Oceanic Currents

Submitted To: Sir. Salman Tariq

Atiqa Ijaz Khan Jan_2013 Weather Forecasting

Semester 7th

Session: 2009-2013

Department of Space Sciences

University of the Punjab

Ocean Currents by Atiqa Page 1

Chapter 1 Welcome to Oceanic Currents 02

Tidal Currents 02

Waves Currents 03

Long-shore Currents 04

Rip Currents 04

Upwelling Currents 05

The Coriolis Force 06

The Ekman Spiral 07

Thermo-haline Movement 07

The Global Conveyer Belt 08

Oceanic Gyres 11

Types of Oceanic Currents 12

Chapter 2 Overview of El Nino and La Nina 14

El Nino 15

La Nina 17

Cold and Warm Episodes by Seasons 18

Chapter 3 References 22

Ocean Currents by Atiqa Page 2

Welcome to Currents

When used in association with water, the term "current" describes the motion of the water.

Some currents you may be familiar with are the motion of rainwater as it flows down the

street, or the motion of the water in a creek, stream, or river flowing from higher elevation

to lower elevation. This motion is caused by gravity. The speed and direction (velocity) of

currents can be measured and recorded.

Tidal currents

They occur in conjunction with the rise and fall of the tide. The vertical motion of the tides

near the shore causes the water to move horizontally, creating currents. When a tidal

current moves toward the land and away from the sea, it “floods.” When it moves toward

the sea away from the land, it “ebbs.” These tidal currents that ebb and flood in opposite

directions are called “rectilinear” or “reversing” currents.

Tidal currents are the only type of current affected by the interactions of the Earth, sun,

and moon. The moon’s force is much greater than that of the sun because it is 389 times

closer to the Earth than the sun is. Tidal currents, just like tides, are affected by the different

phases of the moon. When the moon is at full or new phases, tidal current velocities are

strong and are called “spring currents.” When the moon is at first or third quarter phases,

tidal current velocities are weak and are called “neap currents.”

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Waves

Coastal currents are intricately tied to winds, waves, and land formations. Winds that

blow along the shoreline—long shore winds—affect waves and, therefore, currents.

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Long-shore Currents

The speed at which waves approach the shore depends on sea floor and shoreline

features and the depth of the water. As a wave moves toward the beach, different

segments of the wave encounter the beach before others, which slows these segments

down. As a result, the wave tends to bend and conform to the general shape of the

coastline. Also, waves do not typically reach the beach perfectly parallel to the

shoreline. Rather, they arrive at a slight angle, called the “angle of wave approach.”

Long-shore currents are generated when a "train" of waves reach the coastline and release

bursts of energy.

Rip Currents

As long-shore currents move on and off the beach,

“rip currents” may form around low spots or breaks

in sandbars, and also near structures such as jetties

and piers. A rip current, sometimes incorrectly called

a rip tide, is a localized current that flows away from

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the shoreline toward the ocean, perpendicular or at an acute angle to the shoreline. It

usually breaks up not far from shore and is generally not more than 25 meters (80 feet)

wide.

Upwelling

Winds blowing across the ocean surface often push water away from an area. When this

occurs, water rises up from beneath the surface to replace the diverging surface water.

This process is known as “upwelling.”

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The Coriolis Effect

Coastal currents are affected by local winds. Surface ocean currents, which occur on the

open ocean, are driven by a complex global wind system. To understand the effects of

winds on ocean currents, one first needs to understand the Coriolis force and the Ekman

spiral.

If the Earth did not rotate on its axis, the atmosphere would only circulate between the

poles and the equator in a simple back-and-forth pattern. Because the Earth rotates on its

axis, circulating air is deflected toward the right in the Northern Hemisphere and toward

the left in the Southern Hemisphere.

This deflection is called the Coriolis Effect.

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The Ekman Spiral

The Ekman spiral, named after Swedish scientist Vagn Walfrid Ekman (1874-1954) who first

theorized it in 1902, is a consequence of the Coriolis Effect. When surface water molecules

move by the force of the wind, they, in turn, drag deeper layers of water molecules below

them. Each layer of water molecules is moved by friction from the shallower layer, and

each deeper layer moves more slowly than the layer above it, until the movement ceases

at a depth of about 100 meters (330 feet). Like the surface water, however, the deeper

water is deflected by the Coriolis Effect—to the right in the Northern Hemisphere and to

the left in the Southern Hemisphere. As a result, each successively deeper layer of water

moves more slowly to the right or left, creating a spiral effect. Because the deeper layers

of water move more slowly than the shallower layers, they tend to “twist around” and

flow opposite to the surface current.

