Ocean Currents and Air Currents, a characteristic of the seawater and air respectively are the most important factors in maintaining atmospheric balance.
The ocean current is the general horizontal movement of a mass of water in a fairly defined directly under the various forces, internal and external. The vertical movement of air columns is known as “currents” (Lal, 2016). Similarly, “wind” is the horizontal movement of air. When the air becomes warmer, it rises and leads to the formation of air currents. The atmosphere and the ocean comprise the total environment of earth, including the lithosphere.
They consist of the elements and processes that drive various the necessary physical phenomena like the hydrological cycle and latitudinal energy transfer. Ocean currents and air currents are the movements of the ocean water masses and air columns, respectively. A study of the mechanisms of these movements over the oceans and atmosphere is necessary for a greater spatial understanding of the earth.
Ocean Currents
It is an important characteristic of seawater (Sharma and Vatal, 2011). Ocean currents move the seawater along a fixed path and direction for thousands of kilometers and play a crucial role in transferring heat energy by moving water masses from the warmer to the colder regions.
Types of Ocean Currents
Ocean currents are mainly grouped into two classes based on their depth- surface ocean currents and deep ocean currents.
Surface ocean currents
Surface ocean currents move in the relatively warm upper layers of the ocean, extending from the water surface up to a depth of approximately 1000 meters. They comprise only 10 percent of the total volume of all oceans.
These currents are the result of wind-drag forces, i.e., the frictional force exerted by the wind over the water surface, which drags the water in its direction. That is why these currents are also known as wind-driven ocean currents.
Surface ocean currents are subdivided according to temperature (warm and cold ocean currents) and speed, velocity, and direction (drifts, currents, and streams). These have been defined below:
Surface ocean currents according to temperature:
Warm ocean currents
These flow from the warmer areas of low and middle latitudes towards the higher latitudes. They generally follow the planetary wind direction (e.g., the trade winds, westerlies, etc.) for their movement. They raise the temperatures of areas through which they pass. E.g., Florida current and Brazil current.
Cold ocean currents
These flow from the cold polar and sub-polar regions towards the lower latitudes. Their movement is north-south and south-north in the northern hemisphere and southern hemisphere, respectively. E.g., Peru current, Labrador current.
Surface ocean currents according to speed, volume, and direction:
Drifts
Those surface currents which move by the forces of prevailing winds are known as drifts. North Atlantic Drift, flowing from south-west to north-east, under the influence of the westerlies in the northern hemisphere is an example.
Currents
Some ocean currents move in a fixed direction with great velocity. They are called currents. E.g., north and south equatorial currents.
Streams
Some movements of the ocean involve an enormous volume of water and also velocities, which are higher than the drifts and currents. They are like ocean-rivers. E.g., The Gulf Stream.
Deep Ocean Currents
Below the pycnocline layer (i.e., the zone of the ocean where rapid change occurs in water density with depth), the influence of winds on water movement becomes absent, and ocean currents are generated due to density differences between one zone to another.
Density being a function of temperature and salinity of water, they are also known by the name “thermohaline currents” (‘thermo’ for temperature and ‘haline’ for salinity). Areas with the low temperature where water is denser, experience downwelling of ocean water, thus creating a deep ocean current. This deep ocean current then moves as a horizontal flow towards the warmer areas.
Factors Affecting Ocean Currents
Ocean currents are formed and modified due to different types of factors, which may be physical properties of the ocean water, rotation of the earth, nature of the coastline, etc.
Factors affecting the Origin of Ocean Currents are:
OCEANIC CAUSE
Earth’s rotation and Coriolis Force
The Coriolis force, which is caused due to earth’s rotation deflects the sea currents in the northern hemisphere towards the right and the southern hemisphere towards left. The degree of deflection is higher at the poles and is directly related to the speed of ocean currents.
