7.1 Ocean currents and global circulation patterns

Ocean Circulation Patterns

Global Conveyor Belt

Thermohaline circulation is the engine behind global ocean circulation. The name tells you what drives it: thermo (temperature) and haline (salinity). These two factors determine water density, and density differences are what make the whole system move.

Cold, salty water is denser, so it sinks. Warm, fresher water is less dense, so it rises. This constant sinking and rising creates a massive circulation loop that connects all major ocean basins: the Atlantic, Pacific, Indian, and Southern Oceans.

• A single parcel of water takes roughly 1,000 years to complete the full global circuit
• The conveyor belt redistributes heat, nutrients, and dissolved gases (like $CO_2$ and $O_2$) around the planet
• This heat redistribution is a major reason why some high-latitude regions have milder climates than you'd expect

Surface Currents and Gyres

Gyres are large systems of circular ocean currents formed by the interaction of global wind patterns and Earth's rotation (the Coriolis effect). There are five major gyres: North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean.

Not all currents within a gyre behave the same way. The contrast between western and eastern boundary currents is worth knowing:

• Western boundary currents are fast, narrow, and deep. They flow along the western edges of ocean basins and are intensified by the Coriolis effect and wind stress. Examples: Gulf Stream (Atlantic), Kuroshio Current (Pacific), Agulhas Current (Indian).
• Eastern boundary currents are slower, broader, and shallower. Examples: California Current (Pacific), Canary Current (Atlantic). These currents often bring cool water equatorward and are associated with coastal upwelling.

In the Northern Hemisphere, gyres rotate clockwise. In the Southern Hemisphere, they rotate counterclockwise. This pattern is a direct result of the Coriolis effect.

Deep Ocean Circulation

Deep ocean circulation is driven primarily by density differences rather than wind. In polar regions, surface water cools and becomes saltier (as sea ice forms, salt is left behind). This cold, dense water sinks and spreads along the ocean floor.

Two major deep water masses to know:

• North Atlantic Deep Water (NADW) forms in the Nordic and Labrador Seas and flows southward at depth
• Antarctic Bottom Water (AABW) forms around Antarctica and is the densest water mass in the ocean, spreading northward along the bottom

These deep water masses move slowly, taking centuries to millennia to circulate globally. When deep water eventually rises back to the surface through upwelling, it brings stored nutrients with it, fueling marine productivity.

Major Ocean Currents

Gulf Stream

The Gulf Stream is a fast, powerful western boundary current in the North Atlantic. It originates in the Gulf of Mexico, flows along the U.S. East Coast, and carries warm, salty water northward toward Europe.

• It's part of the larger North Atlantic Gyre circulation
• The Gulf Stream transports enormous amounts of heat from the tropics to higher latitudes, which is a key reason Western Europe has a much milder climate than you'd expect for its latitude (London is at roughly the same latitude as Calgary, Canada, yet has far warmer winters)
• Flow speeds can reach up to 2.5 m/s, and the current transports roughly 30 million cubic meters of water per second

El Niño and La Niña

El Niño and La Niña are opposite phases of the El Niño-Southern Oscillation (ENSO) cycle, a periodic fluctuation in sea surface temperatures and atmospheric pressure patterns across the equatorial Pacific.

During normal conditions, strong easterly trade winds push warm surface water toward the western Pacific, allowing cool, nutrient-rich water to upwell along the South American coast.

During El Niño:

1. Easterly trade winds weaken or even reverse
2. Warm surface water shifts toward the central and eastern Pacific
3. Sea surface temperatures in the eastern Pacific rise above average
4. Upwelling along South America weakens, reducing nutrient supply and harming fisheries
5. Weather impacts include increased rainfall in western South America and drought in Australia and Southeast Asia

During La Niña:

1. Easterly trade winds strengthen beyond normal
2. Warm water is pushed even more strongly toward the western Pacific
3. Sea surface temperatures in the eastern Pacific drop below average
4. Enhanced upwelling along South America boosts marine productivity
5. Weather impacts include increased rainfall in Australia and Southeast Asia and drier conditions in South America

ENSO events typically recur every 2 to 7 years and can disrupt weather patterns worldwide.

Physical Processes Affecting Currents

Coriolis Effect and Ekman Transport

The Coriolis effect is the apparent deflection of moving objects (including ocean currents and winds) caused by Earth's rotation. It deflects currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection is what shapes gyres and intensifies western boundary currents.

Ekman transport builds on this. When wind blows across the ocean surface, friction drags the water along, but the Coriolis effect deflects each successive layer of water slightly more than the one above it. The result is a spiral of decreasing current speed with depth (called the Ekman spiral). The net movement of the water column is perpendicular to the wind direction:

• 90° to the right of the wind in the Northern Hemisphere
• 90° to the left of the wind in the Southern Hemisphere

This perpendicular transport is what drives coastal upwelling and downwelling.

Upwelling and Downwelling

Upwelling is the rise of cold, nutrient-rich water from deeper layers to the surface. Here's how it works along a western continental coast (like Peru or California):

1. Winds blow parallel to the coastline (equatorward)
2. Ekman transport moves surface water offshore (away from the coast)
3. Deep water rises to replace the displaced surface water
4. This deep water carries nutrients like nitrate and phosphate to the sunlit surface layer
5. Phytoplankton thrive on these nutrients, supporting highly productive ecosystems and major fisheries

Downwelling is the opposite process. Surface water converges and sinks, typically along eastern coasts of continents or where wind-driven currents push water toward shore. Downwelling carries oxygen and organic matter to deeper waters but generally produces less biologically productive surface conditions than upwelling zones.