Currents and Tides

The Ocean / NEXT: Geography of the Oceans and the Structure of Planet Water »

Perpetual Ocean: This visualization (best if you change quality to at least 720p for HD!) shows ocean surface currents around the world
during the period from June 2005 through December 2007. Source: NASA/Goddard Space Flight Center Scientific Visualization Studio

Ocean Currents

Ocean currents are a continuous directed flow of ocean water capable of traveling thousands of kilometers. Surface currents move water at the surface of the ocean, while submarine river currents flow deep below. Ocean currents can significantly influence the climate of coastal areas. The Gulf Stream, for example, makes northwest Europe the most temperate of any region at that latitude. The California current in the Hawaiian Islands results in a climate much cooler than other tropical latitudes. El Niño is a phenomenon where an ocean current is reversed, resulting in devastating climate changes from the west coast of South America all the way to Australia. Ocean currents also impact the life cycles of certain marine creatures, such as eels, which relocate thousands of kilometers to spawn based on ocean currents.

Tides

The ocean's surface rises and falls predictably due to changes in gravitational forces originating from the Moon and the Sun. These changes in ocean surface level are known as tides and are evidence of the influence celestial bodies have on our planet. Without these external forces, the ocean's surface would simply exist as a geopotential surface or geoid, where the water is pulled by gravity without currents or tides. The strong gravitational fields of the Moon and the Sun result in significant changing gravitational fields causing tides to rise and fall. Because the Sun is farther away than the Moon, the Sun's pull on the tides is about 40% as strong as the Moon's. Both gravitational and tidal forces weaken as distance increases. Gravitational forces are inverse to the square of the distance and tidal forces are inverse to the cube of the distance.

Introduction to Waves: Khan Academy

Lunar Tides

The two most important forces acting on the Earth to create tides are the Moon's gravitational field pulling the ocean towards itself and the centrifugal force, or outward movement from the axis of rotation, caused by the Earth and Moon revolving in their orbit. At the center of both the Earth and the Moon, centrifugal and gravitational forces cancel each other out, preventing the Moon from falling on the Earth and the Earth from flying out of orbit.

Both the Earth and the Moon revolve around the one center of both of their masses. This common point is called the barycenter and is located at about ¾ of the radius from the Earth's center. Although the Moon is the Earth's only satellite, the Earth moves slightly when they both orbit the barycenter. In addition, the barycenter is not always in the same place due to the elliptical rotation of the Moon. There is one revolution of the Moon every 27 days, so the angular velocity is the same at all places and the centrifugal force lines up with the radius from the barycenter. The effects of the Moon on the Earth have mostly to do with the ocean, since in contrast to the crust which moves as a whole, each ocean particle is free to be pulled by gravity as a liquid. Tidal force is determined by the difference in forces at the Earth's center and surface. Net forces below and above the Earth are equal and point away from the center.

Gravitational pull of the Moon is stronger near the moon and weaker away from the Moon. The pull also varies slightly (only inversely quadratic in contrast to the cube or square) across the Earth's radius due to differences between center and surface.

NASA JPL: Ocean Surface Topography from Space

Important ocean currents include:

» NOAA Ocean Surface Current Analyses - Realtime (OSCAR)
» TAO/TRITON - Graphical displays and animations of realtime and retrospective data from the TAO / TRITON array of El Nino buoys in the tropical Pacific Ocean which measure El Nino and La Nina data.
» Wikipedia: Ocean currents

Types of Tides

High tide is when the water is at its highest level and low tide is the water at its minimum level. Ebb or falling tide is when the water seems to flow back out and happens between a high tide and a low tide. When the water flows back in between low tide and high tide, this period of time is known as flow, flood or rising tide.

Since the earth rotates relative to the moon in one lunar day (24 hours, 48 minutes), each of the two bulges travel around at that speed, leading to two high tides and two low tides each day with high tides occurring 12 hours and 24 minutes apart. Another way to look at it is that 12 hours is half of a solar day and 24 minutes is half of a lunar extension. Two high tides occur simultaneously; one high tide comes from the ocean being pulled by the moon's gravity and the other resulting from the inertia of the ocean water in orbit and that the center of the Earth is pulled towards the barycenter, leaving the ocean dragging behind.

