The Shifting Flows of Our Overheated Oceans

From a Wall Street Journal story by Helen Czerski headlined “The Shifting Flows of Our Overheated Oceans”:

A half century ago, the Apollo missions brought back two photos that redefined our concept of home, one dubbed “Earthrise” and the other “The Blue Marble.” Those vivid images of the Earth showed humanity a beautiful, mostly blue ball of richness and wonder, spinning through the vast emptiness of the universe.

It was a magnificent revelation, but it also led to a kind a collective blindness—a habit of not really looking at the blue itself. When people talk about the ocean, it is primarily about the things in it—fish, whales, pollution, flocks of lost rubber duckies and so on. Even our global maps tend to sideline the oceans so that we can better see the land (though the geophysicist Atheistan Spilhaus came up with a projection that flips the view to display a single, connected global ocean).

Today we tend to see the ocean as a remote void, an absence rather than a presence. We fail to see its dynamic variety, which is especially important to understand as climate change advances. Over the summer, some waters off the coast of Florida soared to 100 degrees, far higher than normal, and it isn’t just a localized phenomenon. Average global ocean surface temperatures have been climbing steadily since the 1970s, and “marine heat waves” are now regularly hitting the headlines.

As ocean temperatures rise, we are being forced to see the blue for what it really is: a giant liquid engine that dictates how the whole planet operates. It’s the physics of the ocean that enables the marine life to weave through it, as part of something much grander.

The ocean is too big for all the water in it to be mixed into one uniform pool. Rather, it has an intricate internal anatomy. The uneven distribution of heat and salt creates water masses of different densities, so the ocean is layered like a posh cocktail: the densest liquid at the bottom and the most buoyant parked at the surface. These layers mean that the surface water is mostly disconnected from the depths.

But the most fundamental characteristic of an engine is the conversion of energy to movement. The energy that drives the ocean’s engine comes from sunlight, which heats up the water and also ultimately powers the weather, making winds that push around the surface of the ocean. And all of this is happening on a spinning planet, where the movement of water is interrupted and directed by giant continents.

As this engine turns, the rich internal anatomy of the ocean flows, swirls and mixes. Though our eyes can only see water and more water, modern imaging technology shows us nuanced patterns of heat, salt, nutrients and trace elements that can shift with the seasons and on multiyear cycles. Most marine life is either a passenger carried by these patterns or a voyager navigating through them. All of that life depends on what the engine offers it.

Imagine yourself as one of the great predators of the seas—perhaps a bluefin tuna, swordfish or shark. You are a fast and powerful swimmer, with the endurance to travel hundreds or thousands of miles to find prey and the resilience to withstand a range of temperatures. The global ocean lies before you. Where do you go to find what you need to thrive?

The Gulf Stream is the fast warm current that flows up the eastern side of North America and then turns to cross the Atlantic. It’s one part of a giant carousel that spins around the whole northern ocean, sluggish and wide as it slides down the side of Western Europe and slinks westward just above the equator, and then narrowing and accelerating as it turns north up the American coast.

But since this engine is liquid, the pattern is far richer than a simple ring. As the narrow, fast stream barrels eastward it begins to wobble. The wobbles get bigger until they form loops, and sometimes a loop becomes so extreme that it detaches into a separate spinning circle. There are two ways that this can happen. The loop can push into the cold water to the north, enclosing warm water from the south to form a trapped island of warmth, spinning clockwise. Or the loop can push southward, creating a counterclockwise spinning cold island drifting through warmer water.

These whirls, known as mesoscale eddies, can be up to 100 miles across and a mile deep and can last for many months, carrying the water within them for hundreds or even thousands of miles. They’re particularly dramatic around the Gulf Stream, but there are similar structures throughout the global ocean.

Every distinctive environment in the ocean will host a different ecosystem that depends on where the water originated, what nutrients are in it, what the temperature is and how much sunlight is available. The cold, counterclockwise islands are especially full of life, and predators like bigeye and yellowfin tuna will travel long distances to take advantage of that. The warm, clockwise islands generally have less life near the surface, but bluefin tuna are common visitors, and blue and white sharks hunt in them for fish, taking advantage of the warmth to venture deeper into the ocean’s interior. Swordfish have lower energy needs and will happily feed on squid rather than fish, so they tend to be found outside the eddies, where there’s less food but also less competition.

There are plenty of other examples. In the southern ocean, along the long boundary called a front where cold and warm water meet, the mixture of nutrients and life from both sides produces the ocean equivalent of a densely populated city. King penguins travel hundreds of miles to hunt for fish there, and the abundance of krill feeds Antarctic fur seals and whales. In the Indian Ocean, silvertip sharks congregate just above submerged mountains called seamounts; the cold currents flowing over the obstacle are forced up toward the surface along with the nutrients they carry.

The point is that instead of thinking of the ocean as a big empty pond, we should see it as the complex swirling engine that it is, a place rich in character and variety with distinct patterns that are dictated by the physics of how the engine moves. With this perspective, the prospect of ocean warming takes on a different flavor.

The main reason that the ocean is warming is that the carbon we emit traps energy, which is mostly stored as heat, and more than 90% of that extra heat ends up in the ocean because water is really effective at storing energy. It takes a huge amount of energy to heat water up by even one degree, so if the ocean warms by even a little bit, that represents a gigantic amount of extra energy.

That energy can power more extreme weather, but it can also have serious effects inside the ocean. As the surface water warms, it becomes more buoyant. This makes it harder for nutrients to mix upward into the sunlight from the layers below, limiting the flow of raw material that life needs to thrive. The extra energy can also change how the ocean engine operates, shifting the speed and shape of currents, and thus the nature of features like the mesoscale eddies.

Marine heat waves and hot spots can have severe local effects, as ocean life struggles to survive the increasing temperatures or moves to find cooler water, disrupting the ecosystems in those places. But the bigger picture is that any change to the ocean engine will matter, because the way this engine hums dictates conditions for the rest of life on Earth. Living on a blue planet isn’t about the color, lovely though it is. The ocean is the dominant feature of Earth, and we live in its shadow. This is about our identity as citizens of an ocean world, which is broadcast to the universe, written in blue light.

Helen Czerski, a physicist and oceanographer, writes the “Everyday Physics” column for The Wall Street Journal. This essay is adapted from her new book, “The Blue Machine: How the Ocean Works,” which will be published by W.W. Norton on Oct. 3.

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