The difference in atmospheric pressure
Currents move from areas of low pressure to areas of high pressure. Higher air pressure depresses the surface water level of the oceans. On the other hand, in areas where air pressure is low, there is a slightly higher sea level. Thus, the areas of high pressure with low water levels experience an inflow of waters from low-pressure areas.
Wind-drag forces
As already stated, surface ocean currents are dependent on the prevailing wind direction. There is a close association between the patterns of wind circulation and ocean current movement across the world.
Precipitation and Evaporation conditions
Just like the effect of atmospheric pressure in enhancing or reducing the seawater levels, precipitation conditions similarly affect oceanic circulation. Higher precipitation in the equator, though, increases the water level, but it also reduces the salinity of seawater, thus making the water lighter. So, there is a surface flow of ocean current from the equator to the poles.
Also, if evaporation rates are very high, resulting in reduced water-levels in a region, there is a net inflow of ocean currents from outside. Such an incident happens in the Mediterranean Sea, where water loss due to evaporation is compensated by the flow of currents from the Atlantic Ocean.
Sub-oceanic Causes
Temperature gradient
High temperature of water in the equatorial regions causes the water particles to expand and move outwards in the north and south directions. Movement of the warm Kuroshio current towards the north in the Pacific ocean is an example.
Salinity and Density variations
Surface ocean currents move from areas of lower salinity to areas of higher salinity. As discussed above, less dense tropical waters move to the poles as a surface current and, highly dense polar waters sink to the sea bottom and move towards the tropics as deep water current.
Melting of Ice
In the ice-covered regions such as the Arctic Ocean in the north, the melting of ice leads to the formation of some cold currents, e.g., the East Greenland Current.
Factors affecting the Modification of Ocean Currents
Some factors modify the usual course of the ocean currents. They are discussed below:
The shape of the coastline
The nature of the topography guides the direction of ocean currents. For example:- In the Atlantic Ocean, the South equatorial current after striking with the protruding eastern coast of South America ( Cape Sao Roque) bifurcates into two branches.
Season
Variations in season can also lead to change in current patterns. An interesting example is the complete reversal of the currents of the Indian Ocean during the summer and winter seasons due to change in the direction of the monsoon winds.
Bottom topography
Bottom relief features of the ocean, such as submarine ridges, also deflect the ocean currents. Example- the North Atlantic drift is diverted to the right when it crosses the sub-oceanic Wyville Thomson Ridge. According to Ekman, ocean currents follow the bottom topography in the middle and higher latitudes, but they are not influenced by it in the lower latitudes.
Circulation patterns in different oceans
The currents in all the major oceans, i.e., the Pacific, Atlantic, and Indian Oceans, complete two circular paths above and below the equator.
This circular pattern is achieved due to the pressure and temperature variations, nature of the coastlines, and wind direction, among other factors. The circulation pattern prevalent in the Atlantic Ocean has been shown below as an example:
Functions and Importance of Ocean Currents
Ocean currents are nature’s instruments of maintaining the latitudinal heat balance over the globe. Some important functions of the ocean currents are listed here:
Modifications of Coastal Climate
Warm ocean currents encourage rainfall along with the coastal areas which they pass along. Also, they raise the temperature of the coasts of high latitudes. On the other hand, cold currents discourage rainfall and encourage aridity.
Example- Kuroshio (warm) current causes rainfall over eastern Japan. Peru (cold) current is one of the reasons for the dry conditions in the western coast of South America. Also, major events like the El Nino- La Nina affect rainfall patterns over vast areas of the Pacific and Indian oceans. During an extreme El Nino event (and a weak La Nina), heavy rainfall occurs in Peru, Chile, and Ecuador, i.e., the western coast of South America.
At the same time, it weakens the monsoon in South and South-east Asia. Weaker monsoons adversely affect the economies of the countries lying in this region. The opposite happens when the conditions are reversed.
