AP Environmental Science

## Learning Outcomes

- Distinguish between **surface currents** and **deep ocean currents**
- Explain how ocean currents redistribute heat and shape regional climate
- Describe how **upwelling** affects coastal climate and marine productivity
- Compare the climate effects of warm currents and cold currents using named examples
- Explain how **El Niño** and **La Niña** alter trade winds, upwelling, and weather patterns
- Understand how climate change can alter ocean circulation, including possible weakening of the **AMOC**
- Apply ocean-current ideas to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how ocean circulation affects climate, with a focus on the following syllabus points:

- Know that ocean currents redistribute heat around Earth and influence regional and global climate
- Identify the difference between **surface currents** driven by wind and the Coriolis effect, and **deep currents** driven by temperature and salinity
- Explain how warm currents can make nearby coasts warmer and wetter, while cold currents can make nearby coasts cooler, drier, and foggier
- Understand the cause and importance of **upwelling**, especially for fisheries and coastal productivity
- Describe the role of the **Gulf Stream**, **California Current**, and other named currents in shaping climate
- Explain how **El Niño** and **La Niña** change trade winds, sea-surface temperatures, and upwelling
- Understand how warming and freshwater input can weaken Atlantic overturning circulation and alter regional climate
- Distinguish long-term climate effects from short-term weather events

## 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. What is the difference between a surface current and a deep ocean current?
2. How does the Gulf Stream affect the climate of western Europe?
3. Why does upwelling usually increase marine productivity?
4. What happens to upwelling off western South America during an El Niño?
5. How could melting ice in Greenland affect Atlantic circulation?

## Introduction

Ocean currents are one of Earth's major heat-transfer systems. The atmosphere moves heat through winds and weather systems, but the oceans also store and transport huge amounts of thermal energy. That matters for climate because even modest changes in ocean circulation can shift temperatures, rainfall, fog, storm tracks, and marine food webss over large areas.

For AP Environmental Science, ocean currents are not just a geography fact to memorize. They are part of **Earth system interactions**: wind affects ocean movement, ocean movement affects climate, climate change affects ocean circulation, and those circulation changes can then affect biodiversity, fisheries, and sea level.

> **Key Term: ocean currents**
> Continuous, directed movements of ocean water that redistribute heat, nutrients, and dissolved substances around Earth.

## Introduction to ocean circulation

Ocean circulation works because water is always responding to differences in energy and density. Near the surface, winds push water across the ocean. At depth, colder and saltier water tends to sink while warmer and less salty water tends to stay higher. Together, these motions connect the world's oceans into a climate-regulating system.

> **Key Term: heat transport**
> The movement of thermal energy from one place to another, such as from warm equatorial waters toward cooler higher latitudes.

A common exam mistake is to think of the ocean as mostly still water. In reality, the ocean is moving at many scales all the time. This movement matters because water has a high heat capacity, so oceans store large amounts of heat and release it slowly. That is one reason coastal regions often have milder climates than inland regions.

### Surface currents: wind, Coriolis effect, and gyres

Surface currents are driven mainly by **global wind patterns**, the **Coriolis effect**, and the positions of continents. Winds push surface water, but they do not send it in perfectly straight lines because Earth's rotation causes deflection.

> **Key Term: Coriolis effect**
> The apparent deflection of moving air or water caused by Earth's rotation: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Because of this deflection, large circular systems called **gyres** form in the major ocean basins.

> **Key Term: gyre**
> A large circular system of surface ocean currents, usually clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Why does this matter for climate? Because surface currents move warm and cold water horizontally. If warm water flows along a coastline, it transfers heat to the overlying air, which can warm nearby land. If cold water flows along a coast, it cools the air above it and can create cooler, more stable coastal conditions.

A useful comparison:

- **Warm currents**: usually warm nearby coasts, often increase humidity, and can make winters milder
- **Cold currents**: usually cool nearby coasts, often reduce evaporation, and can make climates drier or foggier

The **Gulf Stream** is the standard AP example of a warm surface current. It carries warm water northward along the east coast of the United States and across the Atlantic. That heat transfer helps keep parts of western Europe much warmer than they would otherwise be at those latitudes.

