Ekman Pumping and Suction


Outside Links: Lab Guide

Introduction:


Wind stress on the surface of the ocean drives transport processes that reach surprising depths in the water column, as deep as 50 to 100 meters below the surface. Ekman transport is the horizontal transport caused by the balance between wind stress and the Coriolis Effect and whenever this transport is divergent (convergent) there must be a correspondent suction (pumping). These mechanisms play a major role in global ocean circulation.
However, observations of Ekman processes are difficult to make due to, for example, the time-dependence of wind stress. It is nearly impossible to find an Ekman system at steady-state. For this reason, tank experiments have particular application because they provide an environment in which Ekman layer and transport can be observed in detail.


Experimental setup:


The rotating tank can be used to simulate processes associated with Ekman dynamics. The rotation of the tank creates a non-inertial frameof reference, like the rotation of the Earth, and the Coriolis acceleration is able to play a role in the motion of the water. Two fans attached to the sides of the tank simulate winds blowing on the surface of the ocean. Although other settings might be possible, an important feature of the experiment is the fans blowing in opposite directions, like the Westerlies and Trades in Earth's atmosphere, which creates a curl in the wind pattern.
This wind curl drives either convergence or divergence in the water's surface layer and because of continuity there is, respectively, either downward (pumping) or upward (suction) motion in the center of the tank. Because of continuity again, if there is convergence (divergence) near the surface, there must divergence (convergence) near the bottom.
Figure 1 shows a top view of the Ekman pumping experiment. Fans must be positioned before start rotating the tank and for Ekman pumping (suction) the fans must blow a wind with opposite rotation (same direction) as the tank's. The spin of the tank must be set to 10 rotations per minute. After about 15 minutes, the circulation has achieved steady state and potassium permanganate should be added in two spots equidistant from the center. Because the permanganate sinks quickly, its dissolution occurs near the bottom and that's where most of the dye pink is seen (Figure 1). Because of Ekman pumping, there is divergence near the bottom, that are visible as dye sweeping towards the walls.
tanksetup.png
Figure 1. Green arrow indicates wind direction. Orange arrow indicates the direction of tank rotation.


For the Ekman pumping experiment, as the water along the bottom reaches the edges of the tank, it must go upward, dispersing dye near the surface (Figure 2).

leaching.png
Figure 2. Leaching outside of the circular divide. You want to minimize this so that you get a clear signal up or down the walls of the tank. Also, if the signal is good enough, you will see the dye disperse over the top of the tank (if pumping) and even back down again!



Tips for the Experiment:


  • Make sure tank is very well centered. We noticed oscillations near the bottom when it wasn’t centered.

  • Use the level to make sure the table is balanced.

  • Make sure that center lining is flush with the bottom to prevent leaching. If it isn't all the way down, you might lose your dye to the outside rather than get a clear climbing of the dye up or down the sides.

  • Bleach will neutralize food coloring, but does not get rid of the potassium permanganate signal in the water. If you want to do several simulations, begin with the food coloring and use the potassium permanganate at the final iteration, or be prepared to start over with new water.

  • Food dye can be used to illuminate Ekman pumping in the tank. However, it is harder to perceive suction because the dye does not sink uniformly. It makes filaments on the way down.

  • Use the clear hose to syphon the water out of the tank. Do not suck on the hose! Coil the hose under water in the tank, and visually inspect that there are no bubbles in the hose. Firmly cover one end of the hose with your thumb, and position it in the sink so that it is below the elevation of the tank's bottom. Remove your thumb and the water will evacuate, so long as the other hose end remains submerged in the tank.





Theory:


Ekman pumping (suction) results from convergence (divergence) within the Ekman Layer due to a cross stream gradient in the wind stress. It is a steady state force balance between friction, which communicates wind stress to the wind driven layer, and Coriolis, which causes the net transport to be to the right (left) of the wind in the northern (southern) hemisphere.

If, for instance, a steady westerly wind increases in strength poleward, then the equator-ward wind driven transport also increases in that direction and there is convergence therein. Figure 3, illustrates this case in the northern hemisphere.
EkmanPumping_wiki.png
Figure 3. Northern Hemisphere: blue block arrows represent the meridional Ekman transport per meter East-West (x) that results from a positive gradient in the westerly wind stress (𝜏). Blue vectors indicate the direction of induced vertical velocity (w). Outflow at the southern (y1) face of the control volume is less than the inflow at the northern (y2) face: convergence of horizontal transport causes downward velocity at the bottom of the Ekman layer.


Conversely, a steady westerly wind that decreases in strength poleward causes the equator-ward wind driven transport to decrease with distance from the equator and there is divergence. Figure 4, illustrates this case in the northern hemisphere.



Applications:


Figure 4 shows a schematic diagram of the general wind-driven circulation in the deep ocean far from lateral boundaries. The wind curl provided by the presence of Westerlies and Trades results in convergent Ekman transport near the surface and an associate Ekman pumping. This downward motion squashes the water column in the interior and according to an appropriate simplified balance in the vorticity equation for large scales in the ocean (Sverdrup's balance) there is a correspondent equatorward flow (Sverdrup transport). Essentially, the connection between the atmospheric circulation above the surface of the ocean and the general wind-driven circulationand occurs through Ekman dynamics.

sverdrup_balance.jpg
Figure 4. Sverdrup balance circulationship (Northern Hemisphere). Note the the relationship between wind patterns, Ekman transport, Ekman pumping and meridional transport in the ocean interior (from Talley et al, 2011).


Ekman dynamics also has implications on the primary production in the world's ocean, and indirectly, on the marine food web. Light penetration into the ocean and nutrient availability are the main limiting factors on photosynthesis, which occurs mostly in the top 100 meters of the ocean due to light attenuation with depth. Therefore, nutrients are mostly consumed near the surface and their concentration increase with depth. Ekman upwelling is then a mechanism to bring nutrient-rich waters at depth closer to the surface, increasing stimulating primary production. Conversely, downwelling is associated with reducing primary production. From a climatology of near surface chlorophyll-a concentration (Figure 5) we can observe the subtropcial gyres, where Ekman downwelling occurs, are associated with low concentrations while higher latitudes have high concentrations (associated with Ekman upwelling depicted in north part of figure 4). High chlorophyll-a near the equator and on the eastern side of ocean basins is also associated with Ekman upwelling.

global_c.jpg
Figure 5. Global climatology of near surface chlorophyll-a derived from satellite measurements (from NASA's ocean color webpage).




References:


Talley L.D., Pickard G.L., Emery W.J., Swift J.H., 2011. Descriptive Physical Oceanography: An Introduction (Sixth Edition), Elsevier, Boston, 560 pp.
http://oceancolor.gsfc.nasa.gov/cgi/image_archive.cgi?c=CHLOROPHYLL