Tides: Up and Down and Round and Round
1. The movement of global tides
At the link below you will find an animation of tides around the world, which allows you
to see how they move around ocean basins. What you are seeing (snapshot above) is
the predicted tides for five days in September 2013, from a model that is run at Oregon
State University. You can see the date and time counter at the top of the figure. The
color indicates the water height, in meters above or below mean sea level, so that high
tides appear as orange to red, and low tides appear as cyan to dark blue. Greenish
areas would be at mean sea level.
http://volkov.oce.orst.edu/tides/pic/tpxo8.gif
1a. What direction do the tides move on the west coast of North America? For example,
if you follow the progression of a high tide, where does it start and where does it end?
1b. At the far right you can see the tide moving along the coast of western Europe.
What direction is it moving? Follow one of the high or low tides all the way along its
path. Where does it appear to go at the end?
2. Co-tidal Lines
It appears to us on shore that the tides go “in and out” -- toward and away from the
coastline. From the movement of the tidal bulge due to the pull of the moon and sun, it
appears that the tidal crest should move around the globe from east to west. But as you
can see in the animation, they actually rotate around the outside edges of ocean basins,
up and down the coastlines. When there are land masses breaking up the oceans -continents, islands, ridges -- the tidal bulge canʼt progress smoothly around the earth
like it would if it was just one big ocean.
The path that tidal crests follow in an ocean basin are called co-tidal lines, as seen in
the figure below. Each line shows the location of a tidal crest at a given time, which is
what the numbers signify (hour 1, hour 2, etc.). For example, find the long vertical line
running through the middle of the Pacific Ocean that is marked “3”.
That line shows you where a tidal crest (high tide) will be at hour 3. Every point along
that line will be experiencing a high tide at that time. An hour later, the tidal crest will
have moved westward to the line marked “4”.
2a. Find the co-tidal lines that go up the west coast of North America. About halfway
up, there is a line marked “8” near the border between the U.S. and Canada. What
number is shown on the line that intersects the south end of Baja California? How long
does it take for a high tide to move up the coast from southern Baja California to the
U.S.-Canadian border?
2b. Why do the numbers on the co-tidal lines only go up to “11”? (Hint: how long is a
normal tidal cycle, the time between successive high tides?)
Look at the East coast of the U.S. The nearest co-tidal line is marked hour “11”, and
the one east of that is marked “10”, so we know the tide is moving toward the U.S.
coast, from east to west.
2c. At approximately what hour would the high tide crest be at the east coast? How
does the arrival of the tide on the east coast of the U.S. differ from the way it arrives on
the west coast? To help answer this, look at the global tide animation again, and notice
how the high and low tides progress on the West coast vs. the East coast of the U.S.
3. Amphidromic Points
On the map above, there are red dots at the center of sets of co-tidal lines. If you follow
a line through the red dot, youʼll see that the line on the opposite side is 6 hours later.
So an hour “2” line is opposite an hour “8” line, and an hour “5” line is opposite an “11”.
High and low tides occur about 6 hours apart, so when the high tide crest is on one line,
the other side is experiencing a low tide. For example, when the high tide crest is along
a line marked hour “2”, the line marked “8” is the position of the low tide.
At the position of the red dot, the tide is never really high or low -- the tidal crest is
rotating around that point. This is called the amphidromic point. If we look at why the
tides rotate around ocean basins youʼll be able to see whatʼs going on there.
First, look at the animation again.
3a. What direction are the tidal crests moving in the North Pacific and the North Atlantic,
clockwise or counter-clockwise?
Why would that be? Are you thinking Coriolis force? But the Coriolis force makes
things curve to the ___________ or __________-wise in the northern hemisphere….
Hmmm.
But if you were thinking Coriolis, youʼre correct! When a tidal crest enters an ocean
basin, it is deflected to the right, just like ocean currents or winds. The result is that it
goes to the right side of the basin first.
In box 1 below, the tidal crest enters heading north, and as it moves up the basin it gets
pushed to the right (north-eastward), butting it up against the land on that side of the
ocean, as in box 2. (Look at how that tidal crest is moving around the North Pacific
again, in the animation.) The crest continues trying to move up and to the right
(northeastward) but there is only land in that direction, so it follows the coastline as it
curves up to the north. The Coriolis is still trying to push it to the right, so it stays butted
up against the coast. When it has gone as far north as it can (because thereʼs land
there too) it continues around the basin, now heading south. The Coriolis is still pushing
it to its right (box 4).
So if you look at the water level in boxes 1-4, the high tide moves around the outer
edges in a counter-clockwise direction. Directly opposite the high tide, the other side
has a low tide. And that red dot in the middle doesnʼt move up or down much at all -amphidromic point!
3b. It appears from this box diagram that the tide would be highest where it is butting up
to the edge (the land or coastline). In the animation, what happens to the tidal ranges
as you go from open ocean toward the coast (increase or decrease)? [Tidal range
refers to the total change in water level, from the lowest low to the highest high.]
3c. What area of the North American west coast has the largest tidal range, and how
large is it? [Use the scale at the right of the animation, which is marked in meters. Ex:
an area with tides ranging from –1 meter to +1 meter would have a tidal range of 2
meters.]
3d. Find the amphidromic point on the map above thatʼs nearest to the California coast.
Now note some landmarks on the coastline that you can line it up with, so you can find it
on another map (what is directly north and east of the point?) Go back to the animation
and put your mouse over that point, as near as you can, and estimate the tidal range at
that point.
Landmark directly to East:
Landmark directly to North:
Approximate tidal range at that point:
Do the same thing for the amphidromic point in the South Pacific, almost directly south
of the one near us.
Landmark directly to West:
Landmark directly to North:
Approximate tidal range at that point:
How do these compare to the tidal range in #3c? Would you agree that these are points
where there is very little change in water height?
4. Tides and the Moon
The spring and neap tidal cycle
is the change in tidal range that
we see as the moon and earth
shift their positions relative to
one another. When the moon is
in line with the earth and sun,
the tidal bulges from the sun and
moon overlap and we get
maximum tidal ranges, or spring
tides. When the moon is at right
angles to the earth and sun, the
two tidal bulges donʼt line up and
the tidal range is smaller, neap
tides.
You can see an animation of this
here:
NOAA Earth-Moon-Sun
(http://oceanservice.noaa.gov/education/tutorial_tides/media/supp_tide06a.html)
4a. Looking at the global tide animation (again!) what is the date when the tidal ranges
are the largest? (The date is at the top, 09-01-2013 to 09-05-2103, 1-5 September
2013.)
What do you predict the phase of the moon would be on that date?
Now check it out on a lunar phase calendar:
http://www.calendar-365.com/moon-calendar/2013/September.html
What is the actual phase of the moon on that date? Were you correct?
4b. Here is a tide calendar for San Diego:
http://iod.ucsd.edu/~raz/OceanWeb/assignments/Tide_Calendar.pdf
Say you want to visit the tide pools this year, on the best day possible. Take into
consideration:
• The lowest tide is best (low = large negative number)
• Daylight is helpful (sunrise and sunset times are shown for each week, on Sundays)
What is/was the best possible day this year?
Day:
Time:
Tide height:
If itʼs already passed, whatʼs the next best day thatʼs still to come?
Day:
Time:
Tide height: