|Large masses of moving water are called currents. In the oceans there are major surface currents, subsurface currents, and tidal currents. Each of these mass movements of seawater is slightly different and will be treated in this lesson. Local areas have more complicated current patterns but the global currents are rather easily explained.|
Earth's atmospheric wind cells due to the differential heating and cooling of the atmosphere by the sun. (LJF adapted from NASA image)
|Winds are the primary force causing seawater movement at the surface of the ocean. These surface winds are responsible for the major ocean currents and waves. The causes of the winds are almost completely due to the energy from the sun in the form of heat. As the sun heats the air it becomes less dense and rises. Since the greatest amount of heat is centered at the equator there is a large mass of rising air there. As this heated air rises it cools and spreads out near the top of our atmosphere. Eventually it becomes cool enough to fall at about 30 degrees both north and south of the equator. The falling air hits the surface of Earth and spreads out - some air moving back toward the equator and some air moving toward 60 degrees. The air at the poles is very cold and dense relative to the surrounding air masses so it is sinking and spreading out across the surface of Earth on its way toward 60 degrees. The air at 60 degrees is warm enough (in comparison to the polar cold air) to rise, cool, and spread out in a similar fashion to the equatorial air. This patten, due to the temperature of the air, creates a 3-cell wind pattern on Earth - the circulating cells near the equator are called Hadley cells, the temperate cells are called Ferrel cells and the cells surrounding the poles are called Polar cells. The southern hemisphere is a mirror image - also with the three cells.|
The Coriolis effect is caused by the rotation of Earth. (LJF adapted from NASA image)
|Earth's surface winds are influenced by the rotation of Earth and the Coriolis effect. As Earth rotates the surface of Earth moves under the three circulating cells (Hadley, Ferrel, and Polar) causing a drag on the surface wind. This drag makes moving masses veer right in northern hemisphere and left in southern hemisphere in relation to the surface of Earth. Imagine a giant at the North Pole who prepares to jump to an island in the middle of the Pacific Ocean (say Hawaii). Now, imagine yourself, standing in Hawaii, viewing this giant as it leaps into the air and flys toward you. You see the giant coming right toward you at first but the Earth is rotating and you move with the Earth toward the east. As you watch the giant it appears to you that the giant veers to its right and falls into the ocean (missing Hawaii) - all due to the rotation of Earth. This effect is the Coriolis effect (in simple form) and affects all moving masses going over long distances.|
Earth's surface winds are the result of the wind cells and Coriolis effect. (LJF adapted from NASA image)
|The Trades, Westerlies and Polar Easterlies are Earth's major winds. These winds result from the circulating cells and the Coriolis effect. The Trades (0-30 degrees North and South) are an easterly wind and blow from east to west. (Winds are named for the direction from which they come rather than the direction that they go ... the opposite naming system to the way we name currents.) The Westerlies (30-60 degrees North and South) are a westerly wind, blowing from west to east. The Polar Easterlies (60-90 degrees North and South) are an easterly wind, blowing from east to west. Major weather patterns are moved around Earth by these large scale wind systems.|
Earth's surface ocean currents are caused by the winds, continental land mass obstruction and the Coriolis effect. (NOAA image edited by LJF and GA)
|The major ocean currents result from the winds, presence of continental land masses, and Coriolis effect. The largest surface area subjected to surface winds is around the equator where the water is moved from east to west by the Trades. As this water hits a continental mass, on the western side of the ocean, it piles up and flows right (north) in the northern hemisphere and left (south) in the southern hemisphere due to the Coriolis effect. The warm tropical water is thus moved across the oceans from east to west and divided by the continents where it flows toward the poles and begins cooling. Polar water is pulled toward the equator on the eastern sides of the oceans to replace the moving tropical water. This creates the major surface ocean currents which are clockwise gyres in the northern hemisphere and counterclockwise in the southern hemisphere.|
Western (example = Gulf Stream) and eastern (example = California Current) boundary currents are quite different from the Sargasso Sea and the West Wind Drift. (NOAA image edited by LJF and GA)
|Western Boundary Currents (on western sides of oceans) go north in northern hemisphere and south in southern hemisphere. They are rather narrow, deep, fast, and warm. A good example of a Western Boundary Current is the Gulf Stream in the Atlantic Ocean off the East Coast of the USA.|
Eastern Boundary Currents (on eastern sides of oceans) go south in northern hemisphere and north in the southern hemisphere. These are rather wide, shallow, slow and cold. A good example of an Eastern Boundary Current is the California Current in Pacific Ocean off the West Coast of the USA.
The center of the gyre may be calm like in the Sargasso Sea (*) in the North Atlantic Ocean (where the water circulates round and round)or it may have lateral currents from winds like in the North Pacific. These centers are generally saltier than other areas with more evaporation and less mixing.
Only one current, the West Wind Drift, goes around Earth completely uninterrupted by continents. This current surrounds Antarctica.
Bottom water results from the sinking of polar water (blue = Antarctic, orange = Arctic) and their densities. A general stratification of the Pacific Ocean (left), Atlantic Ocean (middle), and Indian Ocean (right) shows their differences. (GA images)
|Major subsurface currents in the oceans are most often due to differences in the density of water masses. A slow subsurface circulation of water develops with the sinking of cold water at the poles and its creeping across the ocean bottom with the meeting of north polar water and south polar water. There is a layering (due to density) near the equator. This is called thermohaline circulation (due to density differences in seawater caused by temperature and salinity) and some scientists predict it takes around 400 years for the water to complete this cycle. In the Atlantic ocean the Antarctic bottom water is denser than the North Atlantic bottom water and may creep up to 35 degrees north on the bottom. In the Pacific Ocean the North Pacific bottom water is denser and creeps down to nearly 15 degrees south latitude on the bottom. Each water mass has its own signature salinity, temperature and density.|
Major upwelling areas of the world. (NOAA image edited by GA)
|Upwelling is a unique subsurface current that is actually a vertical current, bringing nutrient rich water to the surface. This happens in areas where winds blow on the surface with a relatively strong force. Upwelling areas have high biological productivity as the nutrients enhance the food chain. Most upwelling areas are off the west coasts of continents or in the middle of the equatorial parts of oceans. Often these are seasonal areas and include examples like the equatorial area off Peru and the West Coast of the USA.|
Waves are caused by a disturbance to the surface of the ocean. (GA image)
|Waves are a more localized movement of seawater than currents. They are created by a disturbance to the surface of the ocean which could come from wind, an earthquake, or undersea landslide. Waves travel out in a circle (called wave trains) from the center of the disturbance. Each wave has a crest and a trough. The crest is as high above what was the flat, calm surface of the water as the trough is below that level. Waves are classified as to their wave period (how long it takes a single wave to pass a particular point).|
|Tides are another type of seawater movement which is caused by the gravitational attraction between Earth and the moon and sun.|
All bodies in space exert gravitational forces on each other that may change the surface of the body in space. We call these surface changes "tides" and they can be changes in large bodies of water or crust. (GA image left)
On Earth, we generally think about tides as a change in ocean water level but the tidal forces also deform the crust. The crust moves very little in comparison to large water masses so, from our human perspective, we do not even notice the crustal changes but we do notice the changes in water level relative to our shorelines.
The closer the bodies are together, the greater the force. The larger the body, the greater the force. All the bodies in space influence the tides on Earth however it is the forces of Moon and Sun that are the most important and determine our tidal cycles. The low tide pictured here is a result of both the Moon and the Sun's gravitational forces.
