Factors affecting Ocean Currents and their role in influencing Climate

Ocean Currents

An ocean current is a continuous, predictable, directed movement of seawater generated by forces acting on the mean flow.

• Ocean currents are located at the ocean surface and in deep water below 300 meters (984 feet).
• They can move water horizontally and vertically and occur on both local and global scales.
• The ocean has an interconnected current, or circulation, system powered by wind, tides, the Earth’s rotation (Coriolis effect), temperature, salinity, and water density differences.
• The topography and shape of ocean basins and nearby landmasses also influence ocean currents.
• These forces and physical characteristics affect the size, shape, speed, and direction of ocean currents.

Classification of Ocean Currents- based on the depth

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

1. Surface ocean currents: 
   • Large-scale surface ocean currents are driven by global wind systems that are fueled by energy from the sun.
   • The Sun warms water at the equator more than it does at the high latitude polar regions.
   • The heat travels in surface currents to higher latitudes- from the tropics to the polar regions, influencing local and global climate.
   • A current that brings warmth into a high latitude region will make that region’s climate less chilly. 
     • The warm Gulf Stream originating in the tropical Caribbean, for instance, carries about 150 times more water than the Amazon River.
     • The current moves along the U.S. East Coast across the Atlantic Ocean towards Europe.
     • The heat from the Gulf Stream keeps much of Northern Europe significantly warmer than other places equally as far north.
   • Surface Currents amount to only 8-10% of the global oceanic current flow. 
2. Deep ocean currents:
   • Differences in water density, resulting from the variability of water temperature (Thermo) and salinity (haline), also cause ocean currents.
   • This process is known as thermohaline circulation.
   • Thermohaline circulation drives deep ocean circulation. Differences in temperature and in salinity change the density of seawater.
   • So thermohaline circulation is the result of density differences in water masses because of their different temperature and salinity.
   • In cold regions, such as the North Atlantic Ocean, ocean water loses heat to the atmosphere and becomes cold and dense.
   • When ocean water freezes, forming sea ice, salt is left behind causing surrounding seawater to become saltier and denser.
   • Dense-cold-salty water sinks to the ocean bottom- called the downwelling zone.
   • Surface water flows in to replace the sinking water, which in turn becomes cold and salty enough to sink.
   • Again, to fill the void created by moving of the surface water, deep water upwells through upwelling zone. 
   • In upwelling currents, vertical water movement and mixing brings cold, nutrient-rich water toward the surface while pushing warmer, less dense water downward, where it condenses and sinks. 
   • Where the significant vertical movement of ocean currents is observed, this is known as upwelling and downwelling. 
   • This “starts” the global conveyer belt, a connected system of deep and surface currents that circulate around the globe on a 1000 year time span.
   • The global conveyor belt’s circulation is the result of two simultaneous processes: warm surface currents carrying less dense water away from the Equator toward the poles, and cold deep ocean currents carrying denser water away from the poles toward the Equator.
   • Wind-driven surface currents (such as the Gulf Stream) travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water).
     • This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1000 years) upwell in the North Pacific.
     • Extensive mixing, therefore, takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. 
   • The ocean’s global circulation system plays a key role in distributing heat energy, regulating weather and climate, and cycling vital nutrients and gases.
   • Deepwater currents amount to 90-92% of the global oceanic current flow. 
   • Deep ocean currents are currently being researched using a fleet of underwater robots called Argo.

Classification of Ocean Currents- based on temperature

Ocean currents are classified based on temperature: as cold currents and warm currents:

1. Cold currents:
   • Cold currents bring cold water into warm water areas (from high latitudes to low latitudes).
   • Cold ocean currents are masses of cold water moving from high latitude towards the equator absorbing the heat received in the tropics thus cooling the air above.
   • Cold currents are formed when the air circulating the eastern side of the subtropical high is blown over cold water mass and are then dragged toward the equator.
   • 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.
2. Warm currents:
   • Warm currents are masses of warm water with higher temperatures moving away from the equator.
   • Warm currents are formed when the cold saline water becomes dense and sinks allowing the light warm water to flow in the opposite direction, usually far from the equator.
   • These 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.

