Ocean Gyres

Outside Links: Lab Guide
Please leave tips, examples, and troubleshooting information in the comment section below.


Background:


In this experiment we will create an anticyclonic subtropical gyre in the northern hemisphere (in our case, the North Pacific Subtropical Gyre). To do so, there are 3 key ingredients to model a rotating, spherical earth:

1) Sloped tank bottom/Beta plane: To mimic spherical effects
  • Shallow end represents poles
  • Deep end represents tropics
2) Fans: To create anticyclonic wind
  • Fan on the equatorward side blowing west represents trade winds
  • Fan on the poleward side blowing east represents westerlies
3) Coasts: To represent geometric constraints of the gyre
  • East side of tank represents California Coast
  • West side of tank represents Japan

Experimental Setup:


  1. Place 2 blocks on one side of the tank (roughly 2" high blocks if using a 16"x16"x8" tank).
  2. Fill tank halfway with water.
  3. Insert rectangular plate the same dimensions of the tank to create a sloped bottom- press down on all 4 corners to remove any air bubbles.
  4. Clamp 2 fans to opposite sides of the tank to represent the trade winds and westerlies- water level should be about 1 cm below fans. Add or remove water as necessary to achieve the correct water level.
  5. Turn fans on- fans should be blowing gently enough as to not create waves or ripples.
  6. Rotate tank at ~10 rpm.
  7. Wait 15-20 minutes until solid body rotation is achieved.
  8. Using a pipette, drop dye into NE and SW corners of tank (use different colors).


Figure 1.png
Figure 1: Experimental setup for creating NPSG (plan view)


Figure 2.png
Figure 2: Experimental setup for creating NPSG (section view)

Tips:


  • After putting some water in the tank, make sure to push down the beta plane on all 4 corners and ensure no air is trapped beneath the plate. Use a level to make sure the equatorward side and polar side are level.
  • Make sure fans are centered directly across from each other, and are at the same height/same distance from water surface.
  • Use a pipette to distribute the dye if available- it will “shoot” the dye out faster so it will go a bit deeper. If using the food-coloring dropper the dye will exit more slowly, and some may be carried along the surface because of the fans. The pipette reduces interference with the “winds”.
  • We used a 2” block in our 16”x16” tank to create the sloped bottom representing the beta plane (12.5% slope). We tried it once using a 1” block and it did not work as well (but perhaps that was from something else).

Expected Results:


We expect the dye dropped in the NE corner of the tank to slowly disperse towards the equator. In contrast, the dye introduced to the SW corner of the tank should be swiftly picked up by a western boundary current (in our case, the Kuroshio Current). This dye should turn eastward once it hits the northern end of the tank, then recirculate throughout the tank. See Figure 3 for expected results.


Figure 3.png
Figure 3: Expected results (Red dye represents boundary current introduced to the SW corner, blue dye represents southward interior flow, introduced to the NE corner)

Optional Calculations:


Using video data from three runs of the experiment, we made estimates for the velocity of the dye after it was dropped into the NE and SW corners. Our objective was to how much more quickly the western boundary current moved than the rest of the subtropical gyre. We estimated the amount of time it took the dye introduced to the NE corner to move halfway down the tank (approximately 8” or 20.32 cm), and the amount of time it took the dye dropped in the SW corner of the tank to move to the top of the western boundary current (approximately 16” or 40.64 cm). Table 1 shows the raw data of the experiment, and Table 2 summarizes the results.

Table 1: Dye Travel Time Data
Run
Start Time (NE Corner)
End Time (Halfway point)
Travel Time (8”)
Start Time (SW Corner)
End Time (Top of WBC)
Travel Time (16”)
1
0:06
4:02
3:56
1:37
2:03
0:26
2
13:15
15:21
2:06
14:40
15:01
0:21
3
9:03
12:47
3:42
9:15
9:56
0:41

Table 2: Summary of results

Distance Traveled (cm)
Average Travel Time (sec)
Velocity (cm/s)
Southward transport
20.32
194
0.10
Western Boundary Current
40.64
29
1.40
From this calculation, it is clear that the dye introduced into the western boundary current moves much more quickly (14x more quickly) than the dye introduced into the NE corner.

