Sverdrup unit

One sverdrup (Sv) is 10⁶ m³ s⁻¹ — a million cubic metres of water per second. It is the standard unit for volume transport in physical oceanography, named after Harald Sverdrup.

The unit exists because ocean transports are awkward in SI. Expressed in cubic metres per second, the major currents are eight- and nine-digit numbers; in sverdrups they are small integers that can be compared at a glance. That is the whole justification, and it works well enough that the unit is universal in the field.

For scale, the combined discharge of every river on Earth is roughly 1 Sv. So the numbers below are worth reading against that: ocean circulation moves water at a rate that makes global river flow a rounding error.

Typical magnitudes

• The Antarctic Circumpolar Current, the largest, carries roughly 130-150 Sv.
• The Gulf Stream reaches around 30 Sv near Florida, growing to 100 Sv or more downstream as it entrains surrounding water.
• The Atlantic meridional overturning circulation is around 15-20 Sv.
• The Pacific Equatorial Undercurrent carries roughly 20-40 Sv depending on longitude and season — the current that matters most for the Galapagos cold pool and, through it, for the archipelago’s climate.

Having these magnitudes to hand is genuinely useful as a sanity check. A transport estimate of 500 Sv for a regional current is wrong by construction, and knowing the scale catches that immediately.

A caveat on the numbers

Transport figures are less precise than quoting them to two or three digits implies. A transport is an integral of velocity over a cross-section, and both the section’s definition and the depth of integration are choices. Different studies of the same current legitimately report values differing by tens of percent because they integrated over different boundaries — the Gulf Stream range above is mostly this, not disagreement about the ocean.

Worth checking what section a number refers to before comparing it with another.

See also: oceanography, the Pacific Equatorial Undercurrent, the Humboldt Current, and upwelling.