Maine Island Kayak Resources and Links

THE OCEAN SCHOOL

Physical Science – Oceanography – Tides and Tidal Currents

Basic Tide Causing Forces

The same forces which hold the earth in its orbit around the sun and the moon in its orbit around the earth cause our tides. Gravity pulls two objects together, with the larger object exerting the greater force. Therefore the sun pulls the earth towards itself, and the earth pulls the moon.

Counteracting the force of gravity – and therefore preventing the earth from crashing into the sun and the moon into the earth - is the centrifugal force generated by the speed which the satellite (such as the earth or the moon) moves around a central axis. Because the earth is very small compared to the sun, the earth moves around the sun. The difference in mass between the earth and the moon is however not so great, so both the earth and the moon rotate around the common axis of the earth-moon system, keeping each other in orbit with their respective gravitational pull on each other.

This axis actually runs through the earth, about 1/3 of the way between the earth’s crust and its center. Because the earth orbits around this axis, it experiences centrifugal force. The centrifugal force and the force of gravity for the earth-moon system and the earth-sun system are exactly balanced in the center of the earth. So our earth moves along stable orbits with the sun and with the moon. However, while the forces are exactly balanced in the center of the earth, they are not balanced on the surface.

Lunar and Solar Tides

The moon’s gravitational force pulls water towards to moon. The gravitational pull is strongest on the side of the earth facing the moon and weaker on the side opposite the moon. The centrifugal force pointing away from the rotational axis (and therefore from the moon) is the same everywhere on the earth. However, the gravitational pull of the moon is stronger than the centrifugal force on the side facing the moon, resulting in a net pull towards the moon, while the centrifugal force is stronger than the gravitational pull on the opposite side, resulting in a net force away from the moon. The forces involved are much too weak to actually lift the water, instead they pull it along the surface of the earth resulting in two bulges – one on the side of the earth facing the moon, and the other on the opposite side. In between is a low-tide girdle which circles the earth. On average each of the bulges is 13.9 inches high, while the center of the low-tide girdle is depressed by 7 inches. The earth rotates underneath the bulges, while the moon proceeds in its orbit, completing one revolution every 29.53 days. Because the moon orbits the earth in the same direction in which the earth spins, it takes a specific point 50 extra minutes every day to catch up with the moon again (lunar retardation). This means that any point on the earth rotates under one of the lunar tidal bulges every 12 hours and 25 minutes. High (or low) tides are therefore roughly 50 minutes later every day than on the previous day.

 

The sun, just like the moon, also creates tidal bulges on the earth by exactly the same mechanism. The sun is much more massive than the moon but also much further away. Its effect is therefore only about half (45.6%) of that of the moon, resulting on average in high-tide bulges of 6.3 inches and a low-tide girdle depressed by 3.2 inches. Since the sun remains stationary, the earth rotates underneath the bulges once every 24 hours, with a specific point passing under one of the solar high-tide bulges every 12 hours.

 

It becomes obvious that the solar and lunar tide bulges line up whenever the moon in its orbit is either in line with or opposite to the sun (at the new moon and full moon respectively – called syzygy). When the moon is half way between those points (first and third quarter – called quadrature), the lunar and solar bulges actually partly cancel each other out. When the bulges line up an extreme tidal range results, called a spring tide.

 

When the bulges partly cancel each other out, very little tidal variation takes place. This is called a neap tide. After syzygy the interaction of forces causes the high tides to arrive faster (priming the tides), while before syzygy the same interaction causes them to arrive later (lagging of the tides).

 

However, neither the moon nor the earth follow precise circular orbits. Instead the orbits are strongly elliptical. On July 2nd the earth is furthest away from the sun (aphelion), and on January 2nd it is closest (perihelion). The difference between the two is three million miles! The moon moves through its closest position (perigee) and furthest position (apogee) once every 29.53 days. The difference between the two is 31,000 miles. So the biggest tides will occur when all 3 factors (perigee, perihelion, and syzygy) occur together.

Diurnal, Semi-Diurnal, and Mixed Tides

Things get more complicated when we take into account that neither the sun nor the moon are always above the equator. The solar tide bulge is not always perpendicular to the rotational axis of the earth since the rotational plane of the earth (the equatorial plane) is offset by 23.5 degrees from the orbital plane of the solar system (the ecliptic). Since the earth moves around the sun once every year, the sun is only perpendicular to the equator twice a year – at the spring and fall equinoxes when the earth rotational axis is aligned sideways (tangential) to the rotational axis of the solar system. At that point the solar tidal bulges are perpendicular to the earth rotational axis. At midsummer the earth rotational axis is tilted towards the rotational axis of the solar system at an angle of 23.5 degrees and the sun is directly overhead at 23.5 degrees latitude north, resulting in a solar tidal bulge that is offset 23.5 degrees to the earth rotational plane. At the winter solstice the opposite effect takes place, with the sun directly overhead at 23.5 degrees latitude south. The solar tidal bulge therefore slowly moves from a 23.5 degree offset to a 0 degree offset, and back to a 23.5 degree offset every 6 months.