Thermo-haline circulation

Winds drive ocean currents in the upper 100 meters of the ocean’s surface. However, ocean

currents also flow thousands of meters below the surface. These deep-ocean currents are

driven by differences in the water’s density, which is controlled by temperature (thermo)

and salinity (haline). This process is known as thermo-haline circulation.

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Ocean Currents by Atiqa Page 9

Global Conveyor Belt

Thermo-haline circulation drives a global-scale system of currents called the “global

conveyor belt.” The conveyor belt begins on the surface of the ocean near the pole in the

North Atlantic. Here, the water is chilled by arctic temperatures. It also gets saltier because

when sea ice forms, the salt does not freeze and is left behind in the surrounding water.

The cold water is now denser, due to the added salts, and sinks toward the ocean bottom.

Surface water moves in to replace the sinking water, thus creating a current.

This deep water moves south, between the continents, past the equator, and down to the

ends of Africa and South America. The current travels around the edge of Antarctica, where

the water cools and sinks again, as it does in the North Atlantic. Thus, the conveyor belt

gets "recharged." As it moves around Antarctica, two sections split off the conveyor and

turn northward. One section moves into the Indian Ocean, the other into the Pacific

Ocean.

These two sections that split off warm up and become less dense as they travel northward

toward the equator, so that they rise to the surface (upwelling). They then loop back

southward and westward to the South Atlantic, eventually returning to the North Atlantic,

where the cycle begins again.

The conveyor belt moves at much slower speeds (a few centimeters per second) than wind-

driven or tidal currents (tens to hundreds of centimeters per second). It is estimated that

any given cubic meter of water takes about 1,000 years to complete the journey along the

global conveyor belt. In addition, the conveyor moves an immense volume of water—

more than 100 times the flow of the Amazon River (Ross, 1995).

The conveyor belt is also a vital component of the global ocean nutrient and carbon

dioxide cycles. Warm surface waters are depleted of nutrients and carbon dioxide, but they

are enriched again as they travel through the conveyor belt as deep or bottom layers. The

Ocean Currents by Atiqa Page 10

base of the world’s food chain depends on the cool, nutrient-rich waters that support the

growth of algae and seaweed.

Cold, salty, dense water sinks at the Earth's northern polar region and heads south along the western Atlantic

basin.

The current is "recharged" as it travels along the coast of Antarctica and picks up more cold, salty, dense

water.

The main current splits into two sections, one traveling northward into the Indian Ocean, while the other

heads up into the western Pacific.

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The two branches of the current warm and rise as they travel northward, then loop back around

southward and westward.

The now-warmed surface waters continue circulating around the globe. They eventually return to the

North Atlantic where the cycle begins again.

Oceanic Gyres

Gyres are usually bounded by the shallow waters of continental shelves. There are five

major gyres in the world's oceans, which are delimited by the continents around them.

Ocean Currents by Atiqa Page 12

These gyres are responsible for much of the world's surface currents. As you can see in the

map above, much of the eastern coast of Africa has a current going from north to south,

part of the Indian Ocean Gyre. This current was a great problem to early European

navigators, trying to go around the Cape of Good Hope (the southern tip of Africa) to

find a trade route to India. Early sailing ships tended to hug the coast, where the currents

are strongest, and they didn't have a lot of motive power in the days of sail. Even today,

ships use these currents to save fuel, since making way against the current is costly. Debris

floating in the ocean also tends to converge in certain zones because of these currents. The

North Atlantic Garbage Patch and the Great Pacific Garbage Patch are places where a lot

of trash dumped into the oceans has aggregated.

Types of Ocean Currents

The ocean currents may be classified based on their depth as surface currents and deep

water currents:

(i) Surface currents constitute about 10 per cent of all the water in the ocean, these

waters are the upper 400 m of the ocean;

(ii) Deep water currents make up the other 90 per cent of the ocean water. These

waters move around the ocean basins due to variations in the density and gravity. Deep

waters sink into the deep ocean basins at high latitudes, where the temperatures are cold

enough to cause the density to increase.

Ocean currents can also be classified based on temperature:

(i) Cold currents bring cold water into warm water areas. These currents are usually

found on the west coast of the continents in the low and middle latitudes (true in

both hemispheres) and on the east coast in the higher latitudes in the Northern

Hemisphere;

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(ii) Warm currents bring warm water into cold water areas and are usually observed

on the east coast of continents in the low and middle latitudes (true in both

hemispheres). In the northern hemisphere they are found on the west coasts of

continents in high latitudes.

