Griffin Glenn, Long ScienceTides & Currents Web-Quest Worksheet

Due: Monday, 28 March 2011

Instructions: Read these instructions carefully before proceeding. Log on to the NOAA site at: http://oceanservice.noaa.gov/education/kits/currents/welcome.html. Download the Tutorial (.pdf) located in the upper right margin of the pages. In this way, if you get disconnected from the Internet, you still have some reference information to work with.

Next, carefully read through each section of this resource, answering the items below as you proceed. Be certain to play all animations, and enlarge and study all diagrams. And yes, I expect each of you to read the whole thing and to answer all the items. You will need to type up your answers to the following questions (do NOT cut and paste) and print your assignment so that you can put it into your lab notebook after it’s been graded.

Tidal Currents

1. What is an ocean current?An ocean current is the motion of the water in an ocean.

2. What are the three main factors driving ocean currents?The three main factors driving ocean currents are the motion of the tides, wind, and thermohaline circulation.

3. How are tidal currents different from other types of currents?Tidal currents are the only currents that change in a regular pattern and can be predicted for the future.

4. Briefly describe a tidal current.Tidal currents are made when water rises vertically near a shore, creating horizontal currents. They move both towards and away from shore, directions called flooding and ebbing.

5. What is a rectilinear tidal current, and where are they usually found?Rectilinear tidal currents are currents that ebb and flood in opposite directions and possess “slack periods”. They are usually found in coastal rivers and estuaries.

6. What is the difference between spring, neap, perigean, and apogean currents? Spring currents are made when the moon is in the full or new phases, and are stronger currents. Neap currents are made when the moon is in first or third quarter phases, and are weaker currents. Perigean currents are made when the moon and the Earth are positioned closest to each other, and are stronger than average. Apogean currents are produced when the Earth and moon are farthest away from each other, and are weaker currents than average.

7. The sun is much more massive than the moon. Why is the moon’s effect on tidal currents so much stronger than that of the sun?

The moon’s effect on tidal currents is due to its proximity to the earth, as the tide-generating force of an object is measured based on the mass of the object being divided by the cube of its distance, making the Moon’s far closer distance cause a much lesser decrease in its tide-generating force.

8. About how fast do tidal currents move?The speed at which tidal currents move depends on a wide variety of factors, from the shape of their environment to the proximity and phase of the moon or sun.

Coastal Currents

Griffin Glenn, Long Science

9. Draw a wave, labeling the wave height, crest, trough, and length (text boxes are your friend). Under your drawing, explain what wave frequency and period are.

Wave frequency is the number of wave crests that pass a point every second. Wave period is the time required for the wave crest at the start of a wave’s length to reach the other end of its length.

10. What three factors contribute the most to the formation of coastal currents?Winds, waves, and land formations contribute most to the formation of coastal currents.

11. What factors affect wave height?Wave height is affected by wind speed, wind duration, and fetch. Fetch is the distance that winds blows over water in a single direction.

12. What causes waves to break as they move closer to shore?Friction between the sea floor and the bottom of the wave causes the bottom of the wave to move slower as it approaches shore. The top of the wave, still moving quickly, overtakes the bottom and forms an increasingly steep angle, eventually breaking.

13. Explain how longshore currents are formed.Longshore currents are formed when a wave reaches a beach or coastline, where it releases a burst of energy. This creates a current running parallel to the shoreline.

14. What effect does wave angle and wave velocity have on longshore currents?When the wave angle is greater or the wave has a higher velocity, then the longshore current will increase in velocity.

15. What is longshore drift?Longshore drift is the capture of sediment that occurs when a wave washes onto a beach and drags the sand back into the ocean.

16. How might longshore currents and longshore drift affect attempts to reopen Midnight Pass?

Longshore currents and longshore drift might affect attempts to reopen Midnight Pass by eroding away any new land placed in the area, based on their past effects. They could undermine buildings and roads, as well as removing beaches that form near Midnight Pass.