Factors influencing Ocean Currents

1. Atmospheric Pressure and Planetary Winds: 
   • Air pressure on the oceanic water causes ocean currents through density variations.
   • The areas of high atmospheric pressure are characterized by a low volume of water and thus lowering of water level.
   • Contrary to this the areas of low atmospheric pressure record a higher volume of water and higher water level.
   • Thus, water moves as surface current from the areas of higher water level (low-pressure areas) to low water level areas (high-pressure areas).
   • Prevailing or planetary winds (e.g., trade winds, westerlies and polar winds) play major roles in the origin of ocean currents.
   • The wind blowing on the water surface also moves water in its direction due to its friction with the water.
   • Most of the ocean currents of the world follow the direction of prevailing winds.
   • For example, equatorial currents flow westward under the influence of N.E. and S.W. trade winds.
   • The Gulf Stream in the Atlantic and the Kuroshio in the Pacific move in northeastern direction under the influence of the westerlies.
2. Coriolis Force:
   • As wind or an ocean current moves, the Earth spins underneath it.
   • As a result, an object moving north or south along the Earth will appear to move in a curve, instead of in a straight line.
   • Ocean water that travels toward the poles from the equator is deflected to the east, while water that travels toward the equator from the poles gets bent to the west.
   • The Coriolis effect bends the direction of surface currents to the right in the Northern Hemisphere and left in the Southern Hemisphere.
   • Due to Coriolis force, currents flow in a clockwise direction in the northern hemisphere and in anti-clockwise direction in the southern hemisphere.
   • This effect causes currents to form circular patterns. 
   • These large accumulations of water and the flow around them are called Gyres.
     • These produce large circular currents in all the ocean basins.
     • 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. 
     • Subtropical gyres are also responsible for concentrating plastic trash in certain areas of the ocean.
3. Temperature: 
   • Heating by solar energy causes the water to expand.
   • The sun provides more heat to the equatorial and tropical regions than polar regions.
   • That is why near the equator the ocean water is about 8 cm higher in level than in the middle latitudes.
   • There is much difference in the temperature of oceanic waters at the equator and the poles.
   • Warm equatorial waters, therefore, move slowly along the surface towards the poles while heavier cold waters of the polar areas creep slowly towards the Equator along the bottom of the sea.
   • Temperature variation also occurs with the depth of the ocean.
   • Temperature decreases with depth as the sun's rays can't penetrate deep oceanic waters.
   • Thus, the difference in the temperature of the ocean waters causes ocean currents.
4. Salinity: 
   • Oceanic salinity affects the density of ocean water and density variation causes ocean currents.
   • If two areas having equal temperature are characterized by varying salinity, the area of high salinity will have a greater density than the area of low salinity.
   • The denser water sinks and moves as subsurface current whereas less saline water moves towards greater saline water as surface current.
   • In other words, ocean currents on the water surface are generated from the areas of less salinity to the areas of greater salinity.
   • Such a system of surface and subsurface currents caused by salinity variation is originated in open and enclosed seas.
   • For example, the current flowing from the Atlantic Ocean to the Mediterranean Sea via Gibraltar Strait is caused because of the difference in salinity.
5. Density: 
   • Differences in water density affect the vertical mobility of ocean currents (vertical currents).
   • Denser water tends to sink, while relatively lighter water tends to rise.
6. Physiography:
   • A physiographic feature would decide if the ocean current would move straight or deflect from its path. 
   • A landmass always obstructs and diverts a current. When a surface current collides with land, the current must change direction. 
   • The Atlantic South Equatorial Current travels westward along the equator until it reaches South America. At Brazil, some of it goes north and some goes south.
     • Because of the Coriolis effect, the water goes right in the Northern Hemisphere and left in the Southern Hemisphere.

Importance of Ocean Currents

The ocean covers 71% of the planet and holds 97% of its water, making the ocean a key factor in the storage and transfer of heat energy across the globe. The movement of this heat through local and global ocean currents affects the regulation of local weather conditions and temperature extremes, stabilization of global climate patterns, cycling of gases, and delivery of nutrients and larva to marine ecosystems.

The earth’s oceans are so vital for life that over 40% of the world’s population live near coastal areas.

The importance of ocean currents can be understood from the following :

