Geology Reference
In-Depth Information
but, as shown above, real-world tides don't always
follow convention and exceptions are known (Open
University Course Team
1999
).
The major tidal cycles discussed under the equilib-
rium model can be understood in the context of the
dynamic model and tidal constituents. Specifically, the
synodic neap-spring cycle is generated through the
interaction of the S
2
and M
2
constituents. In the modern
world, these two tides come into phase and amplify the
resulting tide every 14.77 days. The result is a syn-
odic spring tide. Conversely, every 13.66 days K
1
and
O
1
converge and generate a tropical spring tide.
Whether a spring tide along a specific coastline is
dominated by the synodic spring tide or the tropical
spring tide is determined by the basin geometry. For
instance, the Gulf of Mexico is dominated by the K
1
and O
1
tides, therefore neap-spring tides cycle with the
tropical month (Fig.
1.9
). The east coast of the USA,
however, is dominated by S
2
and M
2
tides resulting in
neap-spring tides that cycle with the synodic month
(Fig.
1.9
). The semimonthly inequality of spring tides
occurs because of the convergence of M
2
and N
2
every
27.55 days. A diurnal inequality is driven by the inter-
action of O
1
and M
2
(in phase once a day) and is noted
in coastal tides when these constituents are of suffi-
cient amplitude.
One can look at the progressive change in relative
intensity of particular tidal constituent along a coast
and see how that affects the resulting tides. For exam-
ple, Figs.
1.10
and
1.11
shows the amplitudes for the
seven dominant tidal constituents for the Gulf of
Carpentaria, Australia and the tidal patterns that result
from changes in the relative amplitudes of the various
constituents (from Kvale
2006
). At the mouth of the
gulf at Booby Island, the tides are dominated by M
2
,
K
1
and O
1
. Given the dominance of O
1
and K
1
, the
neap-spring cycle occurs every 27.32 days and corre-
sponds to the tropical monthly period. However, unlike
many regions whose neap-spring cycles are tropically
driven, there is a relatively strong M
2
tide (but relatively
weak S
2
tide) at the mouth of the gulf. The resultant
Fig. 1.7
Diagram showing the amphidromic circulation for the
M
2
tide in the North Sea. Co-tidal lines indicate times of high
water. And co-range lines indicate lines of equal tidal range.
Figure is modified from Dalrymple (
1992
) which was based on
a map first drawn by J. Proudman and A. T. Doodson (From
information found in Cartwright
1999
) (From Kvale (
2006
) and
used by permission from Marine Geology)
Table 1.1
List of the seven most common tidal constituents, their rotational speed (number of degrees a tidal wave generated by
the constituent can travel around its amphidromic point in 1 h), description, and period in solar hours (Defant
1961
)
Tidal constituent
Speed (degrees/hour)
Origin
Period in solar hours
M
2
28.9841
Principal lunar
12.42
S
2
30
Principal solar
12
N
2
28.4397
Larger elliptical lunar
12.66
K
2
30.0821
Combined declinational lunar
and declinational solar
11.97
K
1
15.0411
Combined declinational lunar
and declinational solar
23.93
O
1
13.943
Principal lunar
25.82
P
1
14.9589
Principal solar
24.07
