Rossby Waves


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Tank Experiment: Rossby Waves in the ocean

Bia Villas Boas, John Boyle, Stephen Holleman


1. Background


Rossby waves are large scale ocean waves that occur in low to mid latitudes. As such, they are tied to the Coriolis effect through conservation of potential vorticity. This experiment is designed to Rossby wave dependence on potential vorticity by using a beta plane (a rotating, inclined plane), to illustrate the general motion of Rossby waves in the ocean, through the motion of an ice cube.




2. Westward Propagation of Rossby Waves Slide1.jpg


Perturbation of a parcel of water, i.e. from wind, from its original planetary vorticity position (latitude) induces a relative vorticity in the parcel. The movement of the parcel displaces the surrounding water, inducing a relative vorticity for those parcels.


3. Methods


To model Rossby waves, a cone with an approximate slope of 1-to-1 was placed concentrically with a cylindrical boundary inside the rotating tank. We then filled the tank with room temperature water until approximately 8 centimetres from the rim of the container. Once the container was filled, we rotated the tank at approximately 15 revolutions per minute. The fluid will have approximately solid body rotation after rotating for 15 to 20 minutes. Gently place a coloured ice cube into the tank (halfway between the centre of the tank and the edge was found to have optimal results). Plastic dots may also be placed near to the ice cube and on the opposite side of the tank to show the rotation of the water near the ice cube and that the fluid still maintains solid body rotation. Viewed from the top, the tank rotates anti-clockwise, thus (from the westward propagation of Rossby waves, as explained in section 2) the ice cube will rotate about the tank in a clockwise fashion. The ice cube will be rotating anticlockwise about its axis, conserving potential vorticity.

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setup3.JPGsetup4.JPGrotation.jpg

Troubleshooting


  • Placing the ice cube near the edge of the tank decreases the possibility that the ice cube will rotate, especially if it is in direct contact with the tank.
  • Another option to perform the experiment is with an inclined plane. However, the desired slope to perform the experiment is approximately 1-to-1. To achieve the desired slope nearly half of the inclined plane needs to be outside of the tank. Therefore, a cone was decided upon to perform the experiment.
  • If the ice cube is not placed gently into the tank, it will disturb more of the water and be much slower to achieve rotation.

4. Real life examples: Rossby waves in the ocean

4.1 Western boundary intensification


When westward propagating long Rossby waves reach the coast, they reflect back as shortwaves. Since shortwaves are slower than long-waves, the energy and vortcity associated with Rossby waves tends to accumulate at the western boundary, which can explain part of the western boundary intensification predicted by the Stommel’s model (Stommel, 1948) for the wind–driven circulation.
The westward propagation of Rossby waves becomes evident in the animation below, which shows sea level anomaly data from AVISO from 1993 to 2007 in the Atlantic Ocean. It is also noteworthy that as we move to higher latitudes, the response of the ocean to the wind forcing and instabilities tends to be on the form of mesoscale eddies instead of Rossby waves.












Courtesy of Sebastian Krieger, University of São Paulo.


4.2 The impact of Rossby wave on primary production


Because Rossby waves are associated with regions of negative and positive vorticity, they can induce local upwelling and downwelling. Upwelling can either bulge up the thermocline, bringing an existent chlorophyll maximum closer to the surface, or promote the vertical transport of deep, nutrient-rich water to the euphotic zone, inducing a chlorophyll bloom. In such a way, Rossby waves can at least locally impact the primary production, and be detected from ocean color data.


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Ocean color data from SeaWifs averaged for March 2002. Adapted from http://oceancolor.gsfc.nasa.gov

5. Basic Theory


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