Earth Rotation and Horizontal MotionsIn Investigation 1A, videos were viewed that showed the sense of Earth’s rotation varied depending onwhether 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 itssurface, 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 orhttp: //www. ametsoc. org/amsedu/DS-Ocean/googlemaps/Earth_Rotation. wmv. . [Alternative MP4: Equator Rotation – MP4 or http: //www. ametsoc. org/amsedu/DSOcean/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 ofthe image. The red arrow represents the direction and distance a fixed location on the equator moves inone 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/DSOcean/googlemaps/Earth_Rotation_North_Pole. wmv. [Alternative MP4: North Pole Rotation – MP4 or http: //www. ametsoc. org/amsedu/DSOcean/googlemaps/Earth_Rotation_North_Pole. mp4]Here you are reviewing Earth’s rotation from far above the North Pole. The Arctic Circle is shown inyellow2. 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 vantagepoint 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’ssurface in one hour (the time it takes Earth to rotate 15°, or 1/24th of a complete rotation). Differencesin 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/DSOcean/googlemaps/Earth_Rotation_South_Pole. wmv. [Alternative MP4: South Pole Rotation – MP4 or http: //www. ametsoc. org/amsedu/DSOcean/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) showcircular 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 (lineperpendicular 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 acomplete rotation in 24 hours, so a moving object there would be subjected to a maximum CoriolisEffect. At the equator, a vertical is oriented perpendicular to Earth’s axis, so there is no turning in thehorizontal imparted by the planet’s rotation and hence, no Coriolis Effect. Moving from the pole to theequator (toward lower latitudes), the Coriolis Effect lessens from maximum to zero as a vertical becomesless aligned with Earth’s rotational axise have explored the effect of Earth’s rotation on the movement of locations on Earth’s surface. We willnow explore what happens when objects (air and water parcels) move freely across the surface of arotating Earth while their horizontal motions are measured relative to the Earth’s surface. We start byknowing that the effect of Earth’s rotation varies on horizontally moving objects from being zero at theequator 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 fromabove, objects moving freely across Earth’s surface will, relative to the surface, appear to be pulled tothe 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 iscalled the Coriolis Effect. The Planetary-scale Atmospheric CirculationFigure 1 shows the three wind belts that encircle both the Northern and Southern hemispheres. Theyare produced by a combination of factors, including the Coriolis Effect. Note that arrows indicating windflow in the Northern Hemisphere curve to the right while those in the Southern Hemisphere turn to theleft. 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 exhibitdifferent circulation patterns around their centers of high pressure in the Northern and SouthernHemispheres. 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 theopposite direction. Wind-Driven Ocean CirculationThe ocean impacts the atmosphere and the atmosphere impacts the ocean through the exchange ofmatter and energy. One example of the close ties between atmosphere and ocean is the maintenance ofsubtropical ocean gyres, roughly circular wind-driven surface currents centered near 30 degrees latitudein Earth’s Northern and Southern Hemisphere ocean basins. In this part of the investigation we examinethe 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 ofprevailing winds on the ocean surface and Earth’s rotation. Go to: Wind Direction – WMV or http: //www. ametsoc. org/amsedu/DSOcean/googlemaps/1_Wind_Direction. wmv. [Alternative MP4: Wind Direction – MP4 or http: //www. ametsoc. org/amsedu/DSOcean/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 broadscale patterns persist. Look for similarities between the animation and Figure 1. Both show that thesurface wind circulation in the North Atlantic Ocean basin is generally [(clockwise)(counterclockwise)] asseen from aboveWinds blowing over the ocean exert frictional drag that moves surface waters. At the equator the windsmove water directly forward. Away from the equator, where the Coriolis effect makes its presenceincreasingly known as latitude increases, surface waters are moved by as much as 45 degrees to the rightof the wind’s direction in the Northern Hemisphere (and to the left in the Southern Hemisphere). Thesurface waters drag and deflect water layers below, resulting in a net water flow at an angle of 90degrees to the wind direction, to the right in the Northern Hemisphere and to the left in the SouthernHemisphere. This net transport of water due to the coupling of the wind and water is known as Ekmantransport. Go to: Water Mound – WMV or http: //www. ametsoc. org/amsedu/DSOcean/googlemaps/2_Water_Mound. wmv. [Alternative MP4: Water Mound – MP4 or http: //www. ametsoc. org/amsedu/DSOcean/googlemaps/2_Water_Mound. mp4]Run the animation. Starting with the display of the June 2008 monthly-averaged wind field, black arrowsappear which roughly approximate the general wind pattern over the North Atlantic Ocean basin. Bluearrows then emerge to represent the Ekman transport, the net flow of water 90° to the right of the winddirection in the Northern Hemisphere due to wind forcing. Next appear contour lines delineating theresulting mound of water due to the Ekman transport, with the innermost contour enclosing the highestsea 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 sequenceof events being depicted. 