Why is the Gulf Stream on the western side of the Atlantic?

The poleward change in the Coriolis effect forces the return branch of the wind-driven gyre into a narrow, fast jet along the western boundary, which in the North Atlantic is the Gulf Stream.

What is a geostrophic current?

It is a current in which the Coriolis force balances the horizontal pressure-gradient force, so the water flows along, rather than across, lines of constant pressure.

Wind-Driven and Geostrophic Circulation

Surface winds, acting on a rotating Earth, drive the great subtropical and subpolar gyres of the ocean, whose interiors flow in near-geostrophic balance and whose western boundaries intensify into swift currents such as the Gulf Stream and Kuroshio.

Definition

Wind-driven circulation is the large-scale upper-ocean flow forced by surface wind stress, organized into gyres whose interiors obey geostrophic balance — a balance between the horizontal pressure-gradient force and the Coriolis force.

Scope

This topic covers Ekman transport and pumping by wind stress, the Sverdrup balance that sets the interior gyre transport, the dynamics of western boundary currents and their intensification, the geostrophic relation between currents and pressure (or sea-surface height) gradients, and the structure of major surface currents and gyres.

Core questions

How does wind stress, mediated by the Coriolis effect, set up the basin-scale gyres?
Why are western boundary currents like the Gulf Stream so narrow and fast compared to the broad interior flow?
How can geostrophic currents be inferred from measurements of density or sea-surface height?
What determines the total volume transport of a wind-driven gyre?

Key theories

Sverdrup balance: In the ocean interior, the meridional transport integrated over the water column is proportional to the curl of the wind stress, fixing the volume transport of wind-driven gyres.
Westward intensification: The variation of the Coriolis parameter with latitude forces the return flow of a gyre into a narrow, intense western boundary current, explaining features like the Gulf Stream.
Geostrophic method: Because interior currents are nearly geostrophic, the horizontal velocity can be computed from the slope of pressure surfaces derived from density data, relative to an assumed level of no motion.

Mechanisms

Wind stress drives an Ekman layer whose net transport is perpendicular to the wind; spatial variation of this transport pumps water down or up into the geostrophic interior, where the vortex stretching is balanced by meridional motion (the Sverdrup balance). Conservation of potential vorticity then requires that the closing return flow be concentrated at the western boundary, producing intensified currents.

Clinical relevance

The gyre circulation and its boundary currents transport vast amounts of heat poleward, shaping regional climate, fisheries productivity, and shipping routes; sea-surface height from satellite altimetry now maps the geostrophic flow globally in near real time.

History

The theory of wind-driven circulation was assembled between 1947 and 1950 by Sverdrup, who related interior transport to wind-stress curl; Stommel, who explained westward intensification through the variation of the Coriolis parameter; and Munk, who combined these into a complete gyre model.

Key figures

Harald Sverdrup
Henry Stommel
Walter Munk

Seminal works

stommel1948
sverdrup1947

Frequently asked questions

Why is the Gulf Stream on the western side of the Atlantic?: The poleward change in the Coriolis effect forces the return branch of the wind-driven gyre into a narrow, fast jet along the western boundary, which in the North Atlantic is the Gulf Stream.
What is a geostrophic current?: It is a current in which the Coriolis force balances the horizontal pressure-gradient force, so the water flows along, rather than across, lines of constant pressure.