Effects on marine organisms and fishing activities
The upwelling of ocean water onto the surface from greater depths brings various nutrients that are consumed by the phytoplankton and zooplankton. The downwelling of currents, similarly, transports the surface oxygen downwards for the benthic (i.e., bottom-dwelling) organisms.
The mix up of warm and cold ocean currents creates a rich marine environment with planktons and fishes, thereby making such regions ideal fishing grounds. That is why the Grand Banks of New Foundland is a rich fishing ground because it is the confluence of the Labrador current (cold) and the Gulf stream (warm).
Effects on Navigation
Dense fogs are created when cold and warm ocean currents meet at a place. Also, cold currents from the polar regions carry icebergs to the south. These conditions cause problems for the ships by reducing visibility and imposing physical damage, respectively.
Air Currents
The upward movement of air current is called ‘updraft’ whereas the downward motion toward the surface is called ‘downdraft.’ Such vertical movements are necessary for major weather phenomena like the formation of clouds, precipitation, thunderstorms, fogs, etc.
Factors affecting the origin of air currents
Two mechanisms generate air currents. They are either i) thermally induced, or ii) mechanically induced.
Thermally-induced air currents
The amount of heat received at the tropics is greater, and thus, the air over these regions is comparatively warmer than the temperate regions. This warm air being light rises to form vertical currents. Thus, high temperature leads to the formation of vertical air movements in the tropics.
Mechanically induced air currents
A horizontally moving air column can get obstructed by terrain features like a mountain and rise along the slope of the mountain to form air currents.
Such a parcel of air, on condensation, causes rainfall on the windward slope of the highland, which is known as ‘orographic rainfall.’ Also, when a warm air mass and cold air mass meet, the warmer air, being comparatively lighter, rises over the cold air (along an intersection plane called ‘front’), thereby creating warm air currents.
Atmospheric Circulation Pattern (Tricellular Meridional Circulation)
The surplus solar energy received at the equator is dissipated to other cooler regions in the form of air currents and winds, thus maintaining the heat balance of the atmosphere. The tricellular meridional model was proposed by Palmen in 1951 to explain the cycle of this energy.
The model proposes that global atmospheric circulation comprises three meridional cells- 1) Tropical cell (Hadley cell) 2) Polar front cell (Ferrel cell) 3) Polar or subpolar cell.
Tropical cell or Hadley cell
This cell operates between two coordinates, i.e., the equator and 30° N and S latitudes. As stated in the previous sections, the warm equatorial air rises and ascends to the tropopause (i.e., the junction of the troposphere and the stratosphere), diverges, and moves polewards as an upper tropospheric flow and takes the name of the ‘antitrades.’ These air currents start to descend when they reach 30° latitude. These subsidence zones are known as the ‘horse latitudes.’
Surface winds then move from these horse latitudes in two directions- one branch moves towards the poles (to form a part of the polar front cell, discussed below), and another branch moves towards the equator. The equatorward surface winds are called ‘trade winds’- the northeast and the southeast trade winds in the northern and southern hemispheres, respectively.
The trade winds from both the hemispheres connect at the intertropical convergence zone (ITCZ), which is a low-pressure equatorial trough lying between 5°N and S.
Polar front cell or Ferrel cell
This cell is intermediate between the tropical and the polar cells. Unlike the tropical and polar cells which are thermally induced, this cell is induced mechanically, i.e., this cell is formed by the subsiding and rising ends of the Hadley and Polar cells respectively between 30° and 60° N and S.
The direction of circulation is opposite to the Hadley cell. The air which is subsiding at the horse latitudes (as explained in the Hadley cell above) diverges at the surface and flows towards the pole in the form of ‘westerlies.’ These prevailing westerlies are, however, weak winds as they are frequently offset by terrain features and also extratropical cyclones and anti-cyclones.
Polar or subpolar cell
This cell lies between 60° latitudes and the poles. Due to very low temperatures at the poles, the highly-dense air sinks to the surface and then moves as easterlies towards the equator.