8. In the animation, black arrows first appear that approximate the average surface wind flow in theNorth Atlantic Ocean basin based on actual wind observations. The winds over the North Atlantic Oceanbasin exhibit a clockwise circulation as seen from above with its center in [(west-central)(central)(eastcentral)] portion of the basin. 9. Blue arrows then appear to appear to represent the wind-generated Ekman transport. The direction ofthe Ekman transport changes as the averaged wind direction changes across the ocean basin to producea 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 isdepicted highest in the [(west-central)(central)(east-central)] portion of the basin. This shape is acommon 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 aparticular time by a sensor aboard a satellite platform. The reported heights generally confirm themounding of water due to wind forcing as described in the animation. Go to: Ocean Current – WMV or http: //www. ametsoc. org/amsedu/DSOcean/googlemaps/3_Ocean_Current. wmv. [Alternative MP4: Ocean Current – MP4 or http: //www. ametsoc. org/amsedu/DSOcean/googlemaps/3_Ocean_Current. mp4]Run the animation. The animation starts with the color-coded SSH example for ocean background. Thecontour lines portray the mound of water that was generated and is maintained by the Ekman transportresulting 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 waterstreaming down a hillside on land. The flow is initially directly downhill due to the pull of gravity. Thecomponent of the force of gravity that is initiating the flow is called the pressure gradient force. Once thewater is in motion, the underlying rotating Earth produces the Coriolis Effect which is represented by animaginary Coriolis force. In the Northern Hemisphere the Coriolis force is always seen as pulling themoving water 90° to the right of the direction of flow11. Focus on the light blue parcel of water near the center of the mound that is being put into motiondown the sloping water surface by the pressure gradient force shown by a thin blue arrow. As soon asthe water parcel starts moving, a Coriolis force (green arrow) arises to account for the effect of Earth’srotation 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 strengthenswhile always acting at a right angle to the right of the direction of movement. Throughout, the pressuregradient force always remains oriented directly downhill and perpendicular to the contour lines. In thesecond 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, thiscauses 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 continueturning rightward until it arrives at its third position. There, the Coriolis force has increased until it isequal in magnitude and acting in the direction opposite to the pressure gradient force. From the timeonward after the forces balance one another, the animation shows that the water will be flowing (asdenoted 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 straightdown 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 regionof 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 thanelsewhere in the dome, it can be assumed that is where the [(least)(greatest)] pressure gradient forcesexist. This will result in the fastest ocean currents compared to elsewhere around the domeWhen the balance has been achieved between the pressure gradient force and the Coriolis force, thecondition called geostrophic flow has been achieved. Essentially, this condition causes flow “around” thehill of water. This geostrophic flow (light blue arrows) generally gives rise to the ocean currents that areintegral components of ocean gyres. 16. This animation ends by displaying actual ocean surface circulation and currents based on actualobservational data. Note the Florida Current/Gulf Stream system that extends from near the southern tipof Florida to Cape Hatteras, NC and beyond. Its position and flow [(are)(are not)] consistent with thedescription of the North Atlantic gyre examined in this investigation. 17. This animation shows that the Coriolis Effect plays an essential role in the formation andmaintenance of the North Atlantic Subtropical Gyre. The Coriolis Effect plays a similar role in theformation and maintenance of subtropical gyres in the Southern Hemisphere. There, the apparentCoriolis force arising from the Coriolis Effect acts 90° to the left of the direction of motion and producessubtropical 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 occurwhere sea-surface temperatures are relatively high.
Originally posted 2018-07-18 04:53:17. Republished by Blog Post Promoter