Why ocean current flows?

Ocean Water flows in a predictable direction driven by:

 Earth rotation
 Wind flow
 Seawater salt density
 Salinity and pressure
 Rate of evaporation
 Amount of precipitation
 Coriolis force
 and temperature

Tides contribute to coastal currents that travel short distances. Major surface ocean currents
in the open ocean, however, are set in motion by the wind, which drags on the surface of the
water as it blows. The water starts flowing in the same direction as the wind.

But currents do not simply track the wind. Other things, including the shape of
the coastline and the seafloor, and most importantly the rotation of the Earth, influence the
path of surface currents.

In the Northern Hemisphere, for example, predictable winds called trade winds blow from
east to west just above the equator. The winds pull surface water with them, creating currents.
As these currents flow westward, the Coriolis effect—a force that results from the rotation of
the Earth—deflects them. The currents then bend to the right, heading north. At about 30
degrees north latitude, a different set of winds, the westerlies, push the currents back to the
east, producing a closed clockwise loop.

The same thing happens below the equator, in the Southern Hemisphere, except that here the
Coriolis effect bends surface currents to the left, producing a counter-clockwise loop.

Large rotating currents that start near the equator are called subtropical gyres. There are five
main gyres: the North and South Pacific Subtropical Gyres, the North and South Atlantic
Subtropical Gyres, and the Indian Ocean Subtropical Gyre.

These surface currents play an important role in moderating climate by transferring heat from
the equator towards the poles. Subtropical gyres are also responsible for concentrating plastic
trash in certain areas of the ocean.

In contrast to wind-driven surface currents, deep-ocean currents are caused by differences in

water density. The process that creates deep currents is called thermohaline circulation
—“thermo” referring to temperature and “haline” to saltiness.

It all starts with surface currents carrying warm water north from the equator. The water cools
as it moves into higher northern latitudes, and the more it cools, the denser it becomes.

In the North Atlantic Ocean, near Iceland, the water becomes so cold that sea ice starts to
form. The salt naturally present in seawater does not become part of the ice, however. It is
left behind in the ocean water that lies just under the ice, making that water extra salty and
dense. The denser water sinks, and as it does, more ocean water moves in to fill the space it
once occupied. This water also cools and sinks, keeping a deep current in motion.

This is the start of what scientists call the “global conveyor belt,” a system of connected deep
and surface currents that moves water around the globe. These currents circulate around the
globe in a thousand-year cycle.

Ocean current:
Ocean currents are the continuous, predictable, directional movement of seawater driven by
gravity, wind (Coriolis Effect), and water density. Ocean water moves in two directions:
horizontally and vertically. Horizontal movements are referred to as currents, while vertical
changes are called upwellings or downwellings. This abiotic system is responsible for the
transfer of heat, variations in biodiversity, and Earth’s climate system. Currents generally
diminish in intensity with increasing depth.

Ocean currents are similar to winds in the atmosphere in that they transfer significant
amounts of heat from Earth’s equatorial areas to the poles and thus play important roles in
determining the climates of coastal regions. In addition, ocean currents and atmospheric
circulation influence one another. Ocean currents are one of the factors that affect the
temperature of ocean water.

 Warm ocean currents raise the temperature in cold areas

 Cold ocean currents decrease the temperature in warmer areas.

Types:
Ocean currents play an important role in regulating the climate around the world. There are
two main types of ocean currents:

1. Surface Currents--Surface Circulation

•2. Deep Water Currents--Thermohaline Circulation

1.Surface Currents: Surface Currents are driven by Global wind systems due to solar
energy propel large-scale surface ocean currents. These currents affect local and planetary
climate by transferring heat from the tropics to the poles.
•
These waters make up about 10% of all the water in the ocean.
•
These waters are the upper 400 meters of the ocean.

2.Deep Water Currents: Ocean currents are also influenced by fluctuations in water
density brought on by changes in thermal and halo static pressure. This is known as
Thermohaline circulation. It adds to the 90 percent of ocean water that is left. These waters
circulate around the basins of the oceans because of variations in density and gravity. When
temperatures are cold enough to enhance density, deep waters drop into deep ocean basins
at high latitudes. This starts the global conveyor belt, a connected network of deep and
surface currents that travels around the globe over a period of a thousand years. The Earth’s
climate system, as well as the carbon dioxide and nutrient cycles in the ocean, are greatly
impacted by this global network of ocean currents.

•
The density difference is a function of different temperatures and salinity
•
These deep waters sink into the deep ocean basins at high latitudes where the
temperatures are cold enough to cause the density to increase.

