The following are the major currents of the South Pacific Ocean

The following are the major currents of the South Pacific Ocean:

1. Peru Current

2. East Australia Current

3. West Wind Drift

It may be pointed out that besides the above currents; the South Equatorial Current and the Equatorial Counter Current of this part of the Pacific Ocean have already been discussed in the beginning of the Currents of the Pacific Ocean”.

The Peru Current:

It is a cold current that flows along the west coast of South America from the south to the north. It is also called the Humbolt Current after the name of an eminent oceanographer who was the first discoverer of this current.

When this current flows close to the coasts of Peru and Chile, it is called the Peru Current. When this current flows at some distance from the coast, its name is changed as the Peru Oceanic Current.

This current originates in the areas near the Antarctica. The cold water mass of the Antarctic Ocean is carried by the westerlies towards the east, which bends near the west coast of South America towards the north. Thus, the Peru Current comes into existence.

On the basis of observations made by the ship “Discovery” it was found that the total transport by this cold current is estimated to be about 10-15 million m3/second, which comprises the upper surface water and the intermediate Antarctic water mass.

This current is much wider. Near 35°S latitude the current extends upto 900 kilometers from the coast. Before reaching to the Equator, the current deflects towards the west. However, this current flows with a slow speed.

Upwelling of cold and dense water from the deeper layers due to the prevailing westerlies is one of the characteristic features of the Peru Current.

According to Schott and Gunther, in certain parts of the current there is a rapid flow of cold water towards the surface of the ocean. The maximum upwelling occurs between 5°S and 15°S latitudes.

It may be noted that in the coastal areas of Peru and Chile a counter current flows from north to south at a depth of less than 100 meters. Remember that the upwelling of cold water reduces the salinity of the surface water.

However, northward from 25°S latitude there is a slight increase in the salinity.

Near the northern limit of this current certain characteristic changes occur due to changing seasons. In the northern summer, when the wind belts are displaced towards the north, the Peru Current crosses the Equator and converges with the Equatorial Counter Current.

But in winter of the northern hemisphere the Equatorial Counter Current extends southward and starts flowing near the Equador coast and joins the Peru Current after moving beyond the equator. This warm south-flowing current along the coast is called “EI Nino”.

EI Nino:

In January and the middle of February, but sometimes in March or April, the axis of the Equatorial Counter Current (which is supposed to lie to the north of the Equator) is displaced farther to the south.

As a result of this, the warm and saline water flowing along the coast of Equador converges with the cold Peru Current. However, its southern limit lays a few degrees to the south of the Equator. This south-flowing warm current is known as “EI Nino”.

The origin of El Nino involves two meteorological phenomena; First, the Walker Circulation and second, the Southern Oscillation. First, let us consider the Walker Circulation.

When strong surface winds blow, the wind stress on the surface waters of the ocean is strong enough to cause movement of surface water that is warm and light.

Then the warm and light surface waters must be replaced by upwelling of cold water from the deeper layers of the ocean. In this way sequence of events can be started by warm surface waters. Due to warm surface water the atmospheric instability is created.

Remember that whenever the lower layers of the atmosphere pass over a warm surface, the actual lapse rate exceeds the dry adiabatic lapse rate. Such a condition is conducive for making the atmosphere unstable.

Such an atmospheric condition favours the development of storms and high-speed winds, which in turn lead to upwelling of cold water. Due to cold water the unstable atmosphere becomes stable, because cooling from below results in atmospheric stability.

Stable atmospheric conditions immediately put a full stop to winds. A large scale example of this meteorological phenomenon is given by what is known as the Walker Circulation.

The aforementioned sequence of events takes place near the west coast of South America where offshore winds blow to cause upwelling of relatively cooler water.

By coming into contact with the cooler water, the atmosphere over that area becomes stable and is unable move upward to join the Hadley Cell circulation.

Instead, it starts flowing westward in the form of the south­east trade winds across the South Pacific to the relatively warmer West Pacific. In the western part of the Pacific Ocean, the prevailing winds gain both heat as well as moisture from the warm surface water.

Thus, heating from below makes the air unstable and it rises to join the Hadley cell. Flowing eastward it completes the cell.

Every 2-7 years such a disturbance disturbs the circulation pattern, and it results in the suppression of the trade winds. When the trade winds are suppressed, the upwelling of cooler water stops.

This leads to the replacement of cool water by the warm water. Such a periodic appearance of warm water is known as El Nino, which is a Spanish word, and its literal meaning is “the child”, and indirectly it stands for “the Child Christ” because of its coincidence with the Christmas period.

This unusually warm water comes from the equatorial region, as stated earlier. With the appearance of El Nino, the pressure over an extensive area of the South-east Pacific Ocean registers a fall, while the Western Pacific Ocean comes under higher pressure.

