The Cromwell Current (also called Pacific Equatorial Undercurrent or just Equatorial Undercurrent) is an eastward-flowing

subsurface current A subsurface ocean current is an oceanic current that runs beneath surface currents. Examples include the Equatorial Undercurrents of the Pacific, Atlantic, and Indian Oceans, the California Undercurrent, and the Agulhas Undercurrent, the deep t ...

that extends the length of the

equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can al ...

in the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

. The Cromwell Current was discovered in 1952 by Townsend Cromwell, a researcher with the Honolulu Laboratory of the Fish and Wildlife Service (later the

United States Fish and Wildlife Service The United States Fish and Wildlife Service (USFWS or FWS) is an agency within the United States Department of the Interior dedicated to the management of fish, wildlife, and natural habitats. The mission of the agency is "working with othe ...

). It is wide and flows to the east. It is hidden under the surface of the

at the

and is relatively shallow compared to other ocean currents being only from top to base. It is a powerful current with top velocities of up to 1.5 m/s (2.9 knots; 3.4 mph). The current's core coincides with the

thermocline A thermocline (also known as the thermal layer or the metalimnion in lakes) is a thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with ...

and its distance from the parallel

Equatorial Counter Current The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at ...

is approximately . It has 1,000 times the volume of the

Mississippi River The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it ...

and its length is .

Discovery

In 1951 researchers on board a Fish and Fish and Wildlife Service fisheries research vessel were engaged in exploratory

longline fishing Longline fishing, or longlining, is a commercial fishing angling technique that uses a long ''main line'' with fish bait, baited fish hook, hooks attached at intervals via short branch lines called ''snoods'' or ''gangions''.< ...

when they noticed that the gear deep under water drifted eastwards. This was considered unusual because the surface currents of the Pacific Ocean flow westwards at the equator, following the direction of the winds. In 1952 Townsend Cromwell led a research party to investigate how the currents of the ocean varied as a function of depth. They discovered a fast-flowing current that flowed eastwards in the deep surface layers.

Mechanism

It is difficult to explain the Cromwell Current easily. At least two different mechanisms are at work in order to guarantee the constant eastward current: (a) Because the Cromwell Current is located on the equator, the

Coriolis force In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...

is equal to zero and does not act upon a water parcel. This means that the east–west

pressure gradient In atmospheric science, the pressure gradient (typically of air but more generally of any fluid) is a physical quantity that describes in which direction and at what rate the pressure increases the most rapidly around a particular location. The ...

functions without being diverted from the high-pressure area in the west toward the low-pressure area in the east, simply following the gradient. The cause of the pressure gradient is at least partially the

trade winds The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisp ...

blowing from the east westward. (b) Any water parcel flowing eastward from the west that would somehow be perturbed from its path would be diverted northward if north of the equator and southward if south of the equator. In such a case the local Coriolis force would force the parcel immediately back into the main current that flows from west to east.

Detailed data

*Depth: The surface currents flow west. There is reversal point about down, where the water starts to flow east. The current goes down to about . *Flow rate:The total flow is up to around per second. The top speed is around 1.5 m/s (2.9 knots; 3.4 mph), which is about twice as fast as the westerly surface current. *Length:

Interaction with El Niño

El Niño El Niño (; ; ) is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (approximately between the International Date ...

is a reversal of the normal situation in the Pacific Ocean. Surface water is blown westwards by the prevailing winds and deeper water is forced upwards to replace it. Every now and then, the surface water sloshes back across the ocean, bringing warm water temperatures along the eastern coasts of the Pacific. In non-El Niño years, the Cromwell Current is forced to the surface by underwater seamounts near the Galapagos islands (this is called upwelling.) However, during El Nino years the current does not upwell in this way. The waters around the islands are therefore considerably warmer during El Niño years than during normal years.

Effect on wildlife

The Cromwell Current is both oxygen- and nutrient-rich. A large number of fish are concentrated in it.

Upwelling Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted surface water. The nut ...

occurs near the Galapagos Islands. This brings food supplies to the surface for

Galápagos penguin The Galápagos penguin (''Spheniscus mendiculus'') is a penguin endemic to the Galápagos Islands, Ecuador. It is the only penguin found north of the equator. Most inhabit Fernandina Island and the west coast of Isabela Island. The cool water ...

. Upwelling, however, is a sporadic phenomenon; it fails to occur on a regular basis, and so the food supply comes and goes. The

penguin Penguins ( order Sphenisciformes , family Spheniscidae ) are a group of aquatic flightless birds. They live almost exclusively in the Southern Hemisphere: only one species, the Galápagos penguin, is found north of the Equator. Highly adapt ...

s have several adaptations to cope with this, including versatility in their breeding habits.

Possible effect on climate

The effect of this current on world climate is not well understood.