AP Environmental Science

## Learning Outcomes

- Understand how **ENSO** links ocean temperatures and atmospheric pressure in the tropical Pacific
- Identify the normal Pacific pattern before El Niño or La Niña develops
- Explain how changes in trade winds affect upwelling and the thermocline
- Compare El Niño and La Niña as opposite phases of the same climate system
- Describe environmental effects on fisheries, rainfall, storms, and natural habitatss
- Interpret basic ENSO monitoring ideas such as **Niño 3.4** and the **Oceanic Niño Index**

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand Pacific temperature oscillations, with a focus on the following syllabus points:

- Know that **ENSO** is a coupled ocean-atmosphere climate pattern with El Niño, La Niña, and neutral phases
- Identify normal Pacific conditions, including westward-blowing trade winds and upwelling near South America
- Explain how weakened trade winds during El Niño reduce upwelling and deepen the eastern Pacific thermocline
- Explain how strengthened trade winds during La Niña increase upwelling and cool the eastern Pacific
- Understand how ENSO changes marine productivity, especially fisheries off Peru and Ecuador
- Recognise that the **Southern Oscillation** is the atmospheric pressure component of ENSO
- Understand that ENSO influences weather far from the Pacific through **teleconnections**
- Identify **Niño 3.4** sea surface temperature anomalies as a standard way to monitor ENSO conditions

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Under normal Pacific conditions, why does upwelling occur near the west coast of South America?
2. How do trade winds change during El Niño and during La Niña?
3. Why do fisheries off Peru often decline during El Niño events?
4. What is the difference between **ENSO** and the **Southern Oscillation**?
5. How can warmer or cooler Pacific water affect weather in distant regions such as the United States or Australia?

## Introduction

Pacific temperature oscillations are a major part of AP Environmental Science because they connect the atmosphere, oceans, climate, and natural systemss in one system. Questions on this topic often test cause-and-effect chains: wind changes ocean temperature, ocean temperature changes upwelling, upwelling changes nutrient supply, and nutrient supply changes food webs and fisheries.

You need more than the labels **El Niño** and **La Niña**. The exam often rewards students who can explain the sequence clearly from normal conditions to changed conditions, and then link those changes to weather, biodiversity, and human impacts.

> **Key Term: ENSO**
> El Niño–Southern Oscillation, a coupled ocean-atmosphere climate pattern in the tropical Pacific with three phases: El Niño, La Niña, and neutral.

### Normal Pacific conditions: the pattern you must know first

Before you can understand El Niño or La Niña, you must know the baseline pattern. Under normal, or **ENSO-neutral**, conditions, the **trade winds** blow from east to west across the equatorial Pacific. These winds push warm surface water toward the western Pacific, near Indonesia and Australia.

That westward push has two major effects. First, the western Pacific becomes warmer and sea level there becomes slightly higher because warm water piles up. Second, along the west coast of South America, surface water is pulled away and replaced by colder deep water. That process is **upwelling**.

> **Key Term: trade winds**
> Persistent easterly winds in the tropics that normally blow from east to west across the Pacific.
>
> **Key Term: upwelling**
> The movement of cold, nutrient-rich deep water to the ocean surface, often where winds move surface water away.

Upwelling matters because deep water contains nutrients such as nitrates and phosphates from decomposed organic matter. When these nutrients reach the surface, they support phytoplankton. Phytoplankton are the base of marine food webs, so strong upwelling usually supports high fish productivity.

A related idea is the **thermocline**, the boundary between warmer surface water and colder deeper water. In normal Pacific conditions, the thermocline is deeper in the western Pacific and shallower in the eastern Pacific. A shallow eastern thermocline makes it easier for cold nutrient-rich water to rise.

This is the point many students miss: normal conditions already include eastern Pacific upwelling. El Niño is not the “start” of upwelling. It is the weakening of the normal upwelling system.

> **Key Term: thermocline**
> The boundary between warm surface water and colder deep water in a body of water.
>
> **Test Tip:** If an APES question asks what changes during El Niño, start by stating the normal condition first. That prevents a very common error: describing El Niño as if it were the baseline Pacific pattern.

