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Earth Rotation and Horizontal Motions

Question # 00097956
Subject: Geology
Due on: 09/27/2015
Posted On: 08/28/2015 11:50 PM

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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 ]

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

[Alternative MP4: North Pole Rotation - MP4 or]

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

[Alternative MP4: South Pole Rotation - MP4 or]

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 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 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

[Alternative MP4: Wind Direction - MP4 or]

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 ( ). 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

[Alternative MP4: Water Mound - MP4 or]

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

[Alternative MP4: Ocean Current - MP4 or]

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.

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Earth Rotation and Horizontal Motions

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Tutorial Preview …from xxxxx while xxxxx in the xxxxxxxx Hemisphere turn xx the xxxxxxxx xxxxxxxxx Wind-Driven xxxxx Circulation The xxxxx impacts the xxxxxxxxxx and xxx xxxxxxxxxx impacts xxx ocean through xxx exchange of xxxxxx and xxxxxx xxx example xx the close xxxx between atmosphere xxx ocean xx xxx maintenance xx subtropical ocean xxxxxx roughly circular xxxxxxxxxxx surface xxxxxxxx xxxxxxxx near xx degrees latitude xx Earth’s Northern xxx Southern xxxxxxxxxx xxxxx basins xx this part xx the investigation xx examine xxx xxxxxxx responsible xxx ocean gyres, xxxxxxxx on the xxxxx Atlantic xxxxxxxxxxx xxxx as xx example Conditions xxxx give rise xx ocean xxxxx xxx initiated xx a combination xx the frictional xxxxxxx of xxxxxxxxxx xxxxx on xxx ocean surface xxx Earth’s rotation xx to: xxxx xxxxxxxxx - xxx or http://www xxxxxxx org/amsedu/DS-Ocean/googlemaps/1_Wind_Direction wmv xxxxxxxxxxxx MP4: xxxx xxxxxxxxx - xxx or http://www xxxxxxx org/amsedu/DS-Ocean/googlemaps/1_Wind_Direction mp4] xxx animation xxxxx xxxxxxx wind xxxxxxxx based on xxxxxx observations made xxxx one xxxx xxxxx 2007 xxx June 2008), xxxxxxxxx via the xxxxxxxxxxxxx web xxxxxx xxxxxxxxxxxxxxxx gsfc xxxx gov/giovanni/ ) xxx arrows show xxxxxxxxxxxxxxxx wind xxxxxxxxxx xxx speeds x Run the xxxxxxxxx Note that xxxx a xxxx xxxxx is xxxxxxxxxxxx variation in xxxx patterns, but xxxxxxxxxxx patterns xxxxxxx xxxx for xxxxxxxxxxxx between the xxxxxxxxx and Figure x Both xxxx xxxx the xxxxxxx wind circulation xx the North xxxxxxxx Ocean xxxxx xx generally xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx as seen xxxx above Winds xxxxxxx over xxx xxxxx exert xxxxxxxxxx drag that xxxxx surface waters xx the xxxxxxx xxx winds xxxx water directly xxxxxxx Away from xxx equator, xxxxx xxx Coriolis xxxxxx makes its xxxxxxxx increasingly known xx latitude xxxxxxxxxx xxxxxxx waters xxx moved by xx much as xx degrees xx xxx right xx the wind’s xxxxxxxxx in the xxxxxxxx Hemisphere xxxx xx the xxxx in the xxxxxxxx Hemisphere) The xxxxxxx waters xxxx xxx deflect xxxxx layers below, xxxxxxxxx in a xxx water xxxx xx an xxxxx of 90 xxxxxxx to the xxxx direction, xx xxx right xx the Northern xxxxxxxxxx and to xxx left xx xxx Southern xxxxxxxxxx This net xxxxxxxxx of water xxx to xxx xxxxxxxx of xxx wind and xxxxx is known xx Ekman xxxxxxxxx xx to: xxxxx Mound - xxx or http://www xxxxxxx org/amsedu/DS-Ocean/googlemaps/2_Water_Mound xxx xxxxxxxxxxxx MP4: xxxxx Mound - xxx or http://www xxxxxxx org/amsedu/DS-Ocean/googlemaps/2_Water_Mound xxxx xxx the xxxxxxxxx Starting with xxx display of xxx June xxxx xxxxxxxxxxxxxxxx wind xxxxxx black arrows xxxxxx which roughly xxxxxxxxxxx the xxxxxxx xxxx pattern xxxx the North xxxxxxxx Ocean basin xxxx arrows xxxx xxxxxx to xxxxxxxxx the Ekman xxxxxxxxxx the net xxxx of xxxxx xxxx to xxx right of xxx wind direction xx the xxxxxxxx xxxxxxxxxx due xx wind forcing xxxx appear contour xxxxx delineating xxx xxxxxxxxx mound xx water due xx…
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