The **California Current** is a standard cold-current example. It flows southward along the west coast of North America and helps create cooler coastal conditions.

Why students miss this: they memorize "Gulf Stream = warm" and "California Current = cold" but do not explain the mechanism. The mechanism is heat exchange between ocean and atmosphere. Warm water warms the air above it; cold water cools it.

> **Test Tip:** If a question asks why two places at similar latitudes have different climates, think first about ocean currents, prevailing winds, and distance from the ocean.

### Deep currents and thermohaline circulation

Deep ocean currents are different from surface currents because they are mainly **density-driven**, not wind-driven. Density depends mostly on **temperature** and **salinity**.

- Colder water is denser than warmer water
- Saltier water is denser than fresher water

This is why very cold, salty water can sink and begin deep-ocean flow.

> **Key Term: thermohaline circulation**
> Global deep-ocean circulation driven by differences in water temperature and salinity, which affect density.

The name helps:

- **thermo** = temperature
- **haline** = salt

This deep circulation is often called the **global ocean conveyor belt**. In the Atlantic, warm, salty surface water moves northward. As it reaches higher latitudes, it loses heat to the atmosphere. Cooling increases its density. If it is salty enough, it sinks and flows back southward at depth. Later, in other ocean basins, deep water can gradually rise again.

This circulation is slow on a human timescale but huge on a climate scale. It helps regulate polar heat supply and sea-ice patterns. Because the oceans store so much heat, even a modest circulation shift can have large climate consequences.

A good AP distinction:

- **Surface current** driver = wind + Coriolis + continent shape
- **Deep current** driver = density differences from temperature and salinity

Students often confuse the two and say deep currents are caused by wind. That is incorrect.

### Warm currents, cold currents, and regional climate

The climate effects of currents become easiest to understand when you compare places.

The **Gulf Stream** and the North Atlantic Drift move warm water toward the North Atlantic. This moderates climates in parts of western Europe. Without that heat transfer, western Europe would be much colder for its latitude.

The **California Current** brings colder water southward along western North America. This contributes to cooler coastal weather than many inland places at similar latitudes. Cold currents can also help produce fog if warm, moist air moves over cold water and cools to its dew point.

Other useful cold-current examples include:

- **Peru (Humboldt) Current** along western South America
- **Benguela Current** along southwestern Africa

These cold currents are linked to cool coastal conditions and, in some places, dry climates near the coast.

A subtle but important point: cold currents do not always mean low biological productivity. In fact, many cold-current coasts are highly productive because they are associated with upwelling.

### Upwelling: climate and fisheries

Upwelling happens when winds move surface water away from a coast or away from a surface area, allowing deeper water to rise and replace it.

> **Key Term: upwelling**
> The upward movement of cold, nutrient-rich deep water to the ocean surface.

This deep water is colder, so upwelling can cool nearby coastal regions. But its biggest ecological effect is nutrient supply. Deep water often contains nitrates and phosphates released as dead organisms decompose and sink. When that water reaches the surface, phytoplankton growth increases. More phytoplankton means more food for zooplankton, then fish, then larger predators.

Cause-and-effect chain:

- Coastal winds push surface water away
- Deep water rises
- Nutrients increase at the surface
- Phytoplankton increase
- Fishery productivity rises

That is why upwelling zones often support major fisheries.

The AP trap here is confusing upwelling with sinking. Upwelling is rising water, not descending water.

> **Test Tip:** If an FRQ asks why a fishery declined after an ocean-circulation change, connect it to nutrient supply. Less upwelling means fewer nutrients, less phytoplankton, and then fewer fish.

### ENSO: El Niño, La Niña, and climate disruption

The **El Niño–Southern Oscillation (ENSO)** is a major ocean-atmosphere pattern in the tropical Pacific. It changes normal current and wind conditions and can affect weather far from the Pacific.