Sources of Error:


There were many sources of error for this calculation, which is why only used for a velocity comparison. We merely wanted to show how much more quickly the western boundary current moves than the rest of the subtropical gyre. To accurately calculate the horizontal velocity, one would need to take into account the vertical velocity of the dye, and which part of the water column was transported the fastest. One would also need to place the dye in precisely the same location for every run of the experiment.


Other Experiment Ideas:


  • We sprinkled "trash" throughout our tank to mimic the great pacific garbage patches. We used sequins and styrofoam, and the results looked pretty good- the trash collected in 2 main areas (except of course, when we did it in front of the class).
  • It would be interesting to create a southern hemisphere gyre/cyclonic wind driven circulation.
  • See what happens when the fans are turned off.

Real Life Examples of Ocean Gyres:


Gyres.png


Three types of gyres exist in the world's oceans: tropical gyres, subtropical gyres, and subpolar gyres. Tropical gyres are located closest to the equator and have a greater east-west component than north-south circulation. Subtropical gyres circulate anti-cyclonically around high pressure systems in the mid-latitudes and are generated by the equatorial easterly winds and the westerly “roaring 40’s.” Subpolar gyres are above 40⁰ and rotate cyclonically [1].

The five largest gyres are the subtropical gyres that are found in each major oceans spanning the mid-latitudes. These gyres are named for their locations and each has an associated western boundary current. These include the North Pacific Subtropical Gyre (NPSG) with the Kuroshio Current, South Pacific Subtropical Gyre (SPSG) with the East Australian Current, North Atlantic Subtropical Gyre (NASG) with the Gulf Stream, South Atlantic Subtropical Gyre (SASG) with the Brazil Current, and the Indian Ocean Subtropical Gyre (IOSG) with the Agulhas. Each of these gyres extends 5,000-10,000 km both zonally and meridionally. Due to the balancing of the equaterward Sverdrup transport over most of the gyre, these gyres have strong poleward western boundary currents. The Kuroshio, for example, has a transport rate of 60-70 Sv and can reach up to 150 Sv seasonally [1]. These currents play an important role in global climate, as they transport heat from the lower latitudes to the higher latitudes.

The winds driving the gyre circulations cause Ekman transport perpendicular to their direction, resulting in a buildup of water in the center of each gyre. This water pile-up generates high pressure in the middle of the gyre and Ekman pumping (downwelling) to occur in these regions. These high pressure areas can be seen in maps of sea surface height [1]. In the NPSG, the ocean surface is higher near the western boundary in the center of the gyre. Compared to regions outside the gyre, the area inside the NPSG sea surface is higher, with the pressure gradient increasing more steeply closer to the Kuroshio and western boundary. A similar pressure gradient pattern is seen in the other subtropical gyres as well.

Subtropical gyres have gained media attention recently because they concentrate marine debris in the ocean. For example, a convergence zone exists on the northern side of the NPSG that has caused the Eastern Garbage Patch and Western Garbage Patch to form on either side of the Pacific. Macroplastic concentration in the Eastern Garbage Patch is on the order of 0.01 - 0.03 plastic pieces/m^2 or roughly 1 piece of plastic every 50-100 m^2 [2]. Although the marine debris concentration highlights a global pollution problem, the plastic pieces can serve as tracers for surface transport within the gyres and the global oceans.

References:

[1] Talley L.D., Pickard G.L., Emery W.J., Swift J.H., 2011. Descriptive Physical Oceanography: An Introduction (Sixth Edition), Elsevier, Boston, 560 pp.
[2] Goodwin, D.S., 2011. Final Report for S.E.A. Cruise S236. Sea Education Association, Woods Hole, MA 02543, USA. www.sea.edu.