Earth Rotation and Horizontal Motions
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.
again view Earth’s rotation from high above the equator, go to: Equator
Rotation - WMV or http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/Earth_Rotation.wmv.
[Alternative MP4: Equator Rotation - MP4 or
1.While viewing the animation, pause it when
the tail of a red arrow on the equator is in the center of the image. The red
arrow represents the direction and distance a fixed location on the equator
moves in one hour. It shows that as a place on the equator moves, its path [(is straight)(curves left)(curves right)]
as seen from above.
to: North Pole Rotation - WMV or
[Alternative MP4: North Pole Rotation -
you are reviewing Earth’s rotation from far above the North Pole. The Arctic
Circle is shown in yellow
2.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.
3.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).
Differences in arrow lengths reveal that as latitude increases, the eastward
speed of Earth’s surface due to rotation [(increases)(remains
to: South Pole Rotation - WMV or
[Alternative MP4: South Pole Rotation -
are now positioned high above the South Pole. The Antarctic Circle is shown in
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 [(clockwise)(counterclockwise)].
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 Effect. 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. We will 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.
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 the right. 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)(left)].
The deflection of the moving objects when they are viewed in a rotating frame
of reference is called the Coriolis Effect.
Planetary-scale Atmospheric Circulation
Figure 1 shows the three wind belts that
encircle both the Northern and Southern hemispheres. They 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 left. Such curvatures trace their origin
to Earth’s rotation.
6.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 Hemispheres. 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
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.
to: Wind Direction - WMV or
[Alternative MP4: Wind Direction - MP4 or http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/1_Wind_Direction.mp4]
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.gsfc.nasa.gov/giovanni/ ). The arrows show monthly-averaged
wind directions and speeds.
7.Run the animation. Note that over a year
there is considerable variation in wind patterns, but broad-scale 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
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). The 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 Hemisphere. This net transport of water due to
the coupling of the wind and water is known as Ekman transport.
to: Water Mound - WMV or
[Alternative MP4: Water Mound - MP4 or
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. Blue 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 sea surface. 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.
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)(east-central)]
portion of the basin.
9.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.
10.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.
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.
to: Ocean Current - WMV or
[Alternative MP4: Ocean Current - MP4 or http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/3_Ocean_Current.mp4]
the animation. The animation starts with the color-coded SSH example for ocean
background. The 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.
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. The component of the force of gravity that
is initiating the flow is called the pressure gradient force. Once the 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
11.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.
12.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. In the 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)].
13.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.
14.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.
15.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 exist. 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.
16.This animation ends by displaying actual
ocean surface circulation and currents based on actual observational data. 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.
17.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
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.