Rivers in the Ocean
Ocean Currents and Climate
Learning Objectives
Explain how surface currents and deep currents form
Describe how ocean currents distribute heat and affect climate
Analyze the relationship between ocean currents and weather patterns
Rivers in the Ocean
~5 minutesRivers in the Ocean
Imagine rivers flowing through the ocean—not rivers of land, but massive flows of water moving through the sea itself. These are ocean currents, and they're some of the most powerful forces shaping our planet's climate.
The Gulf Stream, for example, moves more water than all the world's rivers combined—and it's just ONE current! These underwater rivers transport heat from the tropics toward the poles, making some places warmer (or cooler) than you'd expect based on their latitude alone.
Without ocean currents, much of Europe would be frozen tundra, and fisheries that feed billions would collapse.
Essential Question: How do ocean currents affect climate and weather patterns?
Surface Currents
~15 minutesSurface Currents
What Drives Surface Currents?
Surface currents are driven primarily by wind. As wind blows across the ocean surface, friction drags the water along. The top 100-400 meters of the ocean moves in response to persistent wind patterns.
Three Key Factors:
1. Global Wind Patterns
• Trade winds blow from east to west near the equator
• Westerlies blow from west to east in mid-latitudes
• These wind belts create predictable current patterns
2. The Coriolis Effect
• Earth's rotation causes moving objects (including water) to curve
• In the Northern Hemisphere: currents curve RIGHT
• In the Southern Hemisphere: currents curve LEFT
• This creates circular patterns called gyres
3. Continental Boundaries
• Continents block and redirect currents
• Water piles up against coasts, then deflects
• Creates the characteristic gyre patterns
Major Ocean Gyres
Ocean currents form five major circular patterns called gyres:
• North Atlantic Gyre (includes Gulf Stream)
• South Atlantic Gyre
• North Pacific Gyre
• South Pacific Gyre
• Indian Ocean Gyre
In the Northern Hemisphere, gyres rotate clockwise. In the Southern Hemisphere, gyres rotate counter-clockwise.
The Gulf Stream is one of Earth's mightiest currents:
• Moves 30 million cubic meters of water per second
• That's 100x the flow of the Amazon River!
• Transports warm tropical water northward along the US East Coast
• Continues across the Atlantic toward Europe
• Keeps Western Europe 5-10°C warmer than it would otherwise be
London is at the same latitude as Calgary, Canada—but London rarely sees the extreme cold that Calgary experiences!
Deep Ocean Currents
~15 minutesDeep Ocean Currents
Thermohaline Circulation
While wind drives surface currents, deep ocean currents are driven by differences in water density. This system is called thermohaline circulation (thermo = temperature, haline = salt).
What Affects Water Density?
Temperature:
• Cold water is denser than warm water
• Cold water sinks; warm water rises
Salinity (Salt Content):
• Salty water is denser than fresh water
• Salty water sinks; fresher water floats
The Global Conveyor Belt
Deep currents and surface currents connect to form a planet-wide circulation system called the global conveyor belt or thermohaline circulation.
How It Works: 1. Warm surface water flows toward the poles (like the Gulf Stream) 2. Near the poles, water cools and becomes denser 3. Sea ice forms, leaving salt behind, making water even saltier and denser 4. This cold, salty water sinks to the deep ocean 5. Deep water slowly flows toward the equator along the ocean floor 6. Eventually, deep water rises (upwells) and warms again 7. The cycle repeats
Time Scale: A single water molecule takes about 1,000 years to complete one full circuit!
Scientists are concerned that climate change could slow or disrupt the thermohaline circulation.
As ice sheets melt, they add fresh water to the North Atlantic. This freshwater is less dense and may not sink as effectively, potentially weakening the conveyor belt.
If this happens, Europe could actually get COLDER even as global temperatures rise!
Ocean Currents and Climate
~10 minutesOcean Currents and Climate
How Currents Affect Coastal Climate
Warm Currents → Warmer, Wetter Climate
• Warm water heats the air above it
• Warm air holds more moisture → more precipitation
• Example: Gulf Stream keeps Western Europe mild and rainy
Cold Currents → Cooler, Drier Climate
• Cold water cools the air above it
• Cool air holds less moisture → less precipitation
• Example: California Current keeps the US West Coast cooler and creates fog
Upwelling: Where Deep Meets Surface
Upwelling occurs when winds push surface water away from a coast, and cold, nutrient-rich deep water rises to replace it.
Effects of Upwelling:
• Brings nutrients from deep ocean to surface
• Creates extremely productive fishing grounds
• Cools coastal areas (cold, foggy summers)
• Examples: Peru coast, California coast, West Africa
The Peru upwelling zone is one of the world's richest fisheries!
El Niño and La Niña
El Niño ("The Boy"):
• Trade winds weaken or reverse
• Warm water spreads eastward across Pacific
• Disrupts normal weather patterns globally
• Peru: warmer water, fewer fish, more rain
• US: warmer winters in north, wetter in south
La Niña ("The Girl"):
• Trade winds strengthen
• Cold water spreads across central Pacific
• Opposite effects of El Niño
• More hurricanes in Atlantic
• Drought in southern US
North Carolina's Connection
The Gulf Stream flows just off the North Carolina coast, particularly near Cape Hatteras.
Effects on NC:
• Moderates coastal temperatures (warmer winters, cooler summers than inland)
• Creates productive fishing grounds (warm and cold water species meet)
• Influences hurricane behavior (warm water = fuel for storms)
• Cape Hatteras is called the "Graveyard of the Atlantic"
Summary
~5 minutesSummary
Key Concepts Review
Surface Currents:
• Driven by wind, Coriolis effect, and continental boundaries
• Form five major gyres (circular patterns)
• Clockwise in Northern Hemisphere, counter-clockwise in Southern
• Gulf Stream: major warm current affecting Eastern US and Western Europe
Deep Currents (Thermohaline Circulation):
• Driven by temperature and salinity differences
• Cold, salty water sinks near poles
• Forms global conveyor belt connecting all oceans
• One complete circuit takes ~1,000 years
Climate Effects:
• Warm currents → warmer, wetter climates
• Cold currents → cooler, drier climates
• Upwelling brings nutrients, creates rich fisheries
• El Niño/La Niña disrupt normal patterns globally
Check Your Understanding
What is the PRIMARY force that drives surface ocean currents?
The Coriolis effect causes ocean currents in the Northern Hemisphere to:
What are the circular ocean current patterns called?
What drives deep ocean currents (thermohaline circulation)?
Why does cold, salty water sink in the ocean?
How long does it take for water to complete one full circuit through the global conveyor belt?
The Gulf Stream keeps Western Europe warmer than expected because:
Upwelling is beneficial for fisheries because:
During an El Niño event:
Match each term with its correct description:
Match each current type with its climate effect:
Match each El Niño/La Niña characteristic:
Which factors affect water density in the ocean? (Select ALL that apply)
Select all that apply.
Put these steps of the global conveyor belt in order: