres in some ports, and that
currents change with the ebb and flow.
Satellite altimetry now provides regular
60°N
30°N
0
60°S
7°30’W
5,5
5°W
6
2°30’W
6,5
7
0
7,5
8
8,5
cm
9
60°E
9,5
120°E
180
120°W
60°W
0
Energy flow of the semi-diurnal, lunar tide wave (M2). The illustration shows the displacement of energy from areas in
which it was generated towards dissipation areas. It has been demonstrated, for instance, that the energy dissipated
on the Patagonia plateau (to the south-east of America), one of the areas in which tides are the highest, comes from
the Pacific and that 40% of the total energy contributed to oceanic tides by the Earth/Moon system, is dissipated
in the North Atlantic
10°N
107°30’E
20
25
30
110°E
35
40
45
112°30’E
50
55
60
Since 1992, TOPEX/POSEIDON
has been providing highly precise
altimetry measurements which has
made it possible to map the
energy dissipated by tides. As was
previously thought, part of this
energy is dissipated in coastal
regions through friction with the
bottom of the sea, but is has also
been observed that a significant
part of the energy (one third) is
dissipated in the deep ocean.
This discovery in turn shed some
light on the possible role of tides
in thermohaline circulation.
115°E
cm
65
70
Amplitude of two main tide components, one semi-diurnal (wave M2), the other
diurnal (wave K1), in the North Atlantic, near to the French coasts and in the China Sea
off the coast of Vietnam (the colour scale is different for each of the figures). While
M2 is clearly dominant on the French coasts (two tides per day) K1 is dominant in the
China Sea where there is only one tide per day.
measurements of sea level in the middle of the
ocean, which means that forecasting of tides has
been improved. In addition to the publication
of tide calendars, we have improved our
understanding of the influence of the Moon on the
Earth and, somewhat unexpectedly, on its climate.
Thermohaline circulation: water from the Tropics cools as it flows into the Norway Sea. It then sinks to the bottom
of the ocean where it propels the deep ocean circulation system before eventually welling up and warming again in
the Tropics, some 1,000 years later.
30°S
5
22°30’N
Models enriched with observations
The model/tide gauge
comparisons show a
remarkable fit between
the model’s forecast
and the actual
observations.
Tides, the ocean
under influences
In addition to their direct effects on maritime and coastal activities, tides appear to have a less well-known
impact on the Earth’s climate. This discovery, which was based on almost ten years of highly precise
altimetry data, has led to new understanding of the way in which the Moon influences our planet.
Creation-conception: CLS / Design: Active Design / Publisher: CNES 2001
51°N
Observing the oceans
from space
Coastal tide gauge
For more information:
Sources:
AVISO/Altimetry: http://www-aviso.cnes.fr
Tides: http://tidesonline.nos.noaa.gov
http://www.shom.fr
CLS, CNES, CNRS/LEGOS, KMS,
NASA/GSFC, SHOM
National Aeronautics
and Space Administration
Jet Propulsion Laboratory