The closest body in space to Earth is the Moon. Because of its closeness, the Moon has the greatest effect of any body in space on the tides of Earth. It is the Moon's gravitational attraction that creates a bulge of water on Earth, directly under the Moon. This bulge is balanced by a bulge on the opposite side of Earth due to centrifugal forces. (NASA image left)
|To understand the tides that result from the Moon (lunar tides) it is easiest to envision Earth covered with a deep ocean from a north polar view. (LJF image left)|
|Then, add the gravitational forces of Moon and the centrifugal forces to create the two bulges of water. (LJF image left)|
|Every day (closer to 25 hours) Earth rotates once on its axis relative to Moon. A person on Earth would experience a high tide while directly under Moon, a low tide six hours later, another high tide while directly opposite Moon six hours later, and a final low tide six hours after that. (LJF image left)|
|Approximately every day there are two high tides and two low tides on the surface of Earth, or two tidal cycles (each tidal cycle is one high tide and one low tide). (In reality the diagrams presented here are out of proportion but serve to illustrate our understanding of tides.) (LJF image left)|
(LJF image above)
Most of Earth's tides are called mixed tides because the two high tides each day are rarely equal and the two low tides each day are also rarely equal. The previous illustration, on daily lunar tides, used a north polar view to illustrate the daily tidal cycles that we call lunar tides.
An equatorial view of Earth relative to Moon will serve to further explain the lunar tides. Although most areas on Earth experience two high and two low tides, their heights are "mixed" because of the declination of Earth relative to Moon. From this equatorial view you can see that as Earth rotates each latitude on Earth would, most likely, not be in an area of the high tide bulge under Moon that was equal to the bulge on the opposite side of Earth. For this reason the tidal cycle is generally referred to as semi-diurnal mixed tides —or, twice daily mixed tides.
(LJF adapted from NASA image above)
The highest of the high tides is called the "high high tide" and the lower high tide is the "low high tide". The lowest of the low tides is called the "low low tide" and the higher of the low tides is called the "high low tide". It is quite a tongue twister and most people have to pause a moment before using this terminology. It is easiest to remember that the word directly next to "tide" refers to it being a high or low tide; the word before that just describes whether it is the higher or lower.
|The Sun has a million mile diameter, yet it has only half the gravitational effect on the tides of Earth when compared to the Moon. This is because it is so far away. Sun's forces act in a similar fashion to Moon's in that a bulge occurs directly under Sun due to gravitational forces and another bulge occurs directly opposite due to centrifugal forces. These forces modify the lunar tides but do not change the fact that there are still two high and two low tides approximately every day. |
(LJF image above)
About every month, Moon moves around Earth once relative to Sun. This is diagrammed here along with the exaggerated tidal bulges. When Earth is between Moon and Sun we see the Moon as a full Moon. When Moon is between Sun and Earth we see the Moon as a new Moon. These phases of the Moon are two weeks apart and are times of extreme high tides and extreme low tides because the forces of Moon and Sun are combined (be they gravitational or centrifugal) to increase the tidal bulges.
All year long, every two weeks, there are days when the high high tides are above average and the low low tides are below average. These times of amplified tides are called spring tides. In fact, average low low tide is called zero sea level and during spring tide weeks the low low tide is often below this, or what we call a minus tide. Minus tides are the best time to go to the shoreline to view the rocky shore because the greatest intertidal area is exposed.
(LJF image above)
During first and third quarter Moon, in the weeks between new Moon and full Moon, the forces of Moon and Sun are not in line with each other. The forces of Sun are pulling at the low tide area of the forces of Moon. Moon's forces still dominate because they are twice as strong as Sun's forces but during these weeks the low tides are not as low and the high tides are not as high. Low low tide is rarely at zero sea level; it is usually a plus tide of a few feet or so. High high tide is not very high —perhaps only as high as average low high tide. These intervening weeks between the spring tide weeks are called neap tide weeks. A much smaller area in the intertidal is exposed during neap tide weeks. Spring and neap tides alternate every week all year long, with a week of spring tides then a week of neap tides and so on.
|Seawater movements are very complicated as you can see. Surface currents, subsurface currents, vertical currents (like upwelling), downwelling (where water is sinking, like near the poles), and tides all contribute to the mixing of the oceans of Earth. Although we know a lot about the general currents (as described in this lesson) each particular place on Earth has its own unique specific currents, many of these seasonal. It is fun to think that if you were a marine animal and knew all the currents in the oceans you could travel everywhere from top to bottom and to every continent and island.|
(Revised 6 August 2007)