Gyres: Ocean Basin Currents
How do the water currents at the ocean’s surface form?
The solar engine driving global wind belts and local wind circulations reaches into the
planetary ocean basins and powers the large oceanic circulations in the Northern and Southern
Hemispheres.
Recall that there is a large belt of atmospheric uplift occurring in the Equatorial zone that
draws in air from the north and the south, creating the easterly Trade winds. Where does that
rising air go? Generally, the belt of air rises thousands of feet above the planet’s surface and
eventually spreads out to the north and the south. As the air rises higher in the atmosphere, it
expands in volume and cools due to lower air pressures; the air loses a lot of moisture in the form
of rainfall, and eventually the air descends back towards the planet’s surface.
A large vertical circulation forms as the air rises above the Equator at 0 degrees of
latitude, spreads out toward both poles, and falls back to the surface at about 30 degrees of
latitude. As the air descends, it warms up and dries out as the air molecules are compressed
under the weight of the atmosphere pushing down on top of it. When the descending air reaches
the planet’s surface, it spreads out from belt centered at about 30 degrees of latitude, generating
winds that head poleward and winds that head back to the Equator.
The winds returning to the Equator feed into the easterly Trade winds belt. The winds
heading poleward are influenced by Coriolis deflection (to the right in the NH, to the left in the
SH), and give rise to the Westerlies in both hemispheres. These winds have some contact with
the planet’s surface, and with that frictional contact, the energy of wind motion is transferred into
the upper layers of the ocean… kind of like pointing a large fan at a swimming pool.
The kinetic energy of the easterly wind belt moves the ocean surface from east to west in
the lower latitudes, and the kinetic energy of the westerly wind belt moves the ocean surface
from west to east in the middle latitudes. The northeast trade winds cause the ocean surface to
flow from the northeast, but Coriolis deflection occurs and the Equatorial surface ocean current
in the Northern Hemisphere then flows directly from east to west. In the middle latitudes, the
prevailing southwesterly winds cause the ocean surface to flow from the southwest, but Coriolis
deflection occurs and the middle latitude surface ocean current then flows from west to east.
Acting together, the easterly currents near the Equator and the westerly currents in the
middle latitudes of the ocean basins put the ocean’s surface water in a clockwise movement
around the basin (the Pacific, for example). These large ocean surface currents are called gyres.
In general, these large ocean currents follow the surface pattern shown just below.