Earth Rotation and Horizontal Motions
In Investigation 1A, videos were viewed that showed the sense of Earth’s rotation varied depending on
whether the viewer was observing the motion from high above the equator, North Pole, or South Pole.
Here we will consider how Earth’s rotation impacts the motion of objects moving freely across its
surface, particularly focusing on winds and water flowing on the ocean surface.
To again view Earth’s rotation from high above the equator, go to: Equator Rotation - WMV or
[Alternative MP4: Equator Rotation - MP4 or http://www.
While viewing the animation, pause it when the tail of a red arrow on the equator is in the center of
The red arrow represents the direction and distance a fixed location on the equator moves in
It shows that as a place on the equator moves, its path [(is straight)(curves left)(curves right)]
as seen from above.
Go to: North Pole Rotation - WMV or http://www.
[Alternative MP4: North Pole Rotation - MP4 or http://www.
Here you are reviewing Earth’s rotation from far above the North Pole.
The Arctic Circle is shown in
As viewed from above the North Pole, all points on Earth’s visible surface (except at the North Pole)
follow a circular path as seen from above as Earth rotates.
The sense of Earth’s rotation from this vantage
point is [(clockwise)(counterclockwise)] around the North Pole as seen from above.
Note the lengths of the red arrows which denote the motion of places at different latitudes on Earth’s
surface in one hour (the time it takes Earth to rotate 15°, or 1/24th of a complete rotation).
in arrow lengths reveal that as latitude increases, the eastward speed of Earth’s surface due to rotation
[(increases)(remains the same)(decreases)].
Go to: South Pole Rotation - WMV or http://www.
[Alternative MP4: South Pole Rotation - MP4 or http://www.
You are now positioned high above the South Pole.
The Antarctic Circle is shown in yellow.
As viewed from above the South Pole, all points on Earth’s visible surface (except at the pole) show
circular motion due to Earth’s rotation, and the sense of Earth’s rotation from this vantage point is
In summary, the sense and impact of Earth’s rotation depends on location.
At the pole, a vertical (line
perpendicular to Earth’s surface and directed through the center of the planet) is oriented along
(coincident to) Earth’s axis.
Therefore, at the pole location a stationary object would experience a
complete rotation in 24 hours, so a moving object there would be subjected to a maximum Coriolis
At the equator, a vertical is oriented perpendicular to Earth’s axis, so there is no turning in the
horizontal imparted by the planet’s rotation and hence, no Coriolis Effect.
Moving from the pole to the
equator (toward lower latitudes), the Coriolis Effect lessens from maximum to zero as a vertical becomes
less aligned with Earth’s rotational axis
e have explored the effect of Earth’s rotation on the movement of locations on Earth’s surface.
now explore what happens when objects (air and water parcels) move freely across the surface of a
rotating Earth while their horizontal motions are measured relative to the Earth’s surface.
We start by
knowing that the effect of Earth’s rotation varies on horizontally moving objects from being zero at the
equator and increases with increasing latitude until reaching a maximum at the poles.
In the Northern Hemisphere, where the sense of planetary rotation is counterclockwise as seen from
above, objects moving freely across Earth’s surface will, relative to the surface, appear to be pulled to
In the Southern Hemisphere, where the sense of rotation is clockwise as seen from above,
objects moving across Earth’s surface will appear to be pulled in the opposite direction, to the [(right)
The deflection of the moving objects when they are viewed in a rotating frame of reference is
called the Coriolis Effect.
The Planetary-scale Atmospheric Circulation
Figure 1 shows the three wind belts that encircle both the Northern and Southern hemispheres.
are produced by a combination of factors, including the Coriolis Effect.
Note that arrows indicating wind
flow in the Northern Hemisphere curve to the right while those in the Southern Hemisphere turn to the
Such curvatures trace their origin to Earth’s rotation.
Evidence of the Coriolis Effect at play is the circulation of subtropical anticyclones (Hs) that exhibit
different circulation patterns around their centers of high pressure in the Northern and Southern
As seen in Figure 1, the Northern Hemisphere high-pressure systems exhibit [(clockwise)
(counterclockwise)] motion as seen from above while those in the Southern Hemisphere turn in the
Wind-Driven Ocean Circulation
The ocean impacts the atmosphere and the atmosphere impacts the ocean through the exchange of
matter and energy.
One example of the close ties between atmosphere and ocean is the maintenance of
subtropical ocean gyres, roughly circular wind-driven surface currents centered near 30 degrees latitude
in Earth’s Northern and Southern Hemisphere ocean basins.
In this part of the investigation we examine
the factors responsible for ocean gyres, focusing on the North Atlantic Subtropical Gyre as an example.
Conditions that give rise to ocean gyres are initiated by a combination of the frictional effects of
prevailing winds on the ocean surface and Earth’s rotation.
Go to: Wind Direction - WMV or http://www.
[Alternative MP4: Wind Direction - MP4 or http://www.
The animation shows surface wind patterns based on actual observations made over one year (July 2007
– June 2008), available via the NASA-GIOVANNI web portal (http://giovanni.
The arrows show monthly-averaged wind directions and speeds.
Run the animation.
Note that over a year there is considerable variation in wind patterns, but broadscale patterns persist.
Look for similarities between the animation and Figure 1.
Both show that the
surface wind circulation in the North Atlantic Ocean basin is generally [(clockwise)(counterclockwise)] as
seen from above
Winds blowing over the ocean exert frictional drag that moves surface waters.
At the equator the winds
move water directly forward.