Variation in Tides

Since there are subtleties to the cycles of the Moon and Sun, there are also variations in the tides. During a new or full Moon, tidal forces from the Sun reinforce those from the Moon causing the tide's range to be at a maximum, also known as spring tide. At the first or third quarter of the Moon, forces from the Sun partially cancel those of the Moon and the tide's range is at its minimum or neap tide.

Other important celestial behavior affecting tide heights is the relative distance of the Moon from the Earth. At perigee, the Moon causes the range to increase, while at apogee the range is diminished. Surfers wait for the spectacular waves that occur every 7 1/4 cycles because at this time perigee and either a new or full Moon coincide causing the most drastic range of tide heights. With the addition of a storm rolling in, the waves can cause severe damage or provide for really exciting surfing experiences.

Tidal Lag

Tidal lag is a somewhat gray area in the predictability of tides. It's caused by the fact that forces on the tides can take time to have a noticeable effect. The Moon's tidal forces have a 12.42 hour period, which is half of the Earth's synodic period of rotation and the resulting complex resonance that occurs is the culprit for the average 6 hour tidal lag. The result of the tidal lag is that low tide often occurs when the Moon is at its Zenith or its nadir, which is opposite of what one may expect. Tidal acceleration, the process of the Earth's movement being slowed down and the Moon being moved farther away is another interesting phenomena resulting from tidal lag and the transfer of momentum between sea and land.

Origin of Tides

Tides originate in the southern oceans, the only place on Earth where a circumventing wave resulting from the tidal force of the Moon can travel with no restrictions from land barriers. The result is that in most places there are delays between phases of the Moon and effects on the tide. For example, spring and neaps are two days behind the new/full moon and first/third quarter in the North Sea.

Standing Waves

Tides are actually standing waves that travel northwards over the Atlantic. A standing wave is a type of wave produced when two waves of identical frequency interfere with each other while traveling in opposite directions. At various points of a standing wave two waves will constructively or destructively interfere (meaning that they add up amplitude or completely destroy amplitude resulting in a node). The activity of the standing wave results in relatively low tidal ranges in some locations and high ranges in other locations.

Tidal Amplitude

Using only the effects of celestial bodies on the tides, it would be calculated that the amplitude of oceanic tide at the equator is about one meter. This value is inaccurate because of the enormous influence of factors like global topography and the period of the oceans being 30 hours (a function of terrestrial gravity and average ocean depth), a large value in comparison to the 57 minute period of the Earth's crust. It has been theorized that if the Moon vanished, ocean levels would continue to oscillate for 30 hours until all of the stored energy was depleted.

Geographical Influences on Tidal Ranges

The local geography has a lot to do with the exact time and height of a tide in a particular location. One particularly remarkable tidal range is the 16 m (or 53 foot) tidal range found in the Bay of Fundy on the east coast of Canada. In this case, the shape of the bay influences the extreme variation in tides. Although it was noted in folklore that a giant whale splashing around caused this immense variation, oceanographers think it results from waves entering the mouth of the bay and then bouncing back, creating a standing wave. Other interesting examples are Southampton in the UK, which has a double high tide due to the Isle of Wight, and Dorset which has a double low tide due to the Isle of Portland. In contrast, extremely small tides occur in the Mediterranean and the Gulf of Mexico due to a very narrow connection with the ocean. A very small tide occurs on the southern coast of Australia due to the very open ocean and a lack of any deep indentation to create interference in the coast.

Tides and Navigation

For accurate navigation of the ocean, tidal flows must be taken into account. Of great importance are tidal heights, which due to the shallow bar at the entrance, can completely prevent a boat from entering harbor at their lowest points. Tidal charts come in sets, with each set covering the hour between one high tide and the other, ignoring the extra 24 minutes. These charts, when used for the area to be traveled, are useful in that they give the average tidal flow for that one hour. Information found on a tidal chart consists of direction and average flow in knots for spring and neap tides.

An alternative to the tidal chart are the tidal diamonds found on most nautical charts, which relate certain points on the chart to a table that will also give direction and speed of tidal flow. A dead reckoning position is calculated along with the direction of travel and these values are marked on the chart. Then a line is drawn from the dead reckoning position in the direction of the tide. An estimated position, marked with a dot in a triangle, can be calculated by measuring the distance the tide will have moved the vessel along this line.