 

The moon’s tidal bulge is also not always perpendicular to the earth rotational axis. The rotational plane of the moon-earth system is offset 5 degrees from the ecliptic, with the moon completing one complete orbit around the earth once a month. While the earth equatorial plane always maintains the same aspect in relation to the galaxy, the rotational plane of the moon-earth system does not. It slowly rotates, completing one revolution every 18.6 years. This means that in its most extreme position, it is tilted to the opposite side of the ecliptic in relation to the earth equatorial plane, resulting in a total angle of 28.5 degrees of the moon-earth system in relation to the equatorial plane of the earth. In its least extreme position it is tilted towards the same side of the ecliptic as the earth’s rotational plane, resulting in a total angle of 18.5 degrees. Since the moon completes one orbit around the earth every month, it moves from being overhead at its most extreme position (between 18.5 and 28.5 degrees north) to being overhead at the equator, to being overhead at the respective angle south, back to being overhead at the equator every month. The offset from the equatorial plane is called lunar declination. The resulting lunar tidal bulge therefore shifts between maximum offset to the equatorial plane and the equatorial plane every half month.


The exact pattern of the solar and lunar tidal bulges lining up or partly canceling each other out repeats itself every 18.6 years (one full tidal cycle).

When the tidal bulge is offset from the equator, the low-tide girdle which normally runs over the poles is shifted sideways. For example at maximum lunar declination a point on the earth at high latitude which rotates under the tidal bulges now encounters only one high-tide bulge and the pole-section of the low-tide girdle during a day. This location then has only one short high and one long low tide. This is called a diurnal tide. At low latitudes a point will pass under a high part of the high tide bulge, through the low tide girdle, under a low part of the high-tide bulge, and through the low-tide girdle during a day, resulting in a high high, a low, a low high, and another low, called a mixed tide. At the equator a point will pass under a lower section of the high-tide bulge, through the low-tide girdle, under the lower section of the high-tide bulge, and again through the low-tide girdle during one day, resulting in a high, a low, another high, and another low. This is called a semidiurnal tide.

Influences of the Earth’s Landmasses

The basic factors described above work in a theoretically bottomless, uniform ocean. In practice the land gets in the way. The tidal bulges are literally a wave with a wavelength that is ½ the circumference of the earth at a particular latitude and a period of 12 hours and 25 minutes (time between the passage of consecutive peaks, i.e. high tides). Each ocean basin, sea, or bay causes waves that travel through them to bounce of the shore and travel back, intercepting themselves and setting up a standing wave. A standing wave is a wave where peaks and valleys do not travel but instead move up and down between stationary points (nodes). It is also called a seiche. Since the standing wave has been created by the incoming and outgoing waves overlapping, the wave height is twice that of the original wave. The characteristic period (time from peak to peak) of the standing wave for each ocean basin depends on the width and depth of that basin. The standing wave can have one or more nodes. A standing wave with one node has a peak on one side of the basin while it has a trough on the other side of the basin. A standing wave with 2 nodes has either a peak or a trough on both sides at the same time. If you add a second node to a standing wave you halve its period.

 

For our tidal wave the period of the tidal wave interacts with the characteristic period of the standing wave the tidal wave has set up in the ocean basin. One of three things now happen: If the characteristic period is much smaller the standing wave has pretty much dissipated by the time the next tidal wave comes around. The tidal wave therefore does not overlap with the standing wave it has caused and the net effect is that the tides are simply what they would be with the tidal wave alone. If the characteristic period is much greater the standing wave has completed less than one oscillation by the time the next tidal wave comes around. This means a valley from the standing wave overlaps with a peak from the next tidal wave, interfering with it. The standing wave actually swallows the tidal wave and the result is a much reduced tidal height, with low water occurring when high water would have normally occurred and opposite. If the characteristic period is however that of the tidal wave, the standing wave and the tidal wave reinforce each other and extreme peak heights result (resonance). The resonating standing wave for the North Atlantic is mononodal while that of the North Pacific is binodal.

Open Ocean Tidal Currents and the Coriolis Effect

The difference between a standing wave and a ‘regular’ wave is that water moves laterally in a standing wave across the nodal point to alternately fill in the troughs and form peaks. Therefore we theoretically have current flowing towards and away from the centerline of the ocean basin. Because the current moves it gets deflected by the coriolis effect (to the right in the northern hemisphere, to the left in the southern hemisphere). So as the water moves across the node to fill in the trough it gets deflected to the right, forming a peak to the right of the old peak, and pulling the trough to the right of the old through around a point on the nodal line. This point becomes the amphidromic point – the point where there is no tide movement. Along lines radiating out from it are the peaks of the standing wave at different hours, completing one rotation in 12 hours and 25 minutes, with the peak always opposite of the trough. Those lines are called the cotidal lines. Because the standing wave rotates around the point where there is no change, the tidal range increases with distance from the nodal point, forming concentric rings called corange rings. The tidal current rotates around the amphidromic point, counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. The Bay of Fundy has the highest tides in the world because the characteristic period of the bay resonates with that of the standing wave around the North Atlantic amphidromic point which pushes into the opening of the bay.