Based on Temperature, It can be classified into 2 types:

1. Cold Currents: These currents transport cold water into places with warm
water. In both hemispheres, these currents are commonly found on the west
coastlines of continents in low and intermediate latitudes and on the east coast in
higher latitudes.
2. Warm Currents: Found frequently on the east coasts of continents in low- and
medium-latitude regions, these currents transport warm water into cold-water
regions. They can be found in the northern hemisphere at high latitudes on the
west coasts of continents.
Types of forces:
1.Primary Forces--start the water moving
The primary forces are: 1. Solar Heating
2. Winds
3. Gravity
4. Coriolis
2.Secondary Forces--influence where the currents flow

Cause of Surface Circulation

 Solar heatingcause water to expand. Near the equator the water is about 8 centimeters
high than in middle latitudes. This cause a very slight slope and water wants to flow down the
slope.
 Windsblowing on the surface of the ocean push the water. Friction is the coupling
between the wind and the water's surface.
 A wind blowing for 10 hours across the ocean will cause the surface waters to flow at
about 2% of the wind speed.
 Water will pile up in the direction the wind is blowing

 Gravity will tend to pull the water down the "hill" or pile of water against the pressure
gradient.
 But the Coriolis Force intervenes and cause the water to move to the right (in the
northern hemisphere) around the mound of water.
 These large mounds of water and the flow around them are calledGyres. The produce
large circular currents in all the ocean basins
Coriolis effect

Major Oceanic Gyres and Sea Currents

Gyre:

An ocean gyre is a large system of circular ocean currents formed by global wind patterns
and forces created by Earth’s rotation.

The movement of the world’s major ocean gyres helps drive the “ocean conveyor belt.” The
ocean conveyor belt circulates ocean water around the entire planet. Also known
as thermohaline circulation, the ocean conveyor belt is essential for
regulating temperature, salinity and nutrient flow throughout the ocean.

How a Gyre Forms

Gyres are formed by three forces: global wind patterns, Earth's rotation, and Earth's
landmasses. Wind drags on ocean surface, causing water to move in the direction of the wind.
Earth's rotation deflects these wind-driven currents, resulting in the Coriolis effect. This
results in an Ekman spiral pattern beneath surface currents, descending about 100 meters.
Earth's continents and landmasses also influence the creation of ocean gyres, with the South
Pacific Gyre being the largest, bounded by Australia, South America, and the Equator.
Most of the world’s major gyres are subtropical gyres. These form between the polar and
equatorial regions of Earth. Subtropical gyres circle areas beneath regions of high
atmospheric pressure.

North Atlantic gyres:

Gyres are comprised of ocean currents that link up as they follow the coastlines of Earth’s
continents. Each gyre has a powerful western boundary current and a weaker eastern
boundary current.

The North Atlantic Gyre begins with the northward flow of the Gulf Stream along the East
Coast of the United States. The Gulf Stream is the western boundary current of the gyre. The
gyre then becomes the North Atlantic Current, which flows across the North Atlantic to
Europe. Still flowing in a circular pattern, the current flows south as far as the northwestern
coast of Africa, where it is known as the Canary Current—the gyre’s eastern boundary
current. The gyre is completed as the North Atlantic Equatorial Current crosses the Atlantic
Ocean to the Caribbean Sea. This entire circle and the water within it is the North Atlantic
Gyre.

Most ocean gyres are very stable and predictable. The North Atlantic Ocean Gyre always
flows in a steady, clockwise path around the North Atlantic Ocean. Some gyres
experience seasonal variation, however.

The Indian Ocean Gyre is a complex system of many currents extending from the eastern
coast of Africa to the western coast of Australia. The northern part of the system circulates
between the Horn of Africa and the Indonesian archipelago. It is sometimes called the
Indian monsoon current.

The Indian monsoon current takes its name from the wind—the monsoon—that drives it. It is
one of the very few currents in an ocean gyre that change direction. In the summer, the
current flows clockwise, as the monsoon blows in from the southwestern Indian Ocean. In the
winter, the current flows counterclockwise, as the wind blows in from the Tibetan plateau in
the northeast.

The temperature in an ocean gyre depends on many factors, including the current. The Gulf
Stream and summer monsoon current are warm currents. They are heated by the warm
tropical waters of the Caribbean Sea (Gulf Stream) and equatorial Indian Ocean (summer
monsoon current). The North Atlantic Current and winter monsoon current are cool currents.
They are cooled by Arctic winds and ocean currents (North Atlantic) and the winter monsoon
blowing from the icy Himalayas (winter monsoon current).

North Atlantic:
The North Atlantic Gyre is a region in the Atlantic Ocean, characterized by warm waters
flowing west towards the Caribbean, Gulf Stream, North Atlantic Current, and Canary
Current. Its center is the Sargasso Sea, a dense accumulation of Sargassum seaweed, and is
shaped by the North Equatorial Current.

South Atlantic:
The South Atlantic Gyre is a region in the Atlantic Ocean, forming the northern boundary of
the gyre. It is influenced by the South Equatorial Current, Brazil Current, Antarctic
Circumpolar Current, and Benguela Current. The Benguela Current experiences the Benguela
Niño event, a climate change similar to El Niño, affecting primary productivity in the
Benguela upwelling zone.

Indian ocean:
The Indian Ocean Gyre, like the South Atlantic Gyre, is surrounded by the Intertropical
Convergence Zone and the Antarctic Circumpolar Current. The South Equatorial Current
forms the northern boundary, split by Madagascar into the Mozambique and East Madagascar
Currents. The Agulhas Current flows south until it joins the Antarctic Circumpolar Current.
Some water in the Agulhas Current leaks into the Atlantic Ocean, impacting global
thermohaline circulation.

North pacific:
The North Pacific Gyre, one of Earth's largest ecosystems, is surrounded by the Intertropical
Convergence Zone and stretches north to 50°N. It is influenced by the North Equatorial
Current, Kuroshio Current, North Pacific Current, Alaska Current, and California Current.
The region is home to the Great Pacific garbage patch, a significant area of plastic waste.

South pacific:
The South Pacific Gyre, a massive ecosystem accounting for 10% of the global ocean
surface, is the farthest from landmass on Earth. Its remoteness makes it historically under-
sampled in oceanographic datasets. The gyre's circulation is influenced by the South
Equatorial Current, East Australian Current, Antarctic Circumpolar Current, and Humboldt
Current. The gyre also has an elevated concentration of plastic waste, known as the South
Pacific garbage patch, discovered in 2016.

The effect of winds on the vertical movement of water

 Upwelling along the coast caused by Ekman transport of waters (waters move to the
right of the wind).
 The waters moved offshore are replaced by waters from below. This brings cold,
nutrient rich waters to the surface.
 Downwelling caused by Ekman transport onshore (movement of water to the right of
the wind direction)

currents, Upwelling and Downwelling

The movement of surface currents also plays a role in the vertical movements of deeper
water, mixing the upper water column. Upwelling is the process that brings deeper water to
the surface, and its major significance is that it brings nutrient-rich deep water to the nutrient-
deprived surface, stimulating primary production (see section 7.3). Downwelling is where
surface water is forced downwards, where it may deliver oxygen to deeper water.
Downwelling leads to reduced productivity, as it extends the depth of the nutrient-limited
layer.

Upwelling occurs where surface currents are diverging, or moving away from each other. As
the surface waters diverge, deeper water must be brought to the surface to replace it, creating
upwelling zones. The upwelled water is cold and rich in nutrients, leading to high
productivity. Many of the most productive regions on Earth are found in upwelling zones. In
the equatorial Pacific, the trade winds blow the North and South Equatorial Currents towards
the west, while Ekman transport causes the upper layers to move to the north and south in
their respective hemispheres. This creates a divergence zone, and a region of upwelling and
high productivity (Figure 9.5.1).

Figure 9.5.1 Equatorial upwelling and increased productivity as a result of divergence

between the north and south equatorial currents (Modified by PW from image by NASA
[Public domain], via Wikimedia Commons).

A similar process occurs near the Antarctic continent, creating one of the most productive
regions on Earth, the Antarctic divergence. In this case, the West Wind Drift (Antarctic
Circumpolar Current) is flowing parallel to, but in the opposite direction of the East Wind
Drift. With both currents occurring in the Southern Hemisphere, Ekman transport will be to
the left, so the eastward-flowing West Wind Drift water will be transported to the north, and
the westward-flowing East Wind Drift water will be transported to the south, creating a
highly productive divergence zone (Figure 9.5.2).
Figure 9.5.2 High nutrient levels in the Antarctic divergence zone, as a result of the
diverging West Wind Drift and East Wind Drift currents creating strong upwelling (Modified
by PW from Plumbago (Own work) [CC BY-SA 3.0], via Wikimedia Commons).