Overview of El Niño and La Niña

El Niño and La Niña are complex weather patterns resulting from variations in ocean

temperatures in the Equatorial Pacific.

El Niño, warmer than average waters in the Eastern equatorial Pacific (shown in orange on the map),

affects weather around the world.

El Niño and La Niña are opposite phases of what is known as the El Niño-Southern

Oscillation (ENSO) cycle. The ENSO cycle is a scientific term that describes the fluctuations

in temperature between the ocean and atmosphere in the east-central Equatorial

Pacific (approximately between the International Date Line and 120 degrees West).

Ocean Currents by Atiqa Page 14

La Niña is sometimes referred to as the cold phase of ENSO and El Niño as the warm

phase of ENSO. These deviations from normal surface temperatures can have large-scale

impacts not only on ocean processes, but also on global weather and climate.

El Niño and La Niña episodes typically last nine to 12 months, but some prolonged events

may last for years. They often begin to form between June and August, reach peak strength

between December and April, and then decay between May and July of the following

year. While their periodicity can be quite irregular, El Niño and La Niña events occur about

every three to five years. Typically, El Niño occurs more frequently than La Niña.

La Niña (December 2000) El Niño (December 1997)

Sea surface temperature anomalies (°C)

El Niño

El Niño means The Little Boy, or Christ Child in Spanish. El Niño was originally

recognized by fishermen off the coast of South America in the 1600s, with the appearance

of unusually warm water in the Pacific Ocean. The name was chosen based on the time of

year (around December) during which these warm waters events tended to occur.

The term El Niño refers to the large-scale ocean-atmosphere climate interaction linked to

a periodic warming in sea surface temperatures across the central and east-central

Equatorial Pacific.

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Typical El Niño effects are likely to develop over North America during the upcoming

winter season. Those include warmer-than-average temperatures over western and central

Canada, and over the western and

northern United States. Wetter-than-

average conditions are likely over

portions of the U.S. Gulf Coast and

Florida, while drier- than-average

conditions can be expected in the

Ohio Valley and the Pacific Northwest.

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La Niña

La Niña means The Little Girl in Spanish. La Niña is also sometimes called El Viejo, anti-

El Niño, or simply "a cold event."

La Niña episodes represent periods of below-average sea surface temperatures across the

east-central Equatorial Pacific. Global climate La Niña impacts tend to be opposite those

of El Niño impacts. In the tropics, ocean temperature variations in La Niña also tend to be

opposite those of El Niño.

During a La Niña year, winter temperatures are warmer than normal in the Southeast and

cooler than normal in the Northwest.

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Notice: Because of the high frequency filter applied to the ERSSTv3b data

(Smith et al. 2008, J.Climate), ONI values may change up to two months after

the initial "real time" value is posted. Therefore, the most recent ONI values

should be considered an estimate.

DESCRIPTION: Warm (red) and cold (blue) episodes based on a threshold of

+/- 0.5oC for the Oceanic Niño Index (ONI) [3 month running mean of

ERSST.v3b SST anomalies in the Niño 3.4 region (5oN-5

oS, 120

o-170

oW)], based

on centered 30-year base periods updated every 5 years. For historical purposes

cold and warm episodes (blue and red colored numbers) are defined when the

threshold is met for a minimum of 5 consecutive over-lapping seasons.