Away from the equator, where the Coriolis effect makes its presence
increasingly known as latitude increases, surface waters are moved by as much as 45 degrees to the right
of the wind’s direction in the Northern Hemisphere (and to the left in the Southern Hemisphere).
surface waters drag and deflect water layers below, resulting in a net water flow at an angle of 90
degrees to the wind direction, to the right in the Northern Hemisphere and to the left in the Southern
This net transport of water due to the coupling of the wind and water is known as Ekman
Go to: Water Mound - WMV or http://www.
[Alternative MP4: Water Mound - MP4 or http://www.
Run the animation.
Starting with the display of the June 2008 monthly-averaged wind field, black arrows
appear which roughly approximate the general wind pattern over the North Atlantic Ocean basin.
arrows then emerge to represent the Ekman transport, the net flow of water 90° to the right of the wind
direction in the Northern Hemisphere due to wind forcing.
Next appear contour lines delineating the
resulting mound of water due to the Ekman transport, with the innermost contour enclosing the highest
Finally a color-coded ocean surface appears showing an actual sea surface height (SSH)
observation at a particular time as an example.
Play the animation several times to explore the sequence
of events being depicted.
In the animation, black arrows first appear that approximate the average surface wind flow in the
North Atlantic Ocean basin based on actual wind observations.
The winds over the North Atlantic Ocean
basin exhibit a clockwise circulation as seen from above with its center in [(west-central)(central)(eastcentral)] portion of the basin.
Blue arrows then appear to appear to represent the wind-generated Ekman transport.
The direction of
the Ekman transport changes as the averaged wind direction changes across the ocean basin to produce
a convergence of water and the [(lowering)(mounding)] of the water surface.
Contour lines are added to approximate the configuration of the water mound.
The mound is
depicted highest in the [(west-central)(central)(east-central)] portion of the basin.
This shape is a
common characteristic of the large sub-tropical gyres of the world ocean.
The animation ends with a color-coded depiction of the sea surface height (SSH) as determined for a
particular time by a sensor aboard a satellite platform.
The reported heights generally confirm the
mounding of water due to wind forcing as described in the animation.
Go to: Ocean Current - WMV or http://www.
[Alternative MP4: Ocean Current - MP4 or http://www.
Run the animation.
The animation starts with the color-coded SSH example for ocean background.
contour lines portray the mound of water that was generated and is maintained by the Ekman transport
resulting from the persistent wind pattern over the North Atlantic Ocean basin.
In response to the mound of water, surface seawater flows down sloping surfaces, just like water
streaming down a hillside on land.
The flow is initially directly downhill due to the pull of gravity.
component of the force of gravity that is initiating the flow is called the pressure gradient force.
water is in motion, the underlying rotating Earth produces the Coriolis Effect which is represented by an
imaginary Coriolis force.
In the Northern Hemisphere the Coriolis force is always seen as pulling the
moving water 90° to the right of the direction of flow
Focus on the light blue parcel of water near the center of the mound that is being put into motion
down the sloping water surface by the pressure gradient force shown by a thin blue arrow.
As soon as
the water parcel starts moving, a Coriolis force (green arrow) arises to account for the effect of Earth’s
rotation and begins to deflect the parcel to the [(right)(left)] of its direction of movement.
The water parcel speeds up as it flows down the sloping surface, so the Coriolis force strengthens
while always acting at a right angle to the right of the direction of movement.
Throughout, the pressure
gradient force always remains oriented directly downhill and perpendicular to the contour lines.
second position of the water parcel, the thick white arrow that appears shows the direction of motion.
As seen in the animation, this causes the parcel to turn further to the right.
In the global view, this
causes the water parcel to turn more towards the [(east)(west)].
The parcel will continue to speed up, causing the Coriolis force to increase.
The parcel will continue
turning rightward until it arrives at its third position.
There, the Coriolis force has increased until it is
equal in magnitude and acting in the direction opposite to the pressure gradient force.
From the time
onward after the forces balance one another, the animation shows that the water will be flowing (as
denoted by the orientation of the motion arrow head) [(across)(parallel to)] the contour lines.
Water at other locations near the center of the dome will follow similar paths, first flowing straight
down hill and then turning rightward as shown by the black arrows in the animation.
The animation shows that the paths of water parcels initially flowing downhill from the central region
of the mound turn to reveal an overall [(clockwise)(counterclockwise)] circulation as seen from above.
Because the contour lines in the western portion of the dome of water are more closely spaced than
elsewhere in the dome, it can be assumed that is where the [(least)(greatest)] pressure gradient forces
This will result in the fastest ocean currents compared to elsewhere around the dome
When the balance has been achieved between the pressure gradient force and the Coriolis force, the
condition called geostrophic flow has been achieved.
Essentially, this condition causes flow “around” the
hill of water.
This geostrophic flow (light blue arrows) generally gives rise to the ocean currents that are
integral components of ocean gyres.
This animation ends by displaying actual ocean surface circulation and currents based on actual
Note the Florida Current/Gulf Stream system that extends from near the southern tip
of Florida to Cape Hatteras, NC and beyond.
Its position and flow [(are)(are not)] consistent with the
description of the North Atlantic gyre examined in this investigation.
This animation shows that the Coriolis Effect plays an essential role in the formation and
maintenance of the North Atlantic Subtropical Gyre.
The Coriolis Effect plays a similar role in the
formation and maintenance of subtropical gyres in the Southern Hemisphere.
There, the apparent
Coriolis force arising from the Coriolis Effect acts 90° to the left of the direction of motion and produces
subtropical ocean gyres that circulate [(clockwise)(counterclockwise)].
It is important to note that there are factors in addition to Ekman transport which affect the topography
(SSH) of the ocean surface.
In particular, water expands when heated so that higher sea levels occur
where sea-surface temperatures are relatively high.