Under normal Pacific conditions:

- Trade winds push warm surface water westward
- Warm water piles up in the western Pacific
- Colder, nutrient-rich water upwells near western South America

During **El Niño**:

- Trade winds weaken
- Warm water shifts eastward
- Upwelling off western South America decreases
- Surface waters in the eastern Pacific become warmer

During **La Niña**:

- Trade winds strengthen
- Warm surface water piles farther west
- Upwelling near western South America increases
- Eastern Pacific surface waters become cooler than normal

Why this matters:

- **El Niño** often reduces fisheries off Peru because reduced upwelling means reduced nutrients
- **La Niña** often strengthens those fisheries because enhanced upwelling increases nutrients

These Pacific changes also alter atmospheric circulation and jet-stream patterns, producing **teleconnections**, meaning climate effects far from their origin.

> **Key Term: El Niño–Southern Oscillation (ENSO)**
> A recurring pattern in the tropical Pacific involving El Niño, La Niña, and neutral conditions that changes sea-surface temperatures, winds, and weather patterns.

Students often say "El Niño means warmer weather everywhere." That is too vague and usually wrong. El Niño means warmer-than-normal sea-surface conditions in the eastern and central tropical Pacific, plus linked atmospheric changes.

### Climate change and ocean circulation change

Climate change is affecting ocean currents because it is changing both **ocean heat content** and **salinity patterns**. As oceans absorb more heat, sea-surface temperatures rise. Since warmer water cannot absorb as much carbon dioxide as cooler water, this can also weaken one part of the ocean's climate-regulating role.

A major Atlantic system to know is the **AMOC**.

> **Key Term: Atlantic Meridional Overturning Circulation (AMOC)**
> A major Atlantic circulation system in which warm, salty surface water moves northward, cools, sinks, and returns southward at depth.

The concern is not that collapse is guaranteed. The exam-safe idea is more careful: warming and extra freshwater can weaken overturning.

Cause-and-effect chain:

- Climate change increases precipitation and ice melt
- More freshwater enters the North Atlantic
- Salinity drops
- Surface water becomes less dense
- Sinking becomes weaker
- AMOC may weaken

This matters because the AMOC helps move heat northward. If it weakens, regional climate patterns around the North Atlantic can shift. Western Europe may receive less ocean-delivered heat than before relative to the global average. A weaker AMOC can also contribute to regional sea-level differences, including extra sea-level rise along parts of the U.S. East Coast.

Be careful not to overstate this. The key AP point is **slowdown risk**, not certain shutdown.

### How ocean currents connect to wider global change

Ocean currents and climate link to many other Unit 9 topics:

- **Polar warming**: currents help move heat toward high latitudes
- **Sea level**: ocean warming causes thermal expansion, and circulation changes can affect regional sea level
- **Ocean acidification**: warmer water holds less dissolved gas and changing circulation affects carbon distribution
- **Biodiversity**: species distributions, fisheries, and breeding conditions depend on water temperature and nutrient supply
- **Storms**: sea-surface temperature patterns influence hurricane intensity and rainfall patterns

This is why APES often frames ocean currents as an interaction between Earth systems rather than as a stand-alone fact list. The hydrosphere, atmosphere, biosphere, and climate system are all connected.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ocean currents redistribute heat around Earth and strongly affect climate
- Surface currents are driven mainly by winds, the Coriolis effect, and continent shape
- Deep currents are driven mainly by density differences caused by temperature and salinity
- Warm currents usually make nearby coasts warmer and often wetter
- Cold currents usually make nearby coasts cooler and often drier or foggier
- The Gulf Stream is a warm current that moderates climates of the eastern United States and western Europe
- The California Current is a cold current that cools the west coast of North America
- Upwelling brings cold, nutrient-rich deep water to the surface and supports productive fisheries
- El Niño weakens trade winds and reduces upwelling off western South America
- La Niña strengthens trade winds and increases upwelling off western South America
- Climate change can alter ocean circulation by warming water and adding freshwater to key regions
- Freshwater input to the North Atlantic can reduce salinity and density and may weaken the AMOC

## Key Terms and Concepts

- ocean currents
- heat transport
- Coriolis effect
- gyre
- thermohaline circulation
- upwelling
- El Niño–Southern Oscillation (ENSO)
- Atlantic Meridional Overturning Circulation (AMOC)


Global Climate Change - Ocean currents and climate