AP Environmental Science

## Learning Outcomes

- Explain how **El Niño–Southern Oscillation (ENSO)** changes regional climate
- Compare normal Pacific conditions with **El Niño** and **La Niña**
- Identify typical U.S. winter effects in the Pacific Northwest, California, and the southern tier
- Describe global ENSO effects in Peru, Australia, Indonesia, India, and Africa
- Explain how altered trade winds, sea-surface temperatures, and jet streams create distant climate effects
- Apply ENSO patterns to fisheries, drought, flooding, wildfire risk, and hurricane activity
- Avoid common exam errors, such as treating ENSO impacts as guaranteed outcomes

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand regional effects of ENSO, with a focus on the following syllabus points:

- Know that ENSO has three phases: **El Niño**, **La Niña**, and neutral conditions
- Identify how normal easterly trade winds support upwelling off western South America
- Explain how weakened trade winds during El Niño reduce upwelling and shift rainfall patterns
- Explain how stronger trade winds during La Niña increase upwelling and reinforce opposite regional patterns
- Understand the typical U.S. winter pattern: **El Niño** tends to favor wetter southern conditions, while **La Niña** tends to favor wetter Pacific Northwest conditions
- Identify global regional effects, including wetter Peru during El Niño and wetter Australia and Indonesia during La Niña
- Understand how ENSO affects tropical cyclone patterns, especially Atlantic hurricanes
- Recognise that ENSO impacts are seasonal tendencies and changed probabilities, not guaranteed weather

## 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. How do normal trade winds affect upwelling off the coast of South America?
2. Why does El Niño often reduce fishery productivity off Peru?
3. What winter weather pattern is typically linked to La Niña in the Pacific Northwest and southern United States?
4. How does ENSO affect Atlantic hurricane activity?
5. Why are ENSO impacts described as probabilistic rather than certain?

## Introduction

ENSO is one of the most tested climate patterns in AP Environmental Science because it links ocean conditions to weather, environmentss, and human impacts across large parts of the world. Students often remember that El Niño is the “warm phase” and La Niña is the “cool phase,” but exam questions usually go further. You need to explain how a temperature change in the tropical Pacific can alter rainfall in Peru, drought risk in Australia, winter storms in the United States, and even hurricane activity in the Atlantic.

The key to doing this well is to think in a cause-and-effect chain. Changes in Pacific **sea-surface temperature** alter atmospheric circulation and tropical rainfall. Those atmospheric changes then shift the **jet stream** and storm tracks, which creates regional climate effects far from the equatorial Pacific. These long-distance links are what make ENSO so important.

> **Key Term: El Niño–Southern Oscillation (ENSO)**
> A recurring ocean-atmosphere pattern in the tropical Pacific with three phases: El Niño, La Niña, and neutral conditions.

### Normal Pacific conditions and why regional effects begin there

To understand regional effects, start with the normal state. Under neutral conditions, **easterly trade winds** push warm surface water westward across the tropical Pacific. That causes warm water to pile up near Indonesia and Australia. In the eastern Pacific, near Peru and Ecuador, the surface water is pulled away from the coast and replaced by colder, nutrient-rich water from below.

This process matters because ocean temperature is not just about heat. It affects nutrients, productivity, rainfall, and air pressure. The eastern Pacific under normal conditions has stronger **upwelling**, which supports phytoplankton growth. That supports fish populations, including commercially important fisheries off South America.

> **Key Term: upwelling**
> The movement of colder, nutrient-rich deep water to the ocean surface, often caused by winds that move surface water away.

The normal Pacific also has an uneven **thermocline**. The thermocline is shallower in the eastern Pacific and deeper in the western Pacific. A shallow eastern thermocline makes it easier for deep, nutrient-rich water to rise.

> **Key Term: thermocline**
> A boundary layer in water where temperature changes rapidly with depth.

Why does this matter for regional climate? Because the warmest ocean water drives the most evaporation and convection. Under normal conditions, that strongest rising air and heavy tropical rainfall are concentrated farther west, near Indonesia and Australia. If the warm water shifts, the rainfall belt shifts too.

A common student mistake is to memorise regional weather effects without understanding this starting point. If you understand normal conditions, the El Niño and La Niña patterns make much more sense.

> **Test Tip:** For FRQs, begin with the Pacific mechanism before naming distant effects. A response like “weaker trade winds reduce upwelling, shift warm water east, alter convection, then shift rainfall and jet-stream patterns” scores better than just listing wet and dry regions.

### El Niño: why the warm phase changes rainfall, fisheries, and U.S. winters

During **El Niño**, trade winds weaken and may partly reverse. Warm water that is normally piled up in the western Pacific spreads eastward into the central and eastern equatorial Pacific. As that happens, the thermocline in the eastern Pacific deepens and upwelling weakens.

That single change creates several linked effects.

First, the eastern Pacific becomes warmer at the surface but poorer in nutrients. Less nutrient supply means less phytoplankton, which reduces productivity through the food web. Fish populations may decline, and fishery yields off Peru can drop sharply. The important point is that the fishery problem is not caused by warmth alone. It is mainly caused by **reduced nutrient upwelling**.

Second, the area of strongest tropical convection and rainfall shifts eastward. Places in the eastern Pacific and parts of western South America, especially Peru and Ecuador, often become wetter than normal. Flooding risk rises. At the same time, Australia and Indonesia often become drier and warmer, raising drought and wildfire risk.

Third, atmospheric circulation changes reach beyond the Pacific through **teleconnections**.

> **Key Term: teleconnection**
> A climate link in which changes in one region, such as the tropical Pacific, affect weather far away.

In the United States, El Niño impacts are strongest and most reliable in the cool season, roughly October to March, especially in winter. A typical El Niño winter has a more southward-shifted jet stream. That tends to send more storms across the southern tier of the United States.

Typical U.S. winter El Niño effects include:

- wetter conditions across much of the southern U.S.
- wetter Gulf Coast and Southeast
- California often wetter, especially during stronger events
- Pacific Northwest often relatively drier and milder
- parts of the southern U.S. often cooler than average

Students often overstate this pattern by saying “El Niño makes California wet.” That is too absolute. The California signal is more reliable in stronger events, not every event.

ENSO also affects hurricanes by changing atmospheric conditions. El Niño generally suppresses Atlantic hurricanes because increased wind shear makes storm development harder there. At the same time, hurricane activity in the eastern and central Pacific often increases.

So the full El Niño chain looks like this:

warmer eastern Pacific SST anomaly -> weaker upwelling and shifted tropical rainfall -> altered jet stream and storm tracks -> regional wet/dry and warm/cool effects -> ecological and economic impacts.

> **Key Term: sea-surface temperature (SST) anomaly**
> The difference between observed ocean surface temperature and the long-term average for that region.

### La Niña: why stronger normal conditions create a different pattern

**La Niña** is not just “the opposite” in a simple one-for-one way, but it does strengthen the normal Pacific pattern. Trade winds become stronger than average, pushing even more warm water westward. That makes the eastern Pacific cooler than average and increases upwelling off South America.

The deeper significance is that La Niña increases nutrient supply in the eastern Pacific. That can support stronger marine productivity and help fisheries tied to upwelling systems. In APES, this is a good contrast with El Niño: El Niño lowers marine productivity off Peru; La Niña often raises it.

Because the warmest water and strongest convection stay farther west, Australia, Indonesia, and parts of Southeast Asia often become wetter than average during La Niña. Flood risk can rise there. India also often sees a stronger or wetter monsoon pattern than during El Niño years, though this varies.

The U.S. winter pattern also shifts. During La Niña, the Pacific jet stream is often positioned farther north. This tends to favour stormier weather in the Pacific Northwest and drier conditions across the southern tier.

Typical U.S. winter La Niña effects include:

- wetter and cooler Pacific Northwest
- warmer and drier southern U.S.
- drier conditions in much of southern California
- drier Gulf Coast and Southeast
- warmer-than-normal conditions across much of the southern tier
- in some years, wetter conditions in parts of the Ohio Valley or Upper Mississippi Valley

La Niña also tends to increase Atlantic hurricane activity, the reverse of El Niño. Reduced wind shear over the Atlantic makes conditions more favourable for hurricane formation and intensification. Meanwhile, eastern and central Pacific hurricane activity tends to be lower.

One subtle point students miss is that La Niña is not a mirror image in every region. Some places do show opposite tendencies, but local geography, season, and other climate drivers still matter.

> **Test Tip:** If a question asks you to predict conditions during La Niña, avoid writing “the exact opposite of El Niño everywhere.” Safer wording is “La Niña often strengthens normal Pacific conditions, tending to produce...” This sounds more scientific and avoids overclaiming.

### Regional effects by area: what to know for APES and how to compare them

For the exam, some regions come up again and again. It helps to organise them side by side.

#### Western South America

Under El Niño, coastal Peru and Ecuador often have warmer ocean water, weaker upwelling, heavier rain, and lower fishery productivity. Flooding risk rises on land, while offshore marine productivity falls.

Under La Niña, the eastern Pacific is cooler, upwelling is stronger, and fishery productivity often improves. The classic El Niño-style coastal flooding signal is less likely.

This is one of the clearest exam regions because it links ocean physics directly to ecology and the economy.

#### Australia and Indonesia

El Niño often brings drier and warmer conditions, increasing drought and wildfire risk. This can reduce agricultural output and stress water supplies.

La Niña often brings wetter conditions, which can improve water availability but also raise flood risk.

This comparison is useful because it shows that “wetter” is not always positive. Extra rainfall can recharge water supplies, but too much can cause erosion, flooding, and nutrient leaching from soils.

#### India and parts of South Asia

El Niño is often linked with a weaker or drier monsoon. That can reduce water available for farming and hydroelectric systems.

La Niña often supports a stronger or wetter monsoon. But again, more rain is not automatically good if it causes flooding.

#### Africa

Regional effects in Africa are more variable, but some broad tendencies are worth knowing. El Niño is often linked with drier conditions in southern Africa and wetter conditions in the Horn of Africa. La Niña often shifts many of these tropical rainfall patterns in the opposite direction.

This is a good example of why students must avoid simplistic global claims. ENSO effects differ by subregion, not continent-wide in one uniform way.

#### United States

The most testable U.S. pattern is winter.

El Niño:

- wetter southern tier
- wetter Gulf Coast and Southeast
- often wetter California in stronger events
- drier or milder Pacific Northwest

La Niña:

- wetter and cooler Pacific Northwest
- warmer and drier southern tier
- drier much of California and Gulf Coast

This pattern is strongest in winter. Outside winter, ENSO still matters, but the signals are often less reliable.

### Why ENSO impacts are probabilistic, seasonal, and sometimes messy

A top-scoring answer does not present ENSO as a weather guarantee. ENSO changes the odds of certain conditions. That means an El Niño winter is more likely to be wet in the southern U.S., but not every single place will be wetter on every date.

There are several reasons for this.

First, ENSO is a coupled ocean-atmosphere pattern, and event strength varies. Some El Niño events are weak; others are strong.

Second, timing matters. Effects in the U.S. are usually strongest in the cool season, not equally strong year-round.

Third, regional geography and other climate drivers matter. The **Indian Ocean Dipole**, background warming, and shorter-term weather patterns can strengthen or weaken ENSO impacts.

A good APES answer uses cautious scientific language such as “tends to,” “often,” or “increases the likelihood.” That reflects how seasonal outlooks work.

You may also see references to NOAA’s **Oceanic Niño Index (ONI)**, which tracks SST anomalies in the Niño 3.4 region.

> **Key Term: Oceanic Niño Index (ONI)**
> A measure of ENSO based on the 3-month running mean of sea-surface temperature anomalies in the Niño 3.4 region.

For APES, you do not usually need to memorise all technical thresholds, but knowing that El Niño and La Niña are identified using sustained SST anomalies is useful context.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- ENSO has three phases: El Niño, La Niña, and neutral conditions
- Normal easterly trade winds push warm water west and support upwelling off western South America
- El Niño weakens trade winds, deepens the eastern thermocline, and reduces upwelling
- Reduced upwelling during El Niño lowers nutrient supply and can reduce fishery productivity off Peru
- La Niña strengthens trade winds and usually increases eastern Pacific upwelling
- El Niño often brings wetter winters to the southern U.S. and drier or milder conditions to the Pacific Northwest
- La Niña often brings wetter, cooler conditions to the Pacific Northwest and warmer, drier conditions to the southern U.S.
- El Niño often brings wetter conditions to Peru and drier conditions to Australia and Indonesia
- La Niña often brings wetter conditions to Australia, Indonesia, and parts of South Asia
- El Niño usually suppresses Atlantic hurricanes, while La Niña usually increases Atlantic hurricane activity
- ENSO effects are strongest on seasonal timescales, especially U.S. winter
- ENSO impacts are probabilistic tendencies, not guaranteed weather outcomes

## Key Terms and Concepts

- El Niño–Southern Oscillation (ENSO)
- upwelling
- thermocline
- teleconnection
- sea-surface temperature (SST) anomaly
- Oceanic Niño Index (ONI)


El Niño and La Niña - Regional effects of ENSO