Due to the many meteorological factors that may influence tides, a depth marking at the lowest possible astronomical tide can be very useful and can be obtained from any nautical chart. Other good information includes heights and times of low and high tide each day and these can be found in tide tables. Using the tide tables and charts together, actual water depth at these times can be calculated. Each port has its own tidal curves to help calculate depths for intervening times. The rule of twelfths can also be used and is simply a way to approximate depth based on the observation that depth in the six hours between low and high tide will increase as follows: first hour- 1/12, second hour- 2/12, third hour- 3/12 and so on until six hours as passed. As a note of caution, the rule of twelfths is only an approximation and should not be used in navigation without proper training.

Other Tides

Oceanic tides are not alone, there are also atmospheric tides and terrestrial or land tides that affect the Earth. Atmospheric tides are drowned out by the much more radical effects of weather and solar thermal tides. The Earth's crust, rises and falls as if it were breathing due to the Moon's pull. On average, terrestrial tides have an amplitude of 1.5 m at the equator and since they are only two hours behind the moon, they reinforce oceanic tides. In 1686, Isaac Newton became the first to give a mathematical explanation of tidal forces in the Philosophiae Naturalis Principia Mathematica. It is interesting to note there are several terms using the word “tide” that have nothing to do with tides. These include: tidal wave, rip tide, storm tide and hurricane tide.

» Wikipedia: Tide
» Tides @ OceanLink.com
» Tidal Misconceptions by Donald E. Simanek

The Ocean / NEXT: Geography of the Oceans and the Structure of Planet Water »

Share your thoughts

Please note: All comments are moderated. Trolls, spammers and those who post offensive material will be reported/banned.

Feedback & Citation

Something to say about this page? Let us know

MLA Citation: "". MarineBio.org. <http://www.marinebio.org/oceans/currents-tides.asp?>. Note: If no author is given under the main page title, simply cite MarineBio.org.

Oceanography News   :: ScienceDaily

Unparalleled views of Earth's coastal zone with HREP-HICO

Scanning the globe from the vantage point of the International Space Station is about more than the fantastic view. While cruising in low Earth orbit, the space station HICO and RAIDS Experiment Payload-Hyperspectral Imager for the Coastal Ocean, or HREP-HICO, gives researchers a valuable new way to view the coastal zone.

Sulfur finding may hold key to Gaia theory of Earth as living organism

Is Earth really a sort of giant living organism as the Gaia hypothesis predicts? A new discovery may provide a key to answering this question. This key of sulfur could allow scientists to unlock heretofore hidden interactions between ocean organisms, atmosphere, and land -- interactions that might provide evidence supporting this famous theory.

Hidden lives of elephant seals: Record-setting dive more than a mile deep

The same researchers who pioneered the use of satellite tags to monitor the migrations of elephant seals have compiled one of the largest datasets available for any marine mammal species, revealing their movements and diving behavior at sea in unprecedented detail.

Arctic seabirds adapt to climate change

The planet is warming up, especially at the poles. How do organisms react to this rise in temperatures? Biologists have now shown that little auks, the most common seabirds in the Arctic, are adapting their fishing behavior to warming surface waters in the Greenland Sea. So far, their reproductive and survival rates have not been affected. However, further warming could threaten the species.

First satellite tag study for manta rays reveals habits and hidden journeys of ocean giants

Using the latest satellite tracking technology, conservationists have completed a ground-breaking study on a mysterious ocean giant: The manta ray.

Push from Mississippi kept Deepwater Horizon oil slick off shore

Geoscientists offer an explanation for why the Deepwater Horizon oil spill didn't have the environmental impact that many had feared. Using publicly available datasets, their study reveals that the force of the Mississippi River emptying into the Gulf of Mexico created mounds of freshwater which pushed the oil slick off shore.

Potential instability in West Antarctic Ice Sheet from newly discovered basin size of New Jersey

Scientists have uncovered a previously unknown sub-glacial basin nearly the size of New Jersey beneath the West Antarctic Ice Sheet near the Weddell Sea. The location, shape and texture of the mile-deep basin suggest that this region of the ice sheet is at a much greater risk of collapse than previously thought.

New weak point discovered in the Antarctic ice sheet

The Filchner-Ronne Ice Shelf fringing the Weddell Sea, Antarctica, may start to melt rapidly in this century and no longer act as a barrier for ice streams draining the Antarctic Ice Sheet, new research shows.

Antarctic octopus study shows West Antarctic Ice Sheet may have collapsed 200,000 years ago

Scientists have found that genetic information on the Antarctic octopus supports studies indicating that the West Antarctic Ice Sheet could have collapsed during its history, possibly as recently as 200,000 years ago.

Plastic trash altering ocean habitats

A 100-fold upsurge in human-produced plastic garbage in the ocean is altering habitats in the marine environment, according to a new study.

Effect of groundwater use: Using water from wells leads to sea level rise, cancels out effect of dams

As people pump groundwater for irrigation, drinking water, and industrial uses, the water doesn't just seep back into the ground -- it also evaporates into the atmosphere, or runs off into rivers and canals, eventually emptying into the world's oceans. This water adds up, and a new study calculates that by 2050, groundwater pumping will cause a global sea level rise of about 0.8 millimeters per year.

70 percent of beaches eroding on Hawaiian islands Kauai, Oahu, and Maui

An assessment of coastal change over the past century has found 70 percent of beaches on the islands of Kaua»i, O»ahu, and Maui are undergoing long-term erosion, according to new results.

Stalagmite research suggests Earth has two modes of responding to change

By analyzing stalagmites, a team of researchers has determined that the climate signature in the tropics through four glacial cycles looks different in some ways and similar in others when compared to the climate signature at high latitudes. The results suggest that Earth's climate system might have two modes of responding to significant changes.

Increasing speed of Greenland glaciers gives new insight for rising sea level

Changes in the speed that ice travels in more than 200 outlet glaciers indicates that Greenland's contribution to rising sea level in the 21st century might be significantly less than the upper limits some scientists thought possible, a new study shows.

Marine food chain becomes clearer with new revelations about prey distribution

A new study has found that each step of the marine food chain is clearly controlled by the trophic level below it -- and the driving factor influencing that relationship is not the abundance of prey, but how that prey is distributed.

Arctic sea-ice loss didn't happen by chance

The ongoing rapid retreat of Arctic sea ice is often interpreted as the canary in the mine for anthropogenic climate change. In a new study, scientists have now systematically examined the validity of this claim. They find that neither natural fluctuations nor self-acceleration can explain the observed Arctic sea-ice retreat. Instead, the recent evolution of Arctic sea ice shows a strong, physically plausible correlation with the increasing greenhouse gas concentration. For Antarctic sea ice, no such link is found – for a good reason.

Ireland's Aran Islands: Old maps and dead clams help solve coastal boulder mystery

Perched atop the sheer coastal cliffs of Ireland's Aran Islands, ridges of giant boulders have puzzled geologists for years. What forces could have torn these rocks from the cliff edges high above sea level and deposited them far inland? While some researchers contend that only a tsunami could push these stones, new research finds that plain old ocean waves, with the help of some strong storms, did the job.

Global warming refuge discovered near at-risk Pacific island nation of Kiribati

Scientists predict ocean temperatures will rise in the equatorial Pacific by the end of the century, wreaking havoc on coral reef ecosystems. But a new study shows that climate change could cause ocean currents to operate in a way that mitigates warming near a handful of islands right on the equator.

Pacific islands on equator may become refuge for corals in a warming climate due to changes in ocean currents

Scientists have predicted that ocean temperatures will rise in the equatorial Pacific by the end of the century, wreaking havoc on coral reef ecosystems. But a new study shows that climate change could cause ocean currents to operate in a surprising way and mitigate the warming near a handful of islands right on the equator. As a result these Pacific islands may become isolated refuges for corals and fish.

Scientists provide first large-scale estimate of reef shark losses in the Pacific Ocean

First study to provide estimates of reef shark losses in the Pacific Ocean are sobering. Researchers noted the enormous detrimental effect that humans have on reef sharks.