These winds are known as ‘polar easterlies.’ The polar easterlies clash with the westerlies of the temperate regions and form a front known as the ‘polar front.’ As the temperate zones are relatively warmer than the poles, the warm air rises travel till the tropopause, and again diverges to sink at the north and south poles.
Upper air circulation- Upper air westerlies and Jet Stream
The air currents, as mentioned in the tricellular meridional circulation, only operate in the lower tropospheric region and do not reach up to the tropopause. Above an altitude of about 1 km from the surface, the upper air circulation prevails, which is very different from the normal cellular circulation, as seen in the Hadley, Ferrel, or Polar cells.
The continuous updraft of air from the equator creates a high-pressure over the equator in the upper troposphere. On the other hand, the constant sinking of cold air masses at the poles creates rarefication or low pressure aloft over the poles. Thus, in greater heights of the atmosphere, the pressure gradient is from the equator to the poles. The upper air has a straight path theoretically.
Still, due to the effect of the Coriolis force, it gets deflected and turns to the right in the northern hemisphere, becoming a geostrophic wind flowing parallel to the isobars. Thus, wind flows from west to the east in the upper atmosphere, and these winds are known as the upper-tropospheric westerly winds.
Jet streams are defined as swift geostrophic airstreams in the upper troposphere that meander in relatively narrow belts (Lal, 2016). These are the strong cores of the upper tropospheric westerlies and follow the same path as the westerlies. These usually are 150-450 km wide, 900-2150 meters thick, with core speed exceeding 300 kmph (Khullar, 2012).
Jet streams, based on their location, have been classified into the following- i) Polar front jet streams ii) Subtropical westerly jet streams iii)Tropical easterly jet streams iv) Stratospherical subpolar jet streams v) Local jet streams
Role of Air Currents
Air currents, or the vertical movements of air, are necessary occurrences in the process of cloud formation and precipitation. Strong vertical updrafts in the equatorial regions lead to the formation of thick cumulonimbus clouds, which cause heavy rainfall in the tropics.
Also, the entire mechanism of thunderstorms is composed of updrafts of air in the initial stages, which causes precipitation and the formation of huge thunderstorm clouds. There are strong downdrafts in the later stages, causing heavy rainfall in areas receiving the downward moving currents.
All cyclonic circulation, like tropical cyclones, tornadoes, and hurricanes, as well as anticyclonic occurrences, are driven by the force of air currents and winds. Air currents also cause frontal rainfall. In the temperate regions, when the warm and cold air masses meet, the warm air is pushed up the cold air along the frontal plane.
Apart from the above roles, an important part and utility of vertical air circulation are in the heat energy transfer across the globe.
Importance of Ocean-Air Interactions
The atmosphere and oceans are linked through diverse physical and chemical processes that sustain the global hydrological cycle, maintain energy balance across the earth, and determine all the weather patterns.
The wind energy forces the ocean currents to move in a particular direction. There is a close relationship between the global wind belts and current ocean patterns in all the oceans. Various weather disturbances, like cyclones, originate and derive their energy from the evaporation of seawater.
The solar energy is guided to the oceans through the atmosphere. Oceans provide moisture inputs to the atmosphere through evaporation, and atmosphere, in turn, supplies that moisture back to the oceans in the form of precipitation.
Conclusion
The interactions between the oceans and the atmosphere are of great importance because these inter-linkages support all the life forms inhabiting the earth. Still, a lot of research is being undertaken to explore the unknown facts about atmospheric and oceanic mechanisms.
Revised studies of atmospheric and oceanic circulation have also become more important at present due to climate change and global warming. There are already apparent observations regarding the impact of global warming on atmospheric phenomena, such as increased intensity and frequency of tropical cyclones around the world.
Thus, there is an immediate need to identify potential factors that inhibit the average circulation of air and ocean currents.
Image Credits
Featured Image: Ocean current- Free-Photos
Kolkata, India
M.Sc Geography
