38. During El Niño:

(A) Cold water of the north-flowing Peru Current is displaced by a low-nutrient warm southward current

(B) Warm water of the north-flowing Peru Current is displaced by a low-nutrient cold southward current

(C) Cold water of the south-flowing Peru Current is displaced by a warm northward current rich in nutrients

(D) Warm water of the south-flowing Peru Current is displaced by a cold northward current rich in nutrients

During El Niño, How Does the Peru Current Change?

Understanding What Happens During El Niño

El Niño is a large-scale ocean–atmosphere phenomenon that develops in the tropical Pacific Ocean and causes major changes in ocean temperature, winds, rainfall, marine productivity, and global weather patterns. One of its most important effects occurs along the western coast of South America, particularly near Peru and Ecuador.

Under normal conditions, the western coast of South America is influenced by the cold Peru Current, also known as the Humboldt Current. This current carries cold water northward along the coast. The region is also characterized by strong coastal upwelling, through which cold, nutrient-rich water rises from deeper layers of the ocean toward the surface.

During El Niño, these normal oceanographic conditions are disrupted. Warm surface water spreads eastward across the equatorial Pacific and reaches the western coast of South America. This warm, relatively nutrient-poor water moves southward along the coast and displaces the normal cold, north-flowing Peru Current.

Therefore, during El Niño, the cold water of the north-flowing Peru Current is displaced by a low-nutrient warm southward current. Hence, Option (A) is correct.

The Normal Peru Current Before El Niño

The Peru Current is a cold ocean current that flows northward along the western coast of South America. It is one of the major eastern boundary currents of the world’s oceans and plays an essential role in maintaining the highly productive marine ecosystem of the region.

The cold nature of the Peru Current is associated with the movement of water from cooler southern latitudes toward the equator. In addition, prevailing winds help move surface water away from the coastline. This allows cold water from deeper ocean layers to rise toward the surface through a process called upwelling.

The deep water brought to the surface is rich in nutrients such as nitrates and phosphates. These nutrients support the rapid growth of phytoplankton, which form the base of the marine food web. As a result, the coastal waters of Peru are normally among the most biologically productive marine regions in the world.

The key features of the normal Peru Current are therefore very clear: it is cold, it flows northward, and the associated upwelling brings nutrient-rich water toward the ocean surface.

What Changes During El Niño?

During an El Niño event, the normal atmospheric circulation over the tropical Pacific weakens. The trade winds that usually push warm surface water toward the western Pacific become weaker, allowing warm water to spread eastward toward the coast of South America.

When this warm water reaches the coast of Peru and Ecuador, it creates a thicker layer of warm surface water. This suppresses the normal upward movement of cold, nutrient-rich deep water. As a result, coastal upwelling becomes weaker and the supply of nutrients to the surface decreases.

At the same time, the normal cold, north-flowing water along the Peruvian coast is displaced by warmer water moving southward. This warm water contains fewer nutrients than the deep water normally brought to the surface by upwelling.

Thus, the sequence during El Niño can be understood as follows:

Normal condition: Cold + northward flow + strong upwelling + nutrient-rich water

El Niño condition: Warm + southward flow + reduced upwelling + low-nutrient water

This is exactly the situation described in Option (A).

Why Does Nutrient Availability Decrease During El Niño?

The reduction in nutrients during El Niño is directly related to the suppression of coastal upwelling. Under normal conditions, deep ocean water contains large quantities of dissolved nutrients produced by the decomposition of sinking organic matter. When this deep water rises to the surface, it provides essential nutrients for phytoplankton growth.

During El Niño, warm surface water forms a deeper layer along the coast. This warm layer acts as a barrier that prevents nutrient-rich deep water from reaching the sunlit surface zone.

As nutrient availability decreases, phytoplankton productivity also declines. Since phytoplankton form the foundation of the marine food chain, this decline can affect zooplankton, fish, seabirds, and marine mammals.

This is why the warm water arriving during El Niño is described as low in nutrients. The problem is not simply the increase in water temperature; the major ecological consequence is the reduction of nutrient supply caused by weakened upwelling.

Detailed Explanation of Each Option

Option (A): Cold Water of the North-Flowing Peru Current Is Displaced by a Low-Nutrient Warm Southward Current

Option (A) is correct. Under normal conditions, the Peru Current carries cold water northward along the western coast of South America. This region is also associated with the upwelling of cold, nutrient-rich deep water.

During El Niño, warm surface water reaches the eastern Pacific and moves southward along the South American coast. This warm water displaces the normal cold water and suppresses nutrient-rich coastal upwelling.

