How Did We Find Out There Are "Rivers" Flowing Inside the Sea?
― A Whaling Captain Knew Why the Mail Ships Ran Late
In the middle of the ocean, with no banks and no levees, a "river" about 100 km wide is flowing. The first person to put it on a map was not a scholar. It was the whalers, who lived on the sea almost every day.
Have you ever walked the wrong way on a moving walkway? You move your legs as usual, but the scenery barely moves. Walk a little slower and you just march on the spot, then slowly get carried backwards.
In the 18th-century Atlantic, exactly this was happening to ships. Mail packets carrying letters from Britain to America somehow arrived about two weeks later than merchant ships. And the packets were the finer vessels.
The people who solved this puzzle knew that an invisible "moving walkway" lay across the sea.
How was the ocean's "river" discovered?
You can't see ocean currents. But a ship that goes against one is slowed, and a ship that rides one arrives sooner. The mail packets' "mysterious delay" was the evidence of a current.
Whales gather at the edges of currents. The captains of the whaling ships that hunted them knew the current's position and width by feel. That knowledge was drawn on paper for the first time.
And this "river" is not found just anywhere in the sea. It runs narrow and fast only along the western edge of a big ocean. The Kuroshio, which flows south of Japan, is one of the same family. Let's take it step by step.
Why were the mail ships two weeks late?
Around 1768, a complaint is said to have reached Benjamin Franklin, who was in London. Franklin held the post of overseeing the mail of the American colonies. The complaint was that mail packets from Britain took about two weeks longer than merchant ships to reach New York.
Franklin consulted his cousin, Timothy Folger. Folger was a whaling captain from Nantucket, an American island. His answer was clear: "The packets are sailing against a strong current."
According to Folger, whaling captains had told the packet captains this many times. But the packet captains would not listen. Perhaps they dismissed it as advice from mere American fishermen.
How much do you lose by going against a current? Use the slider in Figure 1 to change how fast the ship pushes through the water. The bottom arrow shows the speed the ship really makes over the ground (the seabed).
With a current of 1.5 m/s, even a ship doing 2.5 m/s through the water really makes only 1 m/s. On a day of weak wind, a sailing ship is simply pushed back where it floats. Folger is said to have told Franklin that in light winds a ship would be pushed back more than it advanced. The two-week delay was the sum of days like these.
Why did only the whalers know about the current?
Whaling ships spend months going back and forth over the same waters. That is very different from a mail packet rushing straight to its destination. Whales are said to gather at the edges of currents, where food is plentiful. So whalers followed the edges of currents hunting whales, and sometimes crossed them.
Inside a current, the colour and temperature of the water change, seaweed floats by, and the ship is carried sideways. After meeting these clues again and again, the captains built up a "map of the current" in their heads.
Franklin had Folger sketch the current's position and had it printed as a chart. It is one of the earliest charts of the Gulf Stream. But British sailors are said to have ignored it for some time.
Franklin himself, on later crossings of the Atlantic, took the temperature of the sea water with a thermometer. Gulf Stream water is warmer than its surroundings. This is regarded as an early attempt to catch an invisible current in numbers, using temperature.
In 1513, a Spanish expedition off Florida is said to have found its ship sliding backwards despite a following wind. Spanish fleets later rode this current home to Europe. So the current's existence was known. What Franklin and Folger did was map "where it flows, and how wide it is."
Why does the "river" flow only along the ocean's western edge?
In the northern half of the Atlantic, trade winds blow from east to west at low latitudes, and westerlies blow from west to east at high latitudes. Dragged by these two winds, the surface of the sea turns in a big clockwise loop. Take a look at Figure 2.
The odd thing here is that the loop is not symmetrical. The northward flow is squeezed to about 100 km wide at the western edge, and in its fastest parts is said to reach about 2 m/s. The southward return flow, on the other hand, spreads across most of the ocean and moves only slowly.