Downwelling occurs where surface currents converge. The converging water has nowhere to
go but down, so the surface water sinks. Since surface water is usually low in nutrients,
downwelling leads to low productivity zones. An example of a downwelling region is off of
the Labrador coast in Canada, where the Gulf Stream, Labrador, and East Greenland Currents
converge.
Pacific Ocean Currents
1. North Equatorial Pacific Current (warm)
2. Counter Equatorial Current
3. South Equatorial Current
4. Kuroshio system
5. OyashioCurrent (cold)
6. California Current (cold)
7. Peru Current (cold)
8. El Nino or counter Current (warm)
9. East Australia Current (warm)
10. West Wind Drift (cold)
The North Atlantic Ocean features several important gyres, which are large, circular ocean
currents driven by prevailing winds and the Earth's rotation. Here's an overview of the four
main currents within the North Atlantic gyres:

1. *North Atlantic Current (NAC)*:

- The North Atlantic Current is a powerful warm ocean current that flows from the Gulf of
Mexico along the eastern coast of North America towards Europe.

- It transports warm water northward, influencing the climate of regions it passes through,
such as the mild temperatures of western Europe.

2. *Canary Current*:
 - The Canary Current is a cold ocean current that flows southward along the western coast
of Africa, from about 30°N to the equator.

- This current brings cooler waters from higher latitudes towards the equatorial regions.

3. *North Equatorial Current (NEC)*:

- The North Equatorial Current is part of the subtropical gyre in the North Atlantic, flowing
westward across the tropical North Atlantic Ocean.

- It is driven by the trade winds and merges with the Gulf Stream in the Caribbean Sea.

4. *Gulf Stream*:

- The Gulf Stream is a strong, warm ocean current that originates in the Gulf of Mexico and
flows along the eastern coast of the United States towards Europe.

- It plays a crucial role in transporting heat from the tropics towards higher latitudes,
influencing climate and weather patterns in regions it passes through.

These currents are interconnected within the North Atlantic gyres, collectively circulating
warm and cold waters across vast distances, impacting climate, marine life, and oceanic
processes in the region. Understanding these currents is essential for studying ocean
dynamics and climate variability.

1.North Equatorial Current (warm)

A.Antilles current

B.Caribbean current
4. Gulf stream system
Florida current
Gulf stream
North Atlantic Current (drift)
Northern branch-Norway current
Southern branch-Irmingercurrent
Eastern branch
Currents of Mediterranean sea
Rennelcurrent
 West wind drift

The South Atlantic Ocean also features significant gyres and currents that play a key role in
ocean circulation and climate. Here are four main currents within the South Atlantic gyres:

1. *South Equatorial Current (SEC)*:

- The South Equatorial Current flows westward along the equator in the South Atlantic
Ocean, driven by the southeast trade winds.

- This current transports warm surface waters towards the western side of the South
Atlantic.

2. *Brazil Current*:

- The Brazil Current is a warm, southward-flowing ocean current that hugs the eastern coast
of South America.
 - Originating from the subtropical gyre in the South Atlantic, this current brings warm
waters southwards along the Brazilian coastline.

3. *Benguela Current*:

- The Benguela Current is a cold ocean current that flows northward along the western
coast of southern Africa.

- This current is driven by the southeast trade winds and brings cold, nutrient-rich waters
from the deep ocean to the surface, supporting a productive marine ecosystem.

4. *West Wind Drift (Antarctic Circumpolar Current)*:

- While not entirely within the South Atlantic gyre, the West Wind Drift is a major
eastward-flowing current that encircles Antarctica and influences the southern part of the
South Atlantic Ocean.

- This current is the largest and most powerful current in the world ocean, connecting the
Atlantic, Indian, and Pacific Oceans.

These currents interact with each other and with the broader ocean circulation patterns,
impacting climate, marine habitats, and global heat transport. They are essential components
of the South Atlantic gyres, contributing to the complex dynamics of the region's
oceanography.

The North Pacific Ocean is characterized by several gyres, which are large-scale circular
ocean currents driven by winds and the rotation of the Earth. The main gyres in the North
Pacific include the North Pacific Subtropical Gyre and the North Pacific Subpolar Gyre.
Here's a focus on the main large current and associated gyre:
1. *North Pacific Subtropical Gyre*:

- The North Pacific Subtropical Gyre is the largest and most prominent gyre in the North
Pacific Ocean.

- Within this gyre, the primary current is the *Kuroshio Current*.

*Kuroshio Current*:

- The Kuroshio Current is a strong, warm ocean current that flows northward along the
eastern coast of Asia, from the Philippines towards Japan.

- It is the Pacific counterpart of the Gulf Stream in the Atlantic Ocean and plays a critical
role in transporting warm tropical waters towards higher latitudes.

- The Kuroshio Current influences regional climate, marine ecosystems, and weather
patterns along its path.