Therefore, all three important features given in Option (A) are correct: the normal Peru Current is cold, it flows northward, and during El Niño it is displaced by warm, low-nutrient water moving southward.

Hence, Option (A) is the correct answer.

Option (B): Warm Water of the North-Flowing Peru Current Is Displaced by a Low-Nutrient Cold Southward Current

Option (B) is incorrect because it reverses the temperature characteristics of both the normal Peru Current and the El Niño water.

The Peru Current is not a warm current. It is a cold ocean current that normally flows northward along the western coast of South America. During El Niño, it is not displaced by cold water. Instead, unusually warm water spreads toward the eastern Pacific and along the South American coast.

Therefore, although the option correctly refers to the Peru Current as north-flowing, its description of the water temperatures is reversed. The normal current should be described as cold, while the El Niño current should be described as warm.

Option (C): Cold Water of the South-Flowing Peru Current Is Displaced by a Warm Northward Current Rich in Nutrients

Option (C) is incorrect for three important reasons. First, the Peru Current does not normally flow southward; it flows northward along the western coast of South America.

Second, the warm water associated with El Niño does not represent a nutrient-rich northward current. The anomalous warm water moves southward along the coast.

Third, El Niño generally reduces the availability of nutrients in surface waters because warm water suppresses the normal upwelling of cold, nutrient-rich deep water. Therefore, describing the El Niño water as rich in nutrients is also incorrect.

Thus, this option incorrectly describes the direction of the Peru Current, the direction of the anomalous warm current, and the nutrient status of the water.

Option (D): Warm Water of the South-Flowing Peru Current Is Displaced by a Cold Northward Current Rich in Nutrients

Option (D) is incorrect because it essentially describes the reverse of what happens during El Niño.

The normal Peru Current is neither warm nor south-flowing. It is a cold, north-flowing current. During El Niño, this cold-water system is disrupted by the arrival of warm, relatively nutrient-poor water.

The option also suggests that cold, nutrient-rich water becomes more prominent during El Niño. In reality, El Niño suppresses the upwelling of cold, nutrient-rich deep water. Therefore, the surface ocean becomes warmer and less productive.

For these reasons, Option (D) is completely opposite to the actual oceanographic changes observed during an El Niño event.

Ecological Effects of El Niño on the Peruvian Coast

The displacement of cold, nutrient-rich water by warm, low-nutrient water has major ecological consequences. The reduction in nutrient availability causes a decline in phytoplankton production. Because phytoplankton are the primary producers of marine ecosystems, this decline affects the entire food web.

Fish populations that depend directly or indirectly on plankton may decrease or move to other regions in search of suitable conditions. Seabirds and marine mammals that depend on these fish can also be affected.

The Peruvian coastal ecosystem is normally extremely productive because of strong upwelling. Therefore, when El Niño weakens this upwelling, the ecological effects can be especially significant.

Relationship Between El Niño and Coastal Upwelling

Coastal upwelling is central to understanding this question. Normally, winds help move surface water away from the coast of Peru. Cold water from deeper layers then rises to replace it.

This deep water is rich in nutrients, making the coastal ecosystem highly productive. During El Niño, the arrival of a thick layer of warm surface water and changes in atmospheric circulation weaken this normal upwelling system.

As a result, fewer nutrients reach the surface. The water becomes warmer, nutrient concentrations decrease, and biological productivity falls.

Therefore, the term low-nutrient warm southward current in Option (A) accurately describes the major oceanographic change occurring along the South American coast during El Niño.

El Niño and the Peru Current: Key Concept

The easiest way to understand the phenomenon scientifically is to compare normal conditions with El Niño conditions.

Under normal conditions, the western coast of South America experiences a cold northward Peru Current, strong upwelling, abundant nutrients, and high marine productivity.

During El Niño, warm water moves southward, the normal cold-water system is displaced, coastal upwelling weakens, nutrient availability decreases, and marine productivity declines.

Therefore, the central concept tested in this question is the replacement of the cold, north-flowing Peru Current by a warm, low-nutrient southward current during El Niño.

Final Answer

During El Niño, warm surface water reaches the eastern Pacific and moves southward along the western coast of South America. This warm water displaces the normal cold, north-flowing Peru Current and suppresses the upwelling of cold, nutrient-rich deep water.

As a result, the coastal water becomes warmer and relatively poor in nutrients.

Therefore, the correct answer is:

Correct Option: (A) Cold water of the north-flowing Peru Current is displaced by a low-nutrient warm southward current