Tidal currents are the rise and fall of the tide, accompanied by the horizontal flow of water.
The usual terms used to describe the direction of this horizontal movement are ebb and flood.
Ebb currents occur when tidal currents are moving away from the coast.
Flood currents move toward the coast.
In a purely semi-diurnal current, the flood and ebb each last about 6 hours.
Speed of tidal currents depends upon the shape and dimensions of the harbor, coastal areas and ocean bottom.
The configuration also influences vertical range of the tide itself.
Under certain conditions, tidal currents can move more than 10
Energy from the sun is the engine that drives the major ocean basin circulation patterns.
Rising warm air, sinking cold air, and uneven heating of the Earth’s surface create wind, the essential
energy component necessary to move water in a horizontal manner.
Other forces are involved
such as the gravitational pull of the sun and moon.
They have a particular profound influence on coastal waters where tidal ranges are large.
Whatever the force moving the waters, the ocean is in constant motion.
Most currents are persistent global water motions that transport large volumes of surface and subsurface water over vast distances.
They may be horizontal or
vertical, depending on their forcing mechanism.
Horizontal surface currents are
propelled by the frictional force of wind dragging the water.
The subsurface flow of
deep ocean water is called thermohaline circulations; arise from differences in density in seawater.
These sea-surface and deep-ocean currents continually keep the oceans in motion.
Some surface currents are transient and seasonal, a number of them flow with great persistence, setting up a circulation that continues with relatively little change throughout the year.
Because of the influence of wind in creating currents in the
surface-layer circulations of the ocean, there is a relationship between oceanic circulation and the general circulation of the atmosphere.
A notable feature of the
oceanic circulation is that it is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
Currents perform a chore that benefits the entire planet.
Oceans serve as vast heat reservoirs.
They store heat in the summer and release it during the winter.
Currents are the mass transit system that moves large amounts of heat, plus suspended solids and dissolved chemicals, between low and high latitudes, effectively moderating the world’s climate.
Major currents in the Northern Hemisphere include the Gulf Stream in the Atlantic and the Kuroshio Current in the Pacific.
The currents, called western boundary currents, are important links in this heat transfer.
Movement of deep, slow-moving ocean waters can be detected through analyses of temperature and salinity samples drawn from depths that are more than 15,000
These waters can be traced into deep basins of the Atlantic, Pacific, and Indian Oceans because of their unique physical characteristics.
It has been estimated that the dense waters move at a daily rate of
about 3 to 5 miles.
In deep ocean circulations, the differences in seawater density are controlled by variations in temperature and salinity.
The deeper waters are driven by the formation of new, cold dense water masses in polar and sub-
The densest (coldest) seawater
found in the Southern Hemisphere is formed on the continental shelf around Antarctica.
Its water is so cold, and therefore heavier than the surrounding water, that it flows down the
continental slopes of Antarctica, displacing less dense water, which is then caught up and carried around the southern oceans by the
Antarctic Circumpolar Current.
Longshore currents can be found on most beaches, but their strength is seasonally variable (stronger in winter).
They form when waves strike a beach at an angle.
As the wave front enters shallow water, the leading edge of the wave hits the shallow water sooner than the rest of the wave front and slows down, bending the wave as it moves ashore.
The shoreward movement of the wave thus forms a current whose net flow is parallel to the shore
in the surf zone.
The speed of the longshore current increases with increasing wave height, decreasing wave period, increasing angle of wave front to beach, and increasing beach slope.
Once established, the current moves at a speed of about one knot in the same direction as the advancing wave train.
Longshore currents are more prevalent along lengthy straight coastlines.
Sandbars often form in areas
where long shore currents frequently occur.
Longshore currents transport significant amounts of sand and sediment suspended by wave action in the surf zone along the shore.
When the current enters deeper water, forward momentum diminishes and the sediment settles to the bottom.
This can erode the beach in one
area and build it in another. U
Unfortunately a considerable amount of sediment is dumped
into shipping channels and harbours, which requires expensive dredging to remove.
Another consideration of longshore currents is the rip current, often called “rip tide”.
Rip currents are formed when longshore currents, moving parallel to the coastline, are deflected seaward by bottom irregularities, or meet another current deflecting the flow to seaward.
Development depends upon wave
Large incoming waves on a long, straight beach will produce rips.
Rip currents consist of feeders, a neck, and a head.
The feeder is usually the longshore
current that flows parallel to the beach inside the breakers.
The neck is the main channel of
the rip current where feeder currents converge and flow outwards at a speed of one to three
knots through a weak point in the breakers.
The head is where the current widens and slackens outside the breaker line.
A number of swimmers are lost every summer when caught up in rips and swept out to sea.
If trapped in this situation, swim parallel to the shoreline until out of the rip rather than swimming directly into the current, then swim back to shore.