Year DJF JFM FMA MAM AMJ MJJ JJA JAS ASO SON OND NDJ

1950 -1.4 -1.3 -1.2 -1.2 -1.1 -

0.9

-

0.6

-

0.5

-0.4 -0.5 -0.6 -0.7

1951 -

0.8

-0.6 -0.4 -0.2 0.0 0.4 0.6 1.0 1.1 1.2 1.1 0.9

1952 0.6 0.4 0.3 0.3 0.3 0.1 -

0.1

0.0 0.2 0.2 0.2 0.3

1953 0.5 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.8 0.8 0.8 0.8

1954 0.7 0.5 0.1 -0.4 -0.5 -

0.5

-

0.6

-

0.7

-0.8 -0.7 -0.7 -0.7

1955 -

0.7

-0.7 -0.7 -0.8 -0.8 -

0.8

-

0.8

-

0.7

-1.1 -1.4 -1.7 -1.6

Ocean Currents by Atiqa Page 18

1956 -1.1 -0.8 -0.6 -0.5 -0.5 -

0.5

-

0.5

-

0.6

-0.5 -0.5 -0.5 -0.5

1957 -

0.3

0.1 0.4 0.7 0.9 1.0 1.1 1.2 1.2 1.3 1.5 1.8

1958 1.8 1.6 1.2 0.9 0.7 0.6 0.5 0.3 0.3 0.4 0.5 0.6

1959 0.6 0.6 0.5 0.3 0.2 -0.1 -

0.2

-

0.3

-0.1 0.0 0.1 0.0

1960 -0.1 -0.2 -0.2 -0.1 -0.1 0.0 0.1 0.2 0.2 0.1 0.1 0.1

1961 0.0 0.0 0.0 0.1 0.3 0.4 0.2 -

0.1

-0.3 -0.3 -0.2 -0.1

1962 -

0.2

-0.3 -0.3 -0.3 -0.2 -

0.2

0.0 -

0.1

-0.2 -0.3 -0.4 -0.5

1963 -

0.4

-0.2 0.1 0.3 0.3 0.5 0.8 1.1 1.2 1.3 1.4 1.3

1964 1.1 0.6 0.1 -0.4 -0.6 -

0.6

-

0.6

-

0.7

-0.8 -0.8 -0.8 -0.8

1965 -

0.6

-0.3 0.0 0.2 0.5 0.8 1.2 1.5 1.7 1.9 1.9 1.7

1966 1.4 1.1 0.9 0.6 0.4 0.3 0.3 0.1 0.0 -0.1 -0.1 -0.2

1967 -

0.3

-0.4 -0.5 -0.4 -0.2 0.1 0.1 -

0.1

-0.3 -0.3 -0.3 -0.4

1968 -

0.6

-0.8 -0.7 -0.5 -0.2 0.1 0.4 0.5 0.5 0.6 0.8 1.0

1969 1.1 1.1 1.0 0.9 0.8 0.6 0.5 0.5 0.8 0.9 0.9 0.8

1970 0.6 0.4 0.4 0.3 0.1 -

0.2

-

0.5

-

0.7

-0.7 -0.7 -0.8 -1.0

Ocean Currents by Atiqa Page 19

1971 -1.2 -1.3 -1.1 -0.8 -0.7 -

0.7

-

0.7

-

0.7

-0.7 -0.8 -0.9 -0.8

1972 -

0.6

-0.3 0.1 0.4 0.6 0.8 1.1 1.4 1.6 1.9 2.1 2.1

1973 1.8 1.2 0.6 -0.1 -0.5 -

0.8

-

1.0

-

1.2

-1.3 -1.6 -1.9 -2.0

1974 -1.9 -1.6 -1.2 -1.0 -0.8 -

0.7

-

0.5

-

0.4

-0.4 -0.6 -0.8 -0.7

1975 -

0.5

-0.5 -0.6 -0.7 -0.8 -1.0 -1.1 -

1.2

-1.4 -1.5 -1.6 -1.7

1976 -1.5 -1.1 -0.7 -0.5 -0.3 -0.1 0.2 0.4 0.6 0.7 0.8 0.8

1977 0.6 0.6 0.3 0.3 0.3 0.4 0.4 0.4 0.5 0.7 0.8 0.8

1978 0.7 0.5 0.1 -0.2 -0.3 -

0.3

-

0.3

-

0.4

-0.4 -0.3 -0.1 -0.1

1979 -0.1 0.1 0.2 0.3 0.2 0.0 0.0 0.2 0.3 0.5 0.5 0.6

1980 0.5 0.4 0.3 0.3 0.4 0.4 0.3 0.1 -0.1 0.0 0.0 -0.1

1981 -

0.4

-0.6 -0.5 -0.4 -0.3 -

0.3

-

0.4

-

0.4

-0.3 -0.2 -0.2 -0.1

1982 -0.1 0.0 0.1 0.3 0.5 0.7 0.7 1.0 1.5 1.9 2.1 2.2

1983 2.2 1.9 1.5 1.2 0.9 0.6 0.2 -

0.2

-0.5 -0.8 -0.9 -0.8

1984 -

0.5

-0.3 -0.3 -0.4 -0.5 -

0.5

-