Coastal Tidal Curents

We already know that because of the standing wave nature of tidal waves the water flows laterally around the amphidromic point. Local tidal currents are basically caused by this water flow. The tidal wave which is building in an area may also be pushing through a narrow opening or over a shallow section of sea floor. Any time there is more water on one side of an obstruction than on the other, it will flow through, over, or around the obstruction, causing a current.

Reality Check

While all of the above factors give us a reasonable model of the tides in the open ocean, coastal tide prediction depends on an analysis of long-time curves for the local tides (harmonic analysis). The precise astronomic influences on the tides can be broken down into 390 (!) individual wave components, each of which has a precise period (how each of the waves is offset against local time), amplitude (how high/low all of the peaks/troughs in that specific wave are), and phase. The period stays the same, however the phase and the amplitude of each of the components is affected by local factors. So if you analyze your local tide records you can determine how the phase and amplitude of each of the components is affected, and predict tides into the future….

Influences of Meteorological Factors

The two most important meteorological factors influencing tides are wind and barometric pressure. Onshore wind can pile water against a landmass, causing increased tidal heights (called a storm surge), while strong offshore winds can keep water away, causing decreased tidal heights. Low barometric pressure will allow water to rise higher, while high barometric pressure will push down on the water, suppressing tidal height. A low pressure system combined with strong onshore winds and astronomically high tides can cause catastrophic flooding such as the 5’ storm surge recorded during the Halloween Gale in October 1991 (the Perfect Storm) or the 9’ storm surge during the October Gale of 1941 in Germany.

 

Kayaking considerations for tidal conditions

 

Action:
What you can do:

  1. Tides are just one of the tremendous natural forces acting on our planet, producing renewable energy. The energy that comes out of your electric outlets at home however is generated using non-renewable, polluting sources of energy.
    1. Reduce your use of non-renewable energy.
    2. Consider renewable sources of energy for your home (wind, solar)
    3. Decrease the amount of energy lost to the environment through energy-efficient homes, appliances, vehicles.
    4. Ask your local energy provider to purchse energy from renewable sources.
  2. Tides are a very strong force which constantly shifts coastlines in a natural process.
    1. Do not build near shifting coastlines.
    2. Oppose coastal development near shifting coastlines
    3. Do not attempt to stabilize shifting coastlines near your property

What needs to be done globally:

  1. Non-renewable sources of energy need to be phased out and replaced by renewable sources.
  2. Tidal power plants need to be seriously considered as part of a renewable energy program.
  3. Development of shifting coastlines needs to be stopped since it only leads to expensive structures which will either be washed away or, if really massive, inhibit natural current flow and associated ecological processes.

The Ocean School Action Map

Did you do a project at home that promotes renewable energy or opposes coastal development? Tell us about it and we will post it here. Our goal: red dots all over the map, all over the world!
         
Programs

          Introduction

          How Tides Work – 3h (K-4, family)
A classroom introduction to the solar system and the effect of the moon and sun on tides for kids and families. We’ll use a model of the solar system to look at what causes the different phases of the moon, the seasons, and the tides.

Tide Basics – 3h (5-12, adult)
A classroom overview of the basic tide-causing forces. We’ll use a model of the solar system to look at how astronomical factors shape tides as well as introduce seiche conditions.

How Tides Really Work – 6h (5-12, adult)
A classroom program exploring the details of how tides really work including all the astronomical factors, ocean-basin oscillations, amphidromic points, and local tide prediction. For the mathematically inclined.

Exploration

Tidal Currents – day trip (5-12, family, adult)
We’ll measure direction and strength of tidal currents on several points around the island during a day, developing our own map of tidal currents during one full tidal day.

Why did Eastport Fail? – 3- day trip (adult)
The world’s largest tidal power plant was planned in Eastport, Maine, and most of its structures are already in place. As we paddle this area of huge tidal currents and whirlpools (the Old Sow was the world’s second largest whirlpool before a dam altered the current flow) well explore the ecological and physical factors that affected the decision to abort the project.

Real Science

Building a Tidal Gauge – ½ day trip (family, adult)
We’ll build our own local tidal gauge. Details soon.

Action Projects

Coastal Erosion Monitoring – ½ day trip (family, adult)
As we paddle we’ll look for places where human-made structures seem to be in conflict with tidal currents. We’ll analyze the current flow and determine if and how the structure and the sea could coexist.

Further Reading

Fox WT. 1983 At the Sea’s Edge. Prentice Hall, New York

Defant A. 1958 Ebb and Flow. University of Michigan Press, Toronto, Canada

Links

Our Restless Tides (NOAA / NOS) http://140.90.78.170/restles1.html

The Bay of Fundy's Minas Basin - Highest Tides in the World http://www.valleyweb.com/fundytides/

Tides and Tide Prediction http://scilib.ucsd.edu/sio/tide/

WWW Tide and Current Predictor http://tbone.biol.sc.edu/tide/sitesel.html