Earth Rotation and Horizontal Motions

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
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/Earth_Rotation.wmv. .

[Alternative MP4:
Equator Rotation – MP4 or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/Earth_Rotation.mp4 ]

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.

Go to: North Pole
Rotation – WMV or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/Earth_Rotation_North_Pole.wmv.

[Alternative MP4:
North Pole Rotation – MP4 or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/Earth_Rotation_North_Pole.mp4]

Here 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
the same)(decreases)].

Go to: South Pole
Rotation – WMV or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/Earth_Rotation_South_Pole.wmv.

[Alternative MP4:
South Pole Rotation – MP4 or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/Earth_Rotation_South_Pole.mp4]

You are now positioned
high above the South Pole. The Antarctic Circle is shown in yellow.

4.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 [(clockwise)(counterclockwise)].

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 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.

5.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 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.

The 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 Northernand 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 opposite
direction.

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.ametsoc.org/amsedu/DS-Ocean/googlemaps/1_Wind_Direction.wmv.

[Alternative MP4:
Wind Direction – MP4 or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/1_Wind_Direction.mp4]

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.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 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). 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.

Go to: Water Mound –
WMV or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/2_Water_Mound.wmv.

[Alternative MP4:
Water Mound – MP4 or
http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/2_Water_Mound.mp4]

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. 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.

8.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)(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.

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.ametsoc.org/amsedu/DS-Ocean/googlemaps/3_Ocean_Current.wmv.

[Alternative MP4:
Ocean Current – MP4 or http://www.ametsoc.org/amsedu/DS-Ocean/googlemaps/3_Ocean_Current.mp4]

Run 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.

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. 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 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.