This is called the see-saw variation of air pressure. In Meteorology it is known as the Southern Oscillation. This sort of pressure arrangement results in the disappearance of the Equatorial Easterlies.

Now, the equatorial westerlies start flowing with full vigour, and drag warm surface waters towards the east. The warm waters after reaching the coast of South America are deflected towards the south along the coast of Peru and Equador.

This leads to the weakening of the Walker Circulation. The Hadley Circulation, as a result of this becomes stronger, and once again the trade winds start flowing with full force.

Now, warm waters from the western part of the Pacific do not flow across the eastern Pacific and the El Nino comes to an end. However, with the return of the colder water due to upwelling} the Hadley cell becomes weak. Due to the changed conditions, the warm waters return again.

Thus, the immediate cause of the El Nino event appears to be a general weakening of the south-east trade winds and an unusually southerly position of the Inter-tropical Convergence Zone.

Major EI Nino events occurred in 1891, 1925, 1941, 1957, 1965 and 1972. During the last occurrence in 1972, weather-satellite data were available on the basis of which the weather scientists made several investigations of this phenomenon.

In 1972 the occurrence of El Nino prompted a serious drought in the Sahelian region of North Africa, and an abnormal decrease in the amount of monsoon rains in the Indian Subcontinent.

According to Perry and Walker, the far-reaching effects of the sea-surface-temperature anomalies in the eastern Pacific, which are being investigated by empirical studies and computer simulations, may become clearer as the Global Atmospheric Research Programme progresses.

Let us hope that as a result of the joint endeavour of oceanographers and meteorologists the problem may be solved in near future.

It is to be noted that the El Nino event leads to disastrous catastrophes of oceanographic as well as meteoro­logical character. Besides, these events have important ecological and econo­mic implications.

As regards the mete­orological phenomena observed dur­ing the El Nino period, the tropical rain belt shifted a few degrees to the south with the result that there were very heavy downpours on the usually dry coast.

In March, 1925, the amount of rainfall at Trujillo situated at 8°S latitude amounted to 395 mm, whereas the average precipitation there hardly exceeds 4.4m. Such terrific down­pours caused disastrous floods and consequent soil erosion.

Enso:

The biological effects of the El Nino are twofold. First, the anchovies, that are the main food of the guano birds, disappear from the coastal region of Peru and Chile. This results in the death by starvation or migration of these birds.

Excess warming of the coastal water or the absence of upwelling may be the possible reasons. This causes heavy losses to guano industry. Secondly, there is the appearance of red water, which causes large scale mortality of fish.

Red water refers to the red colour of sea water due to the presence of excessive numbers of small planktons. Thus, due to El Nino the organisms in the coastal current, from plankton to fish are destroyed on a very large scale.

The beaches are covered with dead fish, where they decompose as a result of which both the air and the coastal waters produce the most obnoxious foul smell.

Generally, the salinity of the surface water near the coast between 5°S and 15°S latitudes is 35%o. But in the El Nino current the salinity is reduced to 33-34%₀. This decrease in the salinity is perhaps due to mixing of El Nino and the Peru Current.

As regards temperature, it is also reduced towards the south. This is also due to the mixing of El Nino water with that near the coast. However, the scientists have noted great irregularity in the periodicity of the El Nino event.

East Australia Current:

The warm current flowing along the east coast of Australia from north to south is called the East Australian Current. The South Equatorial Current is bifurcated in many branches near New Guinea, the East Austrian Current being the most important.

Flowing along the northern coast of New Guinea this current bends southward and flows along the east coast of Australia. Further, touching the coast of New Zealand it deflects towards the east and finally converges with the South Pacific Current.

The change in the direction of this current is accounted for by the strong westerlies and the deflective force produced by the rotation of the earth.

It is to be kept in mind that since the current flows from the Equatorial region, it is naturally warmer than the adjacent ocean waters, and its salinity is also higher.

The West Wind Drift:

In the absence of land masses, the westerlies blow with terrific speed between 40°S and 55°S latitudes. The cold currents generated by the drag of these strong winds, and flowing from west towards east are known as the West Wind Drift.

It is also given the name of the South Pacific Current. As a matter of fact, this cold current is just a part of the Antarctic Circumpolar Current. It extends from the Island of Tasmania situated in the south of Australia to the west coast of South America.

The velocity of this current, that is very high, is controlled by the Roaring Forties, i.e. the strong westerlies. At about 45°S latitude this current is divided into two branches; one branch flowing to the south of the Cape Horn enters into the South Atlantic Ocean.

While the other branch moves towards the north along the coast of South America and in the final stage converges with the Peru cold Current.