1. Influence on global and regional climate:
   • The movement of the ocean currents has an especially significant factor in the development of the planet’s climate.
   • Major current systems typically flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere, in circular patterns that often trace the coastlines.
   • The oceans directly absorb more than two-thirds of the Sun’s heat, an overall 25% of the planet’s global heat budget is transferred through the actions of ocean currents.
   • Ocean currents act much like a conveyor belt, transporting warm water and precipitation from the equator toward the poles and cold water from the poles back to the tropics.
   • Surface currents carry warm water toward the poles and deep currents bring most of the cold water back toward the equator.
   • Thus, ocean currents regulate global climate, helping to counteract the uneven distribution of solar radiation reaching Earth’s surface.
   • Without currents in the ocean, regional temperatures would be more extreme– super hot at the equator and frigid toward the poles- and much less of Earth’s land would be habitable.
     • For example, the North Atlantic Drift keeps the coasts of the North Sea (western coast of Europe) warm which is unusual for such high latitudes.
     • Similarly, the warm waters of the Kuroshio current in the North Pacific ocean keep the ports of the Alaskan coast ice-free in winter.
     • El Niño event in the Pacific Ocean is an impressive demonstration of how a change in regional ocean currents- in this case, the Peruvian current– can affect climatic conditions around the world. 
   • Precipitation: Warm currents flow along the east coast of continents resulting in warm and rainy climates while cold currents flow along the west coast of continents.
   • Desert Formation: Cold ocean currents have a direct effect on desert formation in west coast regions of the tropical and subtropical continents.
     • For ex., Peru Current is a cold-water current of the southeast Pacific Ocean and a primary reason for the aridity of Atacama desert (driest desert of the world).
   • Tropical cyclones: They pile up warm waters in tropics and this warm water is the major force behind tropical cyclones.
2.  Shipping:
   • Knowledge of surface ocean currents is essential in reducing the costs of shipping since travelling with them reduces fuel costs.
   • In the wind-powered sailing-ship era, knowledge of wind patterns and ocean currents was even more essential.
   • A good example of this is the Agulhas Current (down along eastern Africa), which long prevented sailors from reaching India.
   • In recent times, around-the-world sailing competitors make good use of surface currents to build and maintain speed.
3. Biologically rich:
   • The oceans are an essential part of Earth’s water cycle.
   • Since they cover so much of the planet, most evaporation comes from the ocean and most precipitation falls on the oceans.
   • The oceans are also home to an enormous amount of life. That is, they have tremendous biodiversity.
   • Tiny ocean plants create the base of a food web that supports all sorts of life forms. Marine life makes up the majority of all biomass on Earth. 
4. Distribution of nutrients and food:
   • They carry nutrients and food to organisms that live permanently attached in one place and carry reproductive cells and ocean life to new places.
   • Ocean currents are also very important in the dispersal of many life forms. An example is the life-cycle of the European Eel.
   • Cold ocean water currents flowing from polar and sub-polar regions bring in a lot of plankton that are crucial to the continued survival of several key sea creature species in marine ecosystems.
   • Since planktons are the food of fish, abundant fish populations often live where these currents prevail.
   • The regions where cold and warm currents meet, world's best fishing grounds are found, for example- Newfoundland.
   • Phytoplankton, microscopic plants and animals in the oceans provide the foundation of the global food web of species.
5. Power generation: Ocean currents can also be used for marine power generation, with areas off of Japan, Florida and Hawaii being considered for test projects.

Climate change, the ocean currents and our uncertain future

• There is an elegant balance to the complex movements of the ocean currents, mediated by multiple factors yet maintaining consistent temperature gradients that make most latitudes hospitable to life.
• But anthropogenic climate change is having a profound impact on the Earth’s weather and its climatological balancing systems.
• A hotter planet will inevitably raise temperatures in the sea, which will cause higher sea levels, stronger winds, fiercer and more frequent hurricanes and typhoons, and greater or lesser quantities of rainfall in particular areas that may be profoundly affected by such changes.
• The effects of climate change on currents could be especially troubling.
• As ocean water warms to greater depths, and temperatures climb in the polar regions, major ocean currents may slow down or even stop.
• This could lead to dramatic shifts in temperature everywhere- higher latitudes may be plunged into Ice-Age-like conditions, while heat waves might render equatorial regions virtually uninhabitable.
• This is an extreme result and far from certain.
• But scientists are already observing a slowdown in a powerful Northern hemisphere ocean current system known as the Atlantic Meridional Overturning Circulation (AMOC), which helps moderate temperatures on the European continent.
  • This current system incorporates warm water surface currents and deep-water cold currents, and some models are predicting a 50% or more slowdown by the turn of the next century if greenhouse gas emissions continue at current levels.
  • The last time this current system moved so slowly was between 10,000 and 100,000 years ago when much of Europe and North America was covered by an impenetrable sheet of ice.
• Worst-case scenarios can still likely be avoided if strong action is taken to reduce carbon emissions globally.
• But a failure to take those actions will lead us into unchartered territory, and if that causes ocean currents to slow or shut down it could set off a chain reaction of climate change that will turn hundreds of millions of people (at least) into refugees, fleeing their homes in a desperate search for more habitable regions.

Conclusion

• From helping to keep our planet warm, to influencing precipitation patterns around the world, to playing a critical role in the global carbon cycle- ocean currents are one of the major determinants of weather and climatological conditions.
• Our planet has an interrelated climate system, where the ocean, atmosphere, and the land all interact with each other, influencing global climate.
• Whether it is rain, warm air that creates the wind, or wind that drives ocean currents and determines the distribution of phytoplankton, changes in one area of the global climate system could undoubtedly alter things in the other areas as well.
• With an understanding of this interconnection, it is important to realize the influences that our choices might make on these systems, such as humanity’s choice to continue to burn fossil fuels or the transition to a clean energy future that will safeguard our planet’s climate for future generations.
• We rely on the ocean currents to survive, and if we disrupt them we do so at our own peril.