17. How do rip currents form, and why are they potentially dangerous?Rip currents form as longshore currents move on and off the beach, near low spots in sandbars and jetties or piers. They are potentially dangerous because they can sweep swimmers out into sea very quickly with little warning, and are often very strong.

18. What are upwellings and downwellings, how do they from, and how do they affect fisheries?

Upwellings are areas where water is rising up from below the surface to replace water being blown away by winds. Downwellings are areas where surface water builds up in an area due to winds, and then sinks to the bottom.

19. About how fast do coastal currents move?

Wave Length

Wave Trough

Wave Crest

Wave Height

Griffin Glenn, Long ScienceLongshore currents’ speed depends on the angle at which the wave approaches the beach, the beach height, and the steepness of the beach slope. Rip currents move at speeds of one to two feet per second, but have been measured up to eight feet per second.

Surface Currents

20. What drives surface currents? Surface currents are driven by a global wind system.

21. Explain the influence of the Coriolis effect on wind and water currents. The Coriolis effect causes wind to be diverted and to travel in a curved path as it moves north and south. Thus, water currents are similarly diverted as the wind currents drag them.

22. What is the Ekman Spiral and what is its cause?The Ekman Spiral is a formation that occurs in water as layers of water molecules exert successively more drag on the molecules below them. It is caused when the water is moved by wind, and the top layer is first set into motion, then acting on the other layers.

23. How do ocean gyres form?Ocean gyres form when wind drags on the surface of the water, causing it to move. Then, the Coriolis effect diverts the wind into a circular pattern, creating a gyre.

24. What are the five major ocean gyres?The five major ocean gyres are the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres.

25. About how fast do surface currents move?Surface currents move at speeds of 25 to 75 miles per day.

The Global Conveyor Belt

26. What drives deep ocean currents?Deep ocean currents are driven by differences in the water’s density.

27. Describe flow in the global conveyor belt.The flow of the global conveyor belt begins in the North Atlantic, and moves south along the western Atlantic. It then reaches Antarctica and travels west, breaking into two branches. One of these branches enters the Indian Ocean, and the other enters the Pacific. These branches both warm, rise, and move north, eventually circling back across the globe to the Northern Atlantic, where the cycle begins anew.

28. About how fast do global conveyor belt currents move?Global conveyor belt currents move at speeds of a few centimeters per second.

29. How is the global conveyor belt a vital component of the carbon and ocean nutrient cycling?

The global conveyor belt provides algae and seaweed their food, and supports the base of the world’s food chain. Also, surface waters that are depleted of carbon dioxide and nutrients are enriched as they reach the deeps once more.

30. How might global climate change affect the global conveyor belt?Global climate change could affect the global conveyor belt by causing an influx of warm fresh water that could block the formation of sea ice.

31. What do you think might happen if global climate change affected the global conveyor belt?

The ecosystems of Earth would likely be dealt a strong blow, and weather across the world would be changed.

Griffin Glenn, Long ScienceHow Are Currents Measured?

32. Briefly describe the various ways ocean currents are measured.Ocean currents are measured through one of two methods. In the first, an object is placed in the current as the observer stands on a stationary point, like an anchored ship. The observer then allows the object to move a specific distance, and records the time that the object takes to move this distance. The observer will also note the direction the object travels, and from the data gathered they can find the speed of the current. In another method, an instrument is anchored in the ocean, and the flow of the water is measured as it passes the instrument. Instruments called Davis Drifters are used to measure the speed of wind-driven surface currents, and they drift along with these currents. Some instruments, called “profiling floats”, drift down to great depths in order to measure the speed of those currents as they’re carried along.

33. Discuss the various ways in which ocean currents affect our daily lives.Ocean currents help search-and-rescue personnel define where different people could have ended up, or what they may have done. Ocean currents determine the state of the global fishing market based on the prevalence of upwellings and nutrient movements, and determine where swimmers swim. They also determine the movements of ships, which are responsible for a large part of the world’s commerce, and thus the global economy.