Ocean Currents: Moving Heat and Shaping Precipitation

Let's start with two towns

Happy Valley-Goose Bay in Labrador and London, England, are at nearly the same latitude (London is actually a little farther south, but only by about 2°). In January, London's average temperature is about 5 °C, with rain more often than snow. Goose Bay averages about −17 °C, with deep snow.

Part of the difference is the wind direction (Lesson 3): London receives mild air off the ocean, while Goose Bay often gets frigid air from the continent. But the ocean next to each city is also very different. One is bathed by water that came from the tropics; the other by water that came from the Arctic. Ocean currents are the climate system's great conveyor belts of heat.

Surface currents: pushed by the wind

The prevailing winds (Lesson 3) drag on the sea surface and set the top few hundred metres of water moving. The Coriolis effect turns the moving water (to the right in the Northern Hemisphere), and the continents block it and turn it further. The result is giant loops called gyres, which turn clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

As a result:

• Warm currents carry tropical water poleward along the east coasts of continents: the Gulf Stream off eastern North America and the Kuroshio off Japan.
• Cold currents carry polar water toward the equator: the Labrador Current off Labrador and Newfoundland, the California Current off the west coast of the U.S., and the Humboldt (Peru) Current off South America.

The Gulf Stream is one of the fastest ocean currents, with surface speeds that can exceed 2 m/s. It carries an enormous amount of heat from the Gulf of Mexico and Caribbean region northeastward. Its continuation, the North Atlantic Current (often called the North Atlantic Drift), helps keep western Europe much milder than other places at the same latitude.

How currents shape temperature and precipitation

Air moving over a current takes on its temperature and moisture.

• Warm currents warm the air above them and evaporate lots of water into it. Coasts downwind of warm currents tend to be milder and wetter.
• Cold currents cool the air above them. Cool air near the surface is stable (it doesn't rise easily), so few rain clouds form. Coasts next to cold currents can be cool, foggy and dry. Some of the driest deserts on Earth, such as the Atacama in Chile and the Namib in southwestern Africa, are on coasts with cold currents.
• Where warm, moist air flows over cold water, it cools to its dew point and fog forms. The Grand Banks off Newfoundland, where air that has passed over the Gulf Stream meets the cold Labrador Current, are among the foggiest places in the world.

On Canada's Pacific coast, the North Pacific Current brings relatively mild water toward British Columbia, and the westerlies carry mild, moist ocean air ashore. Combined with the mountains (Lesson 4), this gives coastal B.C. mild, rainy winters.

The deep ocean: the thermohaline circulation

Below the wind-driven surface layer, the ocean moves slowly because of density differences.

• Cold water is denser than warm water.
• Salty water is denser than fresh water. Average seawater has about 35 g of salt per kilogram.

In the far North Atlantic (in places such as the Labrador Sea and the seas near Greenland), surface water gives off heat to the frigid winter air and gets very cold. When sea ice forms, most of the salt is left behind in the water, making it saltier. This cold, salty water becomes dense enough to sink thousands of metres. Similar sinking happens around Antarctica.

The sinking water spreads slowly along the ocean floor, eventually rises (upwells) in other oceans, and returns along the surface. This worldwide loop is called the thermohaline circulation, or the "global conveyor belt". The Atlantic part of it, including the Gulf Stream, is called the Atlantic Meridional Overturning Circulation (AMOC).

Upwelling, El Niño and La Niña

Where winds push surface water away from a coast, cold, deep water wells up to replace it. This water is rich in nutrients, so upwelling zones support huge fisheries.

Across the equatorial Pacific, the trade winds usually push warm surface water west toward Asia and Australia, and cool water wells up near South America. Every 2 to 7 years or so, the trade winds weaken and warm water spreads east: this is El Niño. The opposite, with stronger trade winds and cooler water in the east, is La Niña.

These changes shift where thunderstorms form over the Pacific, which bends the jet stream and changes weather far away. In Canada, El Niño winters are often milder than usual, especially in western and central Canada. La Niña winters are often colder in western Canada. El Niño years also tend to be warmer globally, because the Pacific releases heat to the atmosphere.

Ocean currents in a changing climate

Scientists are watching several changes closely:

• The AMOC: meltwater from Greenland adds fresh water to the North Atlantic, and warming reduces cooling at the surface. Both make surface water less dense and less likely to sink. The IPCC assesses that the AMOC is very likely to weaken during this century. An abrupt collapse before 2100 is considered unlikely, but it can't be ruled out, and it would have major effects, including colder winters in northwestern Europe and changes in rainfall in the tropics.
• Marine heat waves: from about 2014 to 2016, a huge patch of unusually warm water, nicknamed "the Blob", sat in the northeast Pacific, off B.C. and Alaska. It was linked to die-offs of seabirds and changes in fish populations.
• Warmer currents carry warmer water to the poles, which helps melt sea ice and ice shelves from below.