Do the Atlantic and Pacific Oceans Mix?

You may have encountered a video like this: a ship sailing in the open ocean approaching a visible boundary in the water. On one side, the water appears dark blue and clear, while on the other, it looks greenish and murky. These videos often claim this is the dividing line between the Atlantic and Pacific oceans, suggesting the waters defy natural laws and refuse to blend.

However, as you might suspect, water doesn't universally mix in every scenario worldwide, only to suddenly repel like the same poles of a magnet where the Atlantic and Pacific Oceans meet between South America and Antarctica. So, yes, the waters of the Atlantic and Pacific do mix. But what explains the phenomenon in these videos?

Ocean Boundaries

The Drake Passage, a 528-mile-wide (850-kilometer-wide) stretch of ocean between South America and Antarctica, is where the Atlantic and Pacific Oceans converge. Known for its turbulent waters, it has been a feared route for sailors since its discovery in the 1500s.

It’s logical that an early cartographer, examining the vast Atlantic and Pacific Oceans, designated the Drake Passage as the transition point between the two. Drawing a line from Cape Horn, Chile’s southernmost tip, to the Antarctic Peninsula was the simplest way to represent this division.

However, ocean boundaries, much like neighborhood divisions, are largely arbitrary. The waters on either side of the longitudinal line drawn by a cartographer centuries ago in Europe are essentially the same. While lines exist in the ocean, they aren’t the kind that cartographers typically find practical.

Ocean Fronts

"When addressing this question, we need to consider two aspects: the visible surface phenomena, such as lines of varying color or turbidity, and the underlying processes beneath the surface," explains Sally Warner, a physical oceanographer and professor at Brandeis University.

While these videos likely aren’t altered, their exact filming locations remain uncertain. They might depict the Drake Passage, but they could also capture similar phenomena occurring in entirely different regions of the world.

Ocean fronts are formed by surface water masses with distinct temperatures or salinity levels. In the open ocean, these fronts can have remarkably sharp boundaries, sometimes appearing like two distinct ice cream flavors side by side in a container.

For those on land, the most accessible example is where two rivers converge or, even better, where a river meets the ocean. River water, often laden with silt by the time it reaches the ocean, takes on a chocolate milk-like appearance, starkly contrasting with the darker ocean water. Additionally, the difference in density between fresh river water and salty ocean water can create the illusion of separation when viewed from a bridge or boat. However, they will inevitably mix, though it may take a day or two to fully blend.

Around the globe, ocean fronts converge, creating visible lines on the water’s surface. Similar to the mingling of fresh river water and salty ocean water, temperature differences can produce clear boundaries in the open ocean. For example, near the equator, tropical instability waves form where cooler waters from the north and south meet the warm equatorial waters, resulting in distinct visible lines.

Atlantic-Pacific Mixing

The waters of the Atlantic and Pacific absolutely mix, and Warner notes that this mixing may be more pronounced here than in most other oceanic regions worldwide.

The Antarctic Circumpolar Current is a continuous flow of water that circles the globe, staying close to Antarctica. Its path is largely unobstructed, except for the narrow Drake Passage, which creates highly turbulent conditions in this specific area of the ocean.

However, any visible line in the water in this region is likely due to the convergence of waters with different temperatures:

"The waters near Antarctica are significantly colder than those to the north. What people often interpret as the boundary between the Atlantic and Pacific is more likely a front separating the frigid Antarctic waters from the warmer northern waters."