The cause of this lopsidedness is the Earth's rotation. On a spinning Earth, the "effect of rotation" gets stronger the farther north you go. Because of that difference, the return flow can drift slowly across a wide sea. The northward flow, meanwhile, has no choice but to crowd into a narrow channel pressed against the western shore. In 1948, the American oceanographer Henry Stommel showed this mechanism with a simple calculation.
The same thing happens in the North Pacific. The current along the Pacific's western edge, off Japan's southern coast, is the Kuroshio. It, too, was a "river of the sea" that Japanese fishers had long known and named.
The amount of water the Gulf Stream carries is said to be tens of times more than all the world's rivers put together. It is about 100 km wide, and in places nearly 1 km deep. Even so, from the surface of the sea you can hardly tell where the current's edge is.
Summary
Inside the sea flow "rivers" with no banks. The ones who first mapped them were helped by the mail ships' mysterious delay and the experience of captains who chased whales. And these rivers, through the work of wind and the Earth's rotation, gather narrow and fast along the western edge of a big ocean. The Kuroshio, which flows right by Japan, is one of them.
The invisible current first showed up as a delay.
The first to draw the map of the sea were people who lived on it.
How the Earth's rotation affects the direction of flow is covered in "Is it true that bath water swirls the other way in the Southern Hemisphere?", and how land rivers come to bend is covered in "Why don't rivers flow straight? Why do they meander?". For the link between the warm water the Kuroshio carries and fish, try "Why does saury get fatty and delicious in autumn?"
- Fill a bath or a basin with water and stir it by hand to make a slow current going around in one direction.
- Push a small paper boat (or a piece of eraser) with your finger, with the same strength each time, once with the current and once against it.
- Compare the time it takes to cover the same distance in the two directions. With a weaker push, check that the boat is carried back when going against the current.
If the water's movement is hard to see, float a little pepper on it and the flow shows up. Finding an invisible current through a "difference in time" is the same idea Franklin used to reach the Gulf Stream from the mail ships' delay.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics" and "Earth Science"
- High school+Advanced high-school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university courses (physical oceanography, geophysical fluid dynamics)
- ResearchNot taught even at university as settled fact; questions researchers are studying right now
Middle schoolTerms: this phenomenon has names
- Ocean current: a large flow that keeps running along a more or less fixed path in the sea. It is different from the flow caused by the rise and fall of the tides (tidal current).
- Gulf Stream and Kuroshio: strong currents that flow north along the western edges of the North Atlantic and North Pacific. Both are among the largest currents in the world.
- Gyre: a set of currents forming a big loop around a whole ocean. In oceans of the Northern Hemisphere it turns clockwise.
Middle schoolHigh schoolChecking with a formula: how many days late when going against a current?
The ship's speed through the water minus the current's speed gives the speed it really makes. Let's estimate the delay for a case of going 1000 km against the current.
| In symbols | t ＝ L ÷ ( v − u ) |
| In words | Time taken ＝ distance travelled ÷ (ship's speed through the water − current speed) |
| Where it comes from | Combining velocities (relative velocity). A ship's speed seen from land is its speed relative to the water plus the speed of the water itself. An opposing current is subtracted. |
| Symbol | Meaning and unit |
| L | Distance sailed against the current. Assume 1000 km |
| v | Sailing ship's speed through the water. Assume 2.5 m/s |
| u | Current speed. 1.5 m/s (the fast parts of the Gulf Stream are said to reach about 2 m/s) |
| Seconds in a day | 86400 s |
| Distance covered in a day at 1 m/s (km) | 86400 ÷ 1000 ＝ 86.4 |
| Real speed v − u (m/s) | 2.5 − 1.5 ＝ 1.0 |
| Distance covered in a day (km) | 1.0 × 86.4 ＝ 86.4 |
| Days t to go 1000 km against the current | 1000 ÷ 86.4 ≒ 11.6 |
| Distance per day with no current (km) | 2.5 × 86.4 ＝ 216 |
| Days with no current | 1000 ÷ 216 ≒ 4.6 |
| Delay (days) | 11.6 − 4.6 ＝ 7.0 |
Just going against the current makes about a week's delay. On a day when the wind is weak and the ship's speed falls to 1.0 m/s, v − u becomes −0.5, and the ship is pushed back.
| Distance pushed back in a day (km) | 0.5 × 86.4 ＝ 43.2 |
| In familiar terms | You lose almost the length of one full marathon (about 42 km) in a single day |
If days like this pile up, a two-week gap is no surprise. Conversely, if you ride the current, you gain by the same amount. This is why the Spanish fleets used the Gulf Stream on the way home.
