The ocean currents are moving for centuries, around the globe transporting heat and nutrients along the way. No one has a specific answer that when this all started in earth’s history but we do know that how it is important for our survival and origin. Earth has 70% of mass covered in water which contains in 5 ocean. For the sake of ease, these water bodies are given name but in reality, they are all interconnected with each other. The currents are responsible for climate regulation and heat transfer in all over the planet. Read this article What is current by? by NOAA https://oceanservice.noaa.gov/facts/current.html.
Ocean currents are the movement of water. They are mainly of two type Surface and Deep ocean currents. Surface currents are found on the surface of the ocean and driven by wind and Coriolis force. On the other hand, deep ocean currents are driven by density difference, salinity, and temperature.
Types of ocean currents
The ocean currents are named based on where they flow (surface or near bottom) like surface and deep currents. There are also warm currents that originate from equatorial regions and cold currents which originates from polar regions.
surface currents only comprise 10% of ocean circulation. The rest 90% comes under deep ocean circulation. Surface currents are driven mainly by wind and wind is driven by Coriolis force. This cycle starts with temperature. In the equatorial region which receives the highest amount of sunlight and heat, air starts to get heated and rise. This air moves from equator to poles under the principle of movement from lower to higher. Under the effect of heat water withstand expansion which rises it 8 cm in comparison to poles. Because of the lack of temperature in polar region water goes under contraction and fall 8 cm lower. This process creates a slope effect and facilitates the flow of water. So, when this wind contacts with the surface of the ocean water it creates friction and starts a movement. Water moves because of energy transferred by the wind, therefore, resulting in currents.
These currents are affected by many factors like temperature, Coriolis force, physiology of ocean floor, continents. These surface currents move towards poles and carry heat along the way. For example, the Gulf stream of North Atlantic gyre( I will explain it a little later) is the fastest current which carries heat to western Europe. It heavily affects the climate there and responsible for the wet climate of the UK. Because of the gulf stream the UK has is relatively warm as compared to part of Canada which is in the same latitude.
Deep Ocean Currents
deep water currents are triggered by density differences. They are controlled by temperature and salinity differences therefore also called Thermohaline circulation. The surface circulation starts from the equator and goes to poles however deep ocean circulation starts from poles and heads for equator. As per oceanographer, there are 3 spots, two in North and 1 in the south pole from where it starts. Surface currents flow in their respected ocean but deep water circulation connects all ocean under one system therefore it is also called Global Conveyer belt.
Deepwater circulation starts because of the joint effect of both temperature and salinity. Firstly polar water is cold and because of less amount of good intensity light, it goes under contraction. Secondly, in the equator, different strata of water have different temperatures. The surface layer is a lot warmer than the layer near the bottom. But in polar water, there is not much temperature difference between layers of water. Here salinity kicks into action. Due to very low-temperature water started to freeze and as we all know salt does know freeze. Hence, salt is left behind, when water turns into ice making surrounding area denser and saltier. These dense water sink to the bottom and starts thermohaline circulation. This circulation carries a large amount of water across the ocean and only moves a few kilometers/day.
Gyre Formations by ocean currents
Surface currents form a circular pattern of flow which is called Gyre. There is 5 major Gyre in 3 major ocean. They are North Atlantic Gyre, South Atlantic Gyre, North Pacific Gyre, South Pacific Gyre, and Indian Ocean Gyre. The Gyres are formed due to Coriolis force which is a result of the rotation of the earth on its Axis. This force diverts wind and currents to north direction in Northern Hemisphere and south direction in Southern Hemisphere. There are two major types of wind Westerlies and Trade winds that drive surface currents. Westerlies flow from west to east in subtropical region and trade wind flows from east to west in the equatorial region. These winds drive North Equatorial current in the Northern hemisphere and South Equatorial current in the southern region.
As we know winds rose in the equator and goes towards the poles. These wind when came in contact with ocean water because of friction drive water current. Due to the contraction and expansion in the polar and equator region respectively a slope forms which helps in the formation of currents. Due to Coriolis force the wind which had risen in the north instead of going into the straight-line turn right in the northern hemisphere. Under the effect of Coriolis, force westerlies turn to the northeast direction and trade winds turn to southwest direction. According to these two winds surface current also moves in a dominant circular pattern. More than one current make up one Gyre.
North Pacific Gyre:
5 currents make up this Gyre in the North Pacific ocean. These are North equatorial drift, North equatorial current, Oyashio current, Kuroshio current, and California current. Oyashio Current is a cold current that originates from the north pole and it mixes with Kuroshio current which is a warm current. They create an ideal fishing ground on the coast of Japan. California current is cold current a to counter it Alaska current is a warm current.
South Pacific Gyre:
There are 4 currents in this Gyre. South Equatorial current, south pacific drift, east Australian current, and Peru current. East Australian Current is a warm current while per current is a cold current.
South Atlantic Gyre
South equatorial current, south Atlantic drift, Brazilian current, and Benguela current make up this Gyre. Warm Brazilian current creates a superior fishing ground. It flows in the western boundary of the ocean basin and eastern boundary of the continent. It brings humidity and rainfall on the eastern boundary for tropical rainforests. Benguela is a cold current.
North Atlantic gyre
North equatorial current (NEC), gulf stream, north Atlantic drift, Labrador current, and canary current make up this Gyre. It originates with NEC near the equator then flows up to the Gulf of Mexico and turns into a warm gulf stream. Gulf stream is the fastest current which carries nearly 4 billion cubic feet of water/sec. It is a warm current it mixes with Labrador Current which is cold current originates from arctic Canada. Near the coast of Labrador where these two current meets a very productive fishing ground lies. The extension of Gulf Stream is North Atlantic Drift, which carries up to north Europe crossing Spain, Portugal, and England to Scandinavia. The fourth part of this Gyre is cold Canary current which flows near the coast of Africa.