Other notable currents within the North Pacific Ocean include:

- *North Pacific Current*: This current is a broad westward-flowing current that extends
across the North Pacific Subtropical Gyre, connecting with the Kuroshio Current and
eventually joining the North Pacific Subpolar Gyre.

- *California Current*: Although primarily located in the eastern North Pacific, the California
Current is an important cold ocean current that flows southward along the western coast of
North America, from northern regions towards southern California and Baja California.

Understanding these currents and gyres is crucial for studying ocean dynamics, climate
variability, and marine life in the North Pacific Ocean. The Kuroshio Current, in particular, is
a significant feature with far-reaching effects on the surrounding regions.

The South Pacific Ocean contains important gyres and currents that contribute to ocean
circulation and climate patterns in the region. One of the main gyres in the South Pacific is
the South Pacific Subtropical Gyre. Here's an overview of the main large current associated
with this gyre and other notable currents in the South Pacific:
1. *South Pacific Subtropical Gyre*:

- The South Pacific Subtropical Gyre is a large system of circulating ocean currents
dominated by the following major current:

*East Australian Current (EAC)*:

- The East Australian Current is a warm, southward-flowing ocean current that runs along
the eastern coast of Australia, extending from the Coral Sea towards Tasmania and beyond.

- The EAC is a significant current within the South Pacific gyre, transporting warm tropical
waters southward along the Australian coastline and influencing marine ecosystems and
climate in the region.
Other important currents in the South Pacific include:

- *South Equatorial Current (SEC)*: This westward-flowing current runs along the equator in
the South Pacific, transporting warm waters towards the western side of the ocean basin.

- *Peru Current (also known as Humboldt Current)*: While primarily located in the
southeastern Pacific, the Peru Current is a cold, northward-flowing ocean current that runs
along the western coast of South America, influencing climate and marine ecosystems in the
region.

- *South Pacific Current*: This broad, westward-flowing current spans the South Pacific
Subtropical Gyre, connecting various regions within the gyre and contributing to the overall
circulation pattern of the South Pacific Ocean.

Understanding these currents and gyres is important for studying ocean dynamics, climate
variability, and the distribution of marine life in the South Pacific Ocean. The East Australian
Current, in particular, plays a significant role in shaping coastal environments and supporting
diverse marine habitats along the Australian coastline.

Indian Ocean Current

1. North-east Monsoon current (warm)-WINTER

2. Indian Counter current (warm)

3. South-west Monsoon Current (warm)-SUMMER

4. Indian Equatorial Current (warm)

5. Mozambique Current (warm)

6. West Wind Drift (cold)

7. Agulhas Current (cold)

8. West Australia Current (cold)

The Indian Ocean is characterized by several gyres and significant ocean currents that
contribute to global ocean circulation and regional climate patterns. The main gyres in the
Indian Ocean include the Indian Ocean Gyre and the South Indian Ocean Gyre. Here's a
focus on the main large current associated with these gyres and other notable currents:

1. *Indian Ocean Gyre*:

- The Indian Ocean Gyre is a large system of circulating ocean currents that encompasses
the majority of the Indian Ocean. Within this gyre, the primary current is the *South
Equatorial Current (SEC)*.

*South Equatorial Current (SEC)*:

- The SEC flows westward along the equator in the southern Indian Ocean, transporting
warm surface waters from the eastern to the western Indian Ocean.

- This current influences the climate of countries bordering the Indian Ocean and
contributes to the formation of other important currents.

Other notable currents in the Indian Ocean include:

- *Monsoon Currents*:

- During the seasonal monsoon changes, the Indian Ocean experiences reversal of wind
patterns, leading to the formation of the Southwest Monsoon Current and the Northeast
Monsoon Current.

- The Southwest Monsoon Current flows northward during the summer monsoon season
(June-September), bringing heavy rainfall to the Indian subcontinent.

- The Northeast Monsoon Current flows southward during the winter monsoon season
(October-March), affecting regions such as Southeast Asia and the eastern Indian Ocean.

- *Agulhas Current*:

- While primarily located in the southwestern Indian Ocean, the Agulhas Current is a
strong, warm ocean current that flows southward along the eastern coast of Africa.

- It is known for its role in transporting warm, tropical waters into the South Atlantic Ocean
and influencing global ocean circulation.

Understanding these currents and gyres is essential for studying the dynamics of the Indian
Ocean, including its role in global heat transport, monsoon systems, and marine biodiversity.
The South Equatorial Current and the seasonal monsoon currents are particularly important
features that shape the oceanography and climate of the Indian Ocean region.