### El Niño: warm phase of ENSO

**El Niño** is the warm phase of ENSO. During El Niño, sea surface temperatures in the central and eastern equatorial Pacific become warmer than average. The main driver is a weakening, and sometimes reversal, of the trade winds.

When the trade winds weaken, they no longer push as much warm water westward. Warm surface water then spreads eastward across the Pacific. This deepens the thermocline in the eastern Pacific. Once the thermocline is deeper, it becomes much harder for cold deep water to reach the surface near Peru and Ecuador. Upwelling is reduced or may nearly stop.

That one physical change creates a chain of biological effects:

1. Less upwelling means fewer nutrients reach surface waters.
2. Fewer nutrients mean lower phytoplankton productivity.
3. Lower phytoplankton means less food for zooplankton and fish.
4. Fish populations and fishery yields can decline.

This is why El Niño can damage fisheries off western South America. The water may be warmer, but warmth alone does not support high fish abundance. Nutrient supply is the key.

> **Key Term: El Niño**
> The warm phase of ENSO, when the central and eastern tropical Pacific has warmer-than-average sea surface temperatures and reduced upwelling.

The atmospheric part of this change is called the **Southern Oscillation**. Pressure patterns across the tropical Pacific shift, altering winds and rainfall. Because the ocean and atmosphere affect each other, ENSO is not just an ocean temperature event. It is a coupled system.

> **Key Term: Southern Oscillation**
> The atmospheric pressure changes across the tropical Pacific that are linked to El Niño and La Niña.

El Niño also changes weather in distant places through **teleconnections**. In APES terms, these are long-distance climate links caused by shifts in circulation, storm tracks, and jet streams. Typical patterns include wetter conditions in parts of western South America and drier conditions in Australia and Indonesia. In the United States, El Niño is often linked with wetter conditions in parts of the southern U.S. and California, though exact outcomes vary by event.

> **Key Term: teleconnections**
> Climate links in which changes in one region, such as Pacific sea surface temperature, affect weather patterns far away.

El Niño can also reduce hurricane activity in some Atlantic seasons because changed atmospheric circulation may make conditions less favorable there. On the exam, treat this as a tendency, not a guarantee.

A simple cause-and-effect model is:

- Trade winds weaken
- Warm water shifts east
- Eastern thermocline deepens
- Upwelling decreases
- Nutrient supply falls
- Fisheries decline
- Rainfall and storm tracks shift

### La Niña: cool phase of ENSO

**La Niña** is the cool phase of ENSO and is broadly the reverse of El Niño. During La Niña, trade winds become stronger than normal. This pushes even more warm surface water toward the western Pacific and increases the contrast between west and east.

In the eastern Pacific, the thermocline becomes even shallower than normal. Because cold deep water lies closer to the surface, upwelling becomes stronger. That means more nutrients, more phytoplankton, and often stronger marine productivity off western South America.

> **Key Term: La Niña**
> The cool phase of ENSO, when the central and eastern tropical Pacific has cooler-than-average sea surface temperatures and enhanced upwelling.

This is why La Niña is often associated with improved fisheries near Peru compared with El Niño years. Again, the logic is nutrient-driven, not simply temperature-driven.

La Niña also affects rainfall and storms in distant regions. Typical patterns include wetter conditions in Australia and Indonesia and, in many cases, warmer and drier conditions in parts of the southern United States while the Pacific Northwest may be wetter. Some APES materials also note increased hurricane activity during La Niña years.

Students often oversimplify La Niña as “bad weather.” That is not precise enough. It is a climate phase with mixed effects: some regions become wetter, others drier; some food webs benefit, others may be stressed.

To compare the phases clearly:

- **Normal:** normal trade winds, eastern upwelling, cool eastern Pacific
- **El Niño:** weaker trade winds, weaker upwelling, warmer eastern Pacific
- **La Niña:** stronger trade winds, stronger upwelling, cooler eastern Pacific

This comparison is one of the most testable distinctions in the topic.