It is generally observed that on the eastern part of ocean basin cold current flows and on the western part of ocean basin cold currents flow. That is the reason for having rainforests on the eastern part of continents which are connected with western sides of the oceans. For instance, Amazon rain forests situated in the eastern part of South America. Another example is the Sahara desert on the western side of North Africa under the influence of Canary current.
West wind Drift
There is a very deadly surface current encircling Antarctica called west wind drift. Surface currents are affected by the physiography like the shape of the ocean basin and continents. Because Antarctica is almost round and there is no physical boundary for the current, it circles the continent at deadly speed. It is a well-established fact that ocean current determines climates of continents. West wind drift is a cold current that maintains the cold desert conditions in Antarctica. It also blocks the continents from warm currents.
Factors/forces affecting ocean currents:
- Coriolis Force
- Solar Radiation
- Physiography of ocean Basin
Here Wind Coriolis force, solar radiation, gravity has a major effect on surface current. Temperature, salinity, and density affect the deep ocean circulation. The forces which trigger and manage ocean circulations are grouped into two categories; Primary forces and secondary forces. Primary forces initiate the water movement on the other hand secondary forces give direction to currents. Solar radiation wind and density difference come under primary forces while Coriolis force gravity and ocean physiography come under secondary forces.
The combined effect of wind and Coriolis force
As we already know that westerlies and trade wind trigger the surface currents in major ocean basins. These currents are formed by solar radiation. Due to the intensity of sunlight air rose and flow as the wind from less dense to the denser area. By the force of friction, they move water in their direction of flow. Westerlies flow from west to east (northeast) direction and trade winds flow from east to the west (southwest) direction. Coriolis force winds follow a circular pattern and in turn surface current also follows that predetermined pattern. Due to Coriolis force wind and water gets deflected right in the N. hemisphere and left in the S. hemisphere. This pattern leads to Gyre formation in ocean basins.
Gravity and Density behind Thermohaline circulation
Gravity held accountable in the case of contraction and expansion by solar radiation. In the condition of high-intensity areas near the equator the water expands by 8 cm, on the other hand in the polar region, it goes under contraction by 8 cm. This creates a sloping condition that sets off water flow from equator to poles. The shape of the ocean basin and continents affect water flow. Because of no obstruction on way as well as almost circular shape of Antarctica, west wind drift encircles it. The difference in density which activates by temperature and salinity is the main driving force behind Thermohaline circulation. When the ice freezes in the poles it left salt in the surrounding waters which makes it saltier. This water becomes denser and sinks to the floor which sets off the long cycle of Thermohaline circulation.
Some other Phenomena
It is an important phenomenon for restoring the productivity of the ocean basin. Upwelling simply explains the arrival of deep ocean water when surface water has moved away. There can be many reasons for Upwelling like wind, tides, or surface current. The deepwater when moves to the surface bring nutrient. It makes the water more productive and blooms plankton growth. These upwelling zones are some of the best fishing grounds. It happens around coastal areas and the open ocean. The counterpart of the upwelling called downwelling. Downwelling happens when wind piles up water in a coastal area or open ocean. As a result surface water sinks in towards the bottom. Upwelling and downwelling count as vertical movement of water and only transport a few meters of water.
Ekman Spiral happens because of Coriolis force. Dure to the effect of wind water is moved by friction. The topmost layer is moved due to friction and drags the layer beneath it also. This fractional force creates a spiral when the layer beneath is dragged by a layer above it. The force of friction slowed down with every consecutive layer. This spiral can only go up to 100 m. As the friction force slowed as the depth increases the deeper layers move very slow and opposite to surface layers.
Coastal currents depend on the shoreline, coastal winds (like longshore winds that blow alongside the shores), and waves. The speed of current approaching the shoreline depends on the shape of the coastline and depth of water. When a wave reaches coastline and releases the energy it scatters into several waves that run parallel to the shore. They called longshore currents. Rip currents formed when longshore currents met with destruction on the way like mounds, breaks, and jetties. When the current moves back from the shore it passes through these breaks or low spots. Because the passage is very narrow it creates a suction force for water to pass through it. Due to this fact Rip, currents are so deadly and carried to far beyond the shore.
The effect of climate change/global warming on ocean currents
With the increasing threat of global warming and climate change, our oceans are getting affected. It is quite important to understand how our ocean circulation is getting affected. Due to the increase in temperature of earth equator is heating with more intensity this is affecting wind patterns. In recent years hurricane and tropical storms have gotten more intense which is a sign of distortion in the surface current pattern and Gyres. Thermohaline circulation which completes its one cycle in 1 thousand years have far greater effects of climate change. As the glaciers are melting they are diluting the water nearby area. If water will not dense and saltier it won’t sink and hence will affect the deepwater circulation.
The importance of ocean currents
Ocean currents hold an important part in the survival and distribution of species across the globe. The ocean absorbs a far greater amount of heat than the atmosphere and they are the main distributor of heat throughout the planet. For instance, the western is warmer because of heat transfer by the gulf stream. Since centuries the ocean currents have help in navigation and transportation of goods from far away lands. It is important to have a thorough knowledge of ocean current systems for better shipment, trade, and recreational purposes. The upwelling currents are a blessing for fishing industries because they provide nutrients to the ecosystem. It is a necessity to keep our ocean healthy and intact for climate regulation and heat distribution as global warming is destroying its balance.