What if there were no ocean currents?

Ocean water is always on the move. Some of it flows like a giant river in the sea.

What if all that moving water stood still?

The ocean is never still. Huge streams of water called currents flow through it, some at the surface and some far below. NOAA says currents are driven by three main things: the tides, the wind, and differences in how heavy the water is, which depend on its temperature and saltiness.

So what if the ocean's currents stopped? In this thought experiment, you can follow three stories: the Gulf Stream, warm and cold coasts, and the cold, food-rich water that rises from the deep.

NOAA identifies three main drivers of ocean currents: tides (tidal currents, strongest near shore), winds (surface currents, from coastal upwelling to basin-wide gyres) and thermohaline circulation (density differences from temperature and salinity, slower than tidal or surface currents).

This lab is a thought experiment, not a model. It imagines the wind- and density-driven circulation halted and asks what the evidence about today's currents implies, labeling each consequence as an observed fact or an idea.

The Gulf Stream is a warm current in the Atlantic Ocean. It flows north along the coast of Florida.

It carries more water than all the rivers in the world put together!

One of the most famous currents is the Gulf Stream, a fast, warm current in the North Atlantic Ocean. It flows north along the coast of Florida, then turns east off North Carolina and heads across the Atlantic. NOAA says it flows at a few miles per hour, fastest near the surface, and carries more water than all the world's rivers combined.

Ocean currents like it carry warm water away from the equator and cold water away from the poles. NOAA says the warm Gulf Stream helps give Bergen, in Norway, milder winters than New York, which is much farther south.

NOAA describes the Gulf Stream as an intense, warm western boundary current that runs north along Florida, turns east off North Carolina and flows northeast across the Atlantic. NOAA's Gulf Stream fact page gives an average speed of about 6.4 km/h (4 mph), peaking near 9 km/h at the surface and slowing to about 1.6 km/h as it widens northward, while its currents tutorial gives 1 to 3 knots (1.85 to 5.55 km/h): the figure depends on where and how the current is measured. It transports more water than all the world's rivers combined.

Each of the five major ocean gyres is flanked by a strong, narrow western boundary current and a weak, broad eastern one. Together, currents move warm water poleward and cold water equatorward.

Without currents, warm water would stay near the middle of the Earth. Cold water would stay near the poles.

And the food from deep in the sea might not come up.

If the currents stopped, warm water would no longer be carried away from the equator, and cold water would no longer flow away from the poles. Places like Bergen, which get milder winters thanks to the Gulf Stream, might become colder.

The wind would also stop pushing surface water aside along some coasts. That push lets cold water full of nutrients rise from below, a process called upwelling. NOAA says good fishing grounds are usually found where upwelling is common, so those rich waters might lose much of their life.

Halting circulation removes two services NOAA describes. First, heat transport: currents carry warm equatorial water poleward and cold polar water equatorward, and the Gulf Stream keeps Florida's east coast warmer in winter and warms western European countries. Second, nutrient supply: wind-driven upwelling replaces displaced surface water with cold, nutrient-rich water that fertilizes high biological productivity.

Without them, the tropics might hold more heat, high latitudes might lose some, and upwelling regions might become far less productive. These are ideas reasoned from present-day processes, not quantitative predictions.

Wind pushes the top of the sea along. Cold, salty water sinks and moves slowly deep down.

The spinning Earth makes the currents curve.

Wind is one big engine of currents. As it blows across the sea, it drags the top layer of water along, and because Earth spins, the moving water curves: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis effect.

The other engine is deep and slow. Near the poles, very cold, salty water is heavy and sinks, starting a worldwide loop of deep currents called the global conveyor belt. NOAA says a bit of water may take about 1,000 years to go all the way around.

Wind stress drives surface currents. Earth's rotation deflects moving air and water (the Coriolis effect), and friction passes the wind's push to deeper layers, each moving more slowly and further deflected: the Ekman spiral, which fades by about 100 m. In the open ocean, NOAA notes, winds drive currents that circulate water for thousands of miles through the ocean basins, and each basin's circulation forms a large gyre.

Below, thermohaline circulation is driven by density. Brine rejected by forming sea ice makes polar water cold, salty and dense enough to sink, feeding the global conveyor belt; NOAA estimates about 1,000 years for a parcel to complete it.

Scientists watch the ocean's currents with special tools.

They study how a warming world might change them.

Currents cannot really stop: the wind, the tides and Earth's spin keep them going. But scientists do study how they might change. NOAA explains that the deep conveyor belt is strong but easily disrupted.

If warming brought more rain and melting ice to the North Atlantic, the extra fresh water at the surface could get in the way of the cold, salty water sinking. NOAA says this could slow or even stop the conveyor belt, which could bring big temperature changes to Europe. Scientists keep measuring the currents to find out.

NOAA's currents tutorial describes the global conveyor belt as strong but easily disrupted. Increased North Atlantic rainfall and melting glaciers and sea ice could add fresh water at the surface, block sea-ice formation and disrupt the sinking of cold, salty water, which could slow or even stop the conveyor and produce potentially drastic temperature changes in Europe.

A complete halt of all currents is not physically plausible while winds, tides and rotation persist; it is useful here as a way to isolate what currents do. Changes in specific currents remain an active research question.