> **Test Tip:** In multiple-choice questions, eliminate any option that says El Niño increases upwelling off Peru. That is a classic trap. El Niño suppresses upwelling; La Niña enhances it.

### Why Pacific oscillations matter for natural systemss, climate, and APES data

ENSO matters in AP Environmental Science because it shows how physical Earth systems affect living systems and human societies. This topic connects Unit 4 to later units on pollution, biodiversity, agriculture, and global change.

First, marine marine lifes respond strongly to nutrient changes. During El Niño, lower nutrient supply can disrupt food webs, reduce fish catches, and alter animal migration. Some species may move to cooler or more suitable water, while others may suffer population declines. Coral reefs can also be stressed by unusually warm water, increasing bleaching risk.

Second, rainfall changes affect land natural systemss. Drier-than-normal conditions can increase drought, wildfire risk, crop failure, and competition for water. Wetter-than-normal conditions can increase flooding, erosion, and nutrient leaching from soils. In APES, always connect changed precipitation to ecological or agricultural consequences.

Third, warmer ocean water holds less dissolved gas, including less carbon dioxide than cooler water. That means very warm ocean conditions can influence carbon exchange between ocean and atmosphere. For APES, keep the statement careful: warmer water tends to hold less CO₂, which can matter for climate feedbacks.

ENSO also appears in data questions. Two terms may show up:

> **Key Term: Niño 3.4 region**
> A central equatorial Pacific region, from 5°N to 5°S and 120°W to 170°W, commonly used to monitor ENSO sea surface temperature anomalies.
>
> **Key Term: Oceanic Niño Index (ONI)**
> A standard ENSO monitoring index based on the 3-month running mean of sea surface temperature anomalies in the Niño 3.4 region.

At APES level, you do not need advanced climate statistics. What matters is knowing what these terms measure. If ONI is positive enough for long enough, that supports El Niño conditions. If it is negative enough for long enough, that supports La Niña conditions.

A student mistake is to confuse ONI with the **Southern Oscillation Index**. They are not the same. ONI tracks sea surface temperature anomalies. The Southern Oscillation Index tracks atmospheric pressure differences.

Another essential distinction: ENSO is natural short-term climate variability, not the same thing as long-term climate change. Climate change may affect how ENSO-related rainfall extremes play out, but ENSO itself is a recurring natural pattern.

If a free-response question asks for consequences, build your answer as a sequence, for example:

- El Niño weakens trade winds.
- Upwelling decreases.
- Nutrient levels at the surface decline.
- Phytoplankton productivity decreases.
- Fish populations decline.
- Fisheries off Peru suffer economic losses.

That sequence shows mechanism, not just memorized facts.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- **ENSO** is a coupled ocean-atmosphere climate pattern with neutral, El Niño, and La Niña phases
- Under normal Pacific conditions, trade winds blow east to west and push warm surface water toward the western Pacific
- Normal conditions include strong upwelling of cold, nutrient-rich water near the west coast of South America
- The thermocline is normally deeper in the western Pacific and shallower in the eastern Pacific
- **El Niño** is the warm phase, with weaker trade winds and reduced eastern Pacific upwelling
- Reduced upwelling during El Niño lowers nutrient supply and can reduce fisheries off Peru and Ecuador
- **La Niña** is the cool phase, with stronger trade winds and enhanced eastern Pacific upwelling
- Enhanced upwelling during La Niña often supports higher marine productivity off western South America
- The **Southern Oscillation** is the atmospheric pressure component linked to ENSO
- ENSO affects distant weather through **teleconnections**, shifting rainfall, storm tracks, and seasonal climate patterns
- ENSO is natural short-term climate variability, not the same as long-term climate change
- **Niño 3.4** and the **Oceanic Niño Index** are standard ways to monitor ENSO-related sea surface temperature anomalies

## Key Terms and Concepts

- ENSO
- trade winds
- upwelling
- thermocline
- El Niño
- Southern Oscillation
- teleconnections
- La Niña
- Niño 3.4 region
- Oceanic Niño Index (ONI)


El Niño and La Niña - Pacific temperature oscillations