0.3

-

0.2

-0.3 -0.6 -0.9 -1.1

1985 -1.0 -0.9 -0.7 -0.7 -0.7 -

0.6

-

0.5

-

0.5

-0.5 -0.4 -0.4 -0.4

Ocean Currents by Atiqa Page 20

1986 -

0.5

-0.4 -0.2 -0.2 -0.1 0.0 0.3 0.5 0.7 0.9 1.1 1.2

1987 1.2 1.3 1.2 1.1 1.0 1.2 1.4 1.6 1.6 1.5 1.3 1.1

1988 0.8 0.5 0.1 -0.2 -0.8 -1.2 -

1.3

-

1.2

-1.3 -1.6 -1.9 -1.9

1989 -1.7 -1.5 -1.1 -0.8 -0.6 -

0.4

-

0.3

-

0.3

-0.3 -0.3 -0.2 -0.1

1990 0.1 0.2 0.3 0.3 0.2 0.2 0.3 0.3 0.4 0.3 0.4 0.4

1991 0.3 0.2 0.2 0.3 0.5 0.7 0.8 0.7 0.7 0.8 1.2 1.4

1992 1.6 1.5 1.4 1.2 1.0 0.7 0.3 0.0 -0.2 -0.3 -0.2 0.0

1993 0.2 0.3 0.5 0.6 0.6 0.5 0.3 0.2 0.2 0.2 0.1 0.1

1994 0.1 0.1 0.2 0.3 0.4 0.4 0.4 0.4 0.5 0.7 1.0 1.2

1995 1.0 0.8 0.6 0.3 0.2 0.0 -

0.2

-

0.4

-0.7 -0.8 -0.9 -0.9

1996 -

0.9

-0.8 -0.6 -0.4 -0.3 -

0.2

-

0.2

-

0.3

-0.3 -0.3 -0.4 -0.5

1997 -

0.5

-0.4 -0.1 0.2 0.7 1.2 1.5 1.8 2.1 2.3 2.4 2.3

1998 2.2 1.8 1.4 0.9 0.4 -

0.2

-

0.7

-

1.0

-1.2 -1.3 -1.4 -1.5

1999 -1.5 -1.3 -1.0 -0.9 -0.9 -1.0 -

1.0

-1.1 -1.1 -1.3 -1.5 -1.7

2000 -1.7 -1.5 -1.2 -0.9 -0.8 -

0.7

-

0.6

-

0.5

-0.6 -0.6 -0.8 -0.8

2001 -

0.7

-0.6 -0.5 -0.4 -0.2 -0.1 0.0 0.0 -0.1 -0.2 -0.3 -0.3

Ocean Currents by Atiqa Page 21

2002 -

0.2

0.0 0.1 0.3 0.5 0.7 0.8 0.8 0.9 1.2 1.3 1.3

2003 1.1 0.8 0.4 0.0 -0.2 -0.1 0.2 0.4 0.4 0.4 0.4 0.3

2004 0.3 0.2 0.1 0.1 0.2 0.3 0.5 0.7 0.8 0.7 0.7 0.7

2005 0.6 0.4 0.3 0.3 0.3 0.3 0.2 0.1 0.0 -0.2 -0.5 -0.8

2006 -

0.9

-0.7 -0.5 -0.3 0.0 0.1 0.2 0.3 0.5 0.8 1.0 1.0

2007 0.7 0.3 -0.1 -0.2 -0.3 -

0.3

-

0.4

-

0.6

-0.8 -1.1 -1.2 -1.4

2008 -1.5 -1.5 -1.2 -0.9 -0.7 -

0.5

-

0.3

-

0.2

-0.1 -0.2 -0.5 -0.7

2009 -

0.8

-0.7 -0.5 -0.2 0.2 0.4 0.5 0.6 0.8 1.1 1.4 1.6

2010 1.6 1.3 1.0 0.6 0.1 -

0.4

-

0.9

-

1.2

-1.4 -1.5 -1.5 -1.5

2011 -1.4 -1.2 -0.9 -0.6 -0.3 -

0.2

-

0.2

-

0.4

-0.6 -0.8 -1.0 -1.0

2012 -

0.9

-0.6 -0.5 -0.3 -0.2 0.0 0.1 0.4 0.5 0.6 0.2 -0.3

2013 -

0.6

-0.6 -0.4 -0.2 -0.2 -

0.3

-

0.3

-

0.3

-0.3 -0.2 -0.3 -0.4

2014 -

0.6

-0.6 -0.5 -0.2

Ocean Currents by Atiqa Page 22

References:

http://oceanservice.noaa.gov/education/kits/currents/lessons/currents_tutorial.pdf

http://oceanservice.noaa.gov/facts/ninonina.html

http://earthobservatory.nasa.gov/Features/LaNina/

http://kids.earth.nasa.gov/archive/nino/intro.html

http://www.elnino.noaa.gov/lanina.html

http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml