Ocean Currents – Clarifications, Factors and Significations

Ocean Currents

Earth is an energetic or dynamic planet, continuously changing by internal and external forces. For example, convection currents of magma inside Earth move the plates that form the continental crust in a constant process that builds mountains and produces valleys.

These valleys can ultimately become lakes, seas, and oceans. Sunlight is the primary factor that affects the surface of Earth. Living organisms get energy from sunlight, and it makes Earth’s weather and climate by creating temperature gradients in the atmosphere and oceans.

Ocean currents can be triggered by wind, density variations in water masses caused by temperature and differences in salinity, gravity, and even natural calamities such as earthquakes, tsunamis, or storms.

Classification of Ocean Currents- Based on the Depth

Surface Ocean Currents

Global wind systems that are powered by solar energy push surface currents in the ocean. Various surface currents are affected by wind direction, Coriolis forces created from the Earth’s rotation, and the location of landforms that interact with the currents. In addition, surface wind-driven currents create upwelling currents in combination with landforms, producing deepwater currents.

Deep Ocean Currents

Ocean currents may also be produced by variations in the density of water masses due to temperature (thermo) and salinity (haline) differences through a process called thermohaline circulation. These currents move water masses all through the deep ocean by holding nutrients, oxygen, and heat along with them.

Thermohaline ocean circulation:

Warm air is carried by surface currents to the polar regions, where the water ultimately becomes cold enough to freeze. When this change occurs, water molecules are locked into the ice, but salt molecules remain unaltered, and the density of the water increases below the edge of the freezing zone by adding salts. This cold water has a natural tendency to sink due to its density.

Intermittent events such as huge storms and underwater earthquakes (tsunamis) can also generate severe ocean currents, moving masses of water inland when they reach shallow water and coastlines. Earthquakes may also cause rapid downslope movement of water saturated with sediments, generating intense turbidity currents.

Lastly, when a current move over a wide area and is pushed into a restricted space, it (current) may become very strong.

Classification of Ocean Currents- Based on Temperature

Ocean Currents

Ocean currents are classified based on temperature. Ocean currents are divided into cold currents and warm currents:

Cold Currents:

Cold currents bring cold water from high altitudes into warm water areas, i.e., to low latitudes. Cold ocean currents are masses of cold water moving from high latitude to the direction of the equator by absorbing the heat received in the tropics, thus cooling the air above. Formation of cold currents takes place when the air circulating the eastern side of the subtropical high is carried over cold water mass and is then pulled in the direction of the equator.

These currents are generally noticed on the west coast of the continents in the low and middle latitudes and on the east coast in the higher latitudes in the Northern Hemisphere.

Warm Currents:

Warm currents are masses of warm water with elevated temperatures shifting away from the equator.

Warm currents are produced when the cold saline water becomes dense and sinks, which allows the light warm water to flow in the opposite way, normally far away from the equator.

These warm currents are typically seen on the east coast of continents in the low and middle latitudes. In the northern hemisphere, warm currents are found on the west coasts of continents in high latitudes.

Factors Affecting Ocean Currents

Atmospheric Pressure and Planetary Winds:

Air pressure on the oceanic water causes ocean currents to go through density changes.

The characteristics of areas of high atmospheric pressure are that it has a low volume of water and hence, lower water level, whereas the areas of low atmospheric pressure record a higher volume of water and a higher level of water. Hence, water shifts as surface current from the areas of higher water level areas, i.e., low-pressure areas, to areas of low water level, i.e., high-pressure areas.

Prevailing or global winds such as trade winds, westerlies, and polar winds play the most important functions in the origin of ocean currents. The majority of the ocean currents of the world follow the path of prevailing winds.

For example, equatorial currents move westward under the effect of N.E. and S.W. trade winds. The Gulf Stream in the Atlantic and the Kuroshio in the Pacific flow in the northeastern direction under the impact of the westerlies.

Coriolis Force:

When wind or an ocean current moves, the Earth spins below it. Due to this, an object moving in the north or south direction along the Earth will seem to move in a curve instead of in a straight line. For example, ocean water that goes toward the poles from the equator is deflected to the east, whereas water that moves toward the equator from the poles becomes curved to the west.

Due to the Coriolis effect, the path of surface currents bends to the right in the Northern Hemisphere and left in the Southern Hemisphere.

Due to the Coriolis force, the flow of currents will be in a clockwise direction in the northern hemisphere and an anti-clockwise direction in the southern hemisphere. Due to this effect, currents form circular patterns.

These ring-like patterns formed by ocean currents are called Gyres. In other words, Gyre is a circular movement of water caused by wind and formed by Coriolis force. Gyre generates large circular currents in all the ocean basins. Subtropical gyres are large rotating currents that start near the equator.

The five main gyres are – the North Pacific subtropical Gyre, South Pacific Subtropical Gyre, the North Atlantic subtropical Gyre, the South Atlantic Subtropical Gyre, and the Indian Ocean Subtropical Gyre.

Temperature:

Expansion of water takes place by heating of water by sun’s energy. Equatorial and tropical regions receive more solar heat than polar regions. Hence, the ocean water is about 8 cm higher in level near the equator than in the middle latitudes.

There is a lot of variation in the temperature of oceanic waters at the equator and the poles.

Hence warm equatorial waters move slowly along the surface to the poles, whereas heavier cold waters of the polar areas move slowly towards the equator near the bottom of the sea. The depth of the ocean also affects temperature differences. Temperature decreases with depth as the sun’s rays cannot penetrate deep oceanic waters. Hence, the variation in the temperature of the ocean waters affects ocean currents.

Salinity:

Oceanic salinity causes the density of ocean water and changes in density cause ocean currents.

If two areas with the same temperature are characterized by a change in salinity, then the area of high salinity will have a greater density than the area of low salinity.

The dense water sinks and goes as subsurface current, whereas less saline water moves towards greater saline water as surface current. In other words, ocean currents on the water’s surface are produced from the areas of less salinity to the areas of larger salinity. Such a system of surface and subsurface currents is caused by changes in salinity created in open and enclosed seas.

For example, the flow of current from the Atlantic Ocean to the Mediterranean Sea through Gibraltar Strait is produced due to the variation in salinity.

Significance of Ocean Currents

Impact on global and Regional Climate:

The movement of the ocean currents is a major factor in the development of the Earth’s climate.

The majority of the current systems normally flow in the clockwise direction in the northern hemisphere and anticlockwise direction in the southern hemisphere, in circular patterns that frequently trace the coastlines.

More than two-thirds of the Sun’s heat is directly absorbed by oceans. An overall 25% of the planet’s global heat is carried by the actions of ocean currents.

Ocean currents act more like conveyor belts, transferring warm water and precipitation from the equator to the poles and cold water from the poles back to the tropics.

Surface currents bring warm water to the poles, and deep currents bring cold water back to the equator.

Hence, ocean currents adjust global climate and help to neutralize the uneven supply of solar radiation by reaching Earth’s surface.

If there are no currents in the ocean, local temperatures will be extremely hot at the equator and very cold near the poles. Due to these extreme temperatures, very little of Earth’s land will be habitable.

For example, the North Atlantic Drift keeps the coasts of the North Sea warm, which is very uncommon for such high latitudes. In the same way, the warm waters of the Kuroshio current in the North Pacific Ocean keep parts of the Alaskan coast free from ice in the winter season.

El Niño event in the Pacific Ocean is a remarkable presentation of how a change in regional ocean currents can affect climatic situations around the world.

Precipitation: Warm currents flow along the east coast of continents; warm currents flow that result in warm and rainy climates, whereas along the west coast of continents, cold currents flow.

Desert Formation: Cold ocean currents immediately impact the formation of a desert in the west coast regions of the tropical and subtropical continents. For example, Peru current is a cold-water current of the southeast Pacific Ocean, and it is the leading cause of the dryness of the Atacama desert (the driest desert in the world).

Tropical Cyclones:

They pile up warm waters in the tropics; warm water is piled by tropical cyclones, and this warm water is the main force behind tropical cyclones.

Climate change has an impact on all areas around the world. Due to climate change, polar ice shields are melting, and the sea level is increasing. In some regions, severe weather incidents and rainfall are becoming very common, whereas others are experiencing more intense heat waves and drought conditions. These effects are likely to increase in the coming years.

Climate Change: Melting of Ice and Rising of Sea Levels

When the water warms up, it expands, but at the same time, due to global warming, polar ice sheets and glaciers melt. The pattern of these changes is causing the rise in sea levels, resulting in flooding and erosion of coastal and low-lying areas.