Page 60 - (DK) Ocean - The Definitive Visual Guide

58 CIRCULATION AND CLIMATE

Surface Currents

FLOWING FOR ENORMOUS DISTANCES within the upper regions of
the oceans are various wind-driven currents. Many join to produce
large circular fluxes of water, called gyres, around the surfaces of the
main ocean basins. Surface currents affect only about 10 percent of ocean water, but Coriolis
deflection
they are important to the world’s climate (see p.66), because their overall effect is to
wind
transfer huge amounts of heat energy from the tropics to cooler parts of the globe.
They also impact shipping and the world’s fishing industries. direction of
frictional wind drag
Wind on Water resultant direction
of water motion
When wind blows over the sea, it causes the upper ocean to move, creating a
current. However, the water does not move in the same direction as the wind.
Instead, it moves off at an angle—to the right in the Northern Hemisphere and drag imparted
to the left in the Southern Hemisphere. This phenomenon was first explained in from layer above
1902 by a Swedish scientist, Walfrid Ekman, using a model of the effect of wind direction of water
on water now called the Ekman spiral. The model assumes that the movement of motion
water in each layer of the upper ocean is produced by a combination of frictional
drag from the layer above (or, in the top layer, from wind drag) and the Coriolis water motion in this
effect (see p.54). The model predicts that, overall, a mass of water will be pushed layer
at right angles to the wind direction, an effect known as Ekman transport.
drag
EKMAN SPIRAL
N. Atlantic Drift
Labrador Canary The direction of motion in each water
Somali
N. Equatorial E. Greenland layer results from a combination of the
Gulf Stream Agulhas
Oyashio drag from the layer above and a deflection
Alaska caused by the Coriolis effect. This diagram
Kuroshio
N. Pacific shows the Ekman spiral in the Northern
Hemisphere. In the Southern Hemisphere,
California deflection
is to the left of
N. Equatorial wind direction.
Equatorial MAIN CURRENTS
Counter This map shows all
of the world’s main
surface currents,
both warm and cold.
S. Equatorial
E. Australia
W. Australia
Antarctic
Circumpolar
Peru S. Equatorial Benguela S. Equatorial warm current
Brazil cold current

Ocean Gyres

The combination of prevailing winds (see p.54) and Ekman transport
produces large-scale, circular systems of currents known as gyres. All
together there are five ocean gyres—two in each of the Atlantic and
north Pacific gyre Pacific oceans and one in the Indian Ocean. Each gyre
westerly winds consists of several named currents. Thus, the gyre
in the north Pacific is made up of the Kuroshio
northeast
trade winds current in the west, the California current
in the east, and two other linked currents.
Water tends to accumulate at the center
of these gyres—producing shallow
equator
INTRODUCTION westerly south Pacific direction GYRE CREATION
“mounds” in the ocean.
southeast
trade winds

gyre
In the north Pacific, the combination of
westerly and trade winds, always pushing
water to the right (by Ekman transport)
of gyre
produces a clockwise gyre. In the south
winds
Pacific, where winds push water to the left,
direction
a counterclockwise gyre is created.
of wind