High schoolHigh school+Wind moves the sea, rotation bends its direction
High schoolIn Earth Science you learn that the trade winds and westerlies drag the surface water, creating clockwise gyres in the oceans of the Northern Hemisphere. The "relative velocity" of Basic Physics is exactly the formula above.
High school+On a rotating Earth, water in motion feels an apparent force (the Coriolis force) that bends it to the right of its direction of travel in the Northern Hemisphere. So surface water dragged by wind is carried not in the wind's direction itself but to the right of it. This is called Ekman transport. Water piles up between the westerlies and the trade winds, raising the sea surface slightly, and this is the source of the force that drives the gyre.
UniversityWestern intensification: why is only the western edge fast?
The way a gyre becomes strong and lopsided to the west is called western intensification, and the strong flow is called a western boundary current. The balance between the spin the wind gives the sea and the change of the Coriolis force with latitude (the beta effect) is called the Sverdrup balance, and under it the flow in the ocean interior goes wide and slow toward the south. To carry the returning water north, a narrow current is needed at the western edge, where friction can act. That was Stommel's (1948) answer, which Munk later developed into a form including eddy viscosity.
📖 Derivations of the formulas, and more advanced material: Wikipedia (Japanese): "Western boundary current" ／ Wikipedia (Japanese): "Gulf Stream"
ResearchWhat is still not well understood
- Is Europe's mild winter thanks to the Gulf Stream? This is a common explanation, but some studies say the heat carried by the atmosphere matters more, and how much is due to the current is still debated.
- Is the Atlantic's great circulation weakening? There are warnings that if warming makes the northern sea water lighter, the circulation reaching the deep sea could weaken. But the period of observation is short, and views differ on how far it has weakened.
- Why does a large meander of the Kuroshio start, and why does it end? The Kuroshio sometimes bends far to the south off the Kii Peninsula, and stays that way for years. Predicting its start and end in advance is still said to be difficult.
In other words, this article too is "an explanation of what is understood so far." Even now that satellites can measure the height of the sea surface, the position and strength of currents still shift a little every year.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science, Year 2: "Weather and its changes" (movements of air and sea) | The words ocean current and gyre |
| High school | Basic Physics: "Relative velocity" / Earth Science: "Ocean circulation" | Figure 1 and the formula check, Figure 2 |
| High school+ | Advanced Earth Science: "Coriolis force and Ekman transport" | How wind moves the sea |
| University | Physical oceanography, geophysical fluid dynamics | Western intensification, Sverdrup balance |
| Research | Climate change and ocean circulation | The Gulf Stream and Europe's climate, the Kuroshio large meander |
| ― | Links to daily life | Fishing grounds, shipping routes, Japan's climate and the Kuroshio |
- Wikipedia (Japanese) "Gulf Stream" (メキシコ湾流)
- Wikipedia (Japanese) "Western boundary current" (西岸境界流)
- Richardson, P. L. (1980). Benjamin Franklin and Timothy Folger's first printed chart of the Gulf Stream. Science, 207.
- Stommel, H. (1948). The westward intensification of wind-driven ocean currents. Transactions, American Geophysical Union, 29.
- Stommel, H. (1965). The Gulf Stream: A Physical and Dynamical Description. University of California Press.
* This article is a popular-science explanation for general readers. The numbers given are rough guides to help you understand the mechanism. Historical exchanges are presented as the accepted account, based on surviving letters and records.