Ekman driven cyclones/anti-cyclones and wind driven ocean circulation using “Weather in a Tank”
David Beesley, Univ. of Massachusetts, Dartmouth, MA; and J. Olejarz, A. Tandon, and J. Marshall
The “Weather in a Tank” demonstrations are being tried at a group of universities (UW Madison, Penn State, NYU, Millersville University and UMass Dartmouth, led by MIT) to introduce many atmospheric and ocean circulation related concepts to students at various levels. An elegant and easily transportable rotating table has been designed for a series of experiments that simulate diverse processes in the atmosphere-ocean system. We present several new experiments that we prototyped using this setup in the summer of 2007 in collaboration with MIT.
The ocean is forced by blowing wind across its surface. Due to rotation of the earth, the net integrated transport in the Northern hemisphere is at right angles to the wind as Ekman found in 1905. Convergence of this transport produces Ekman downwelling and anticyclonic circulation, while divergence of this transport produces cyclonic circulation on the rotating earth. Another important effect stems from the beta-effect, which results in a strong western boundary current in ocean basins.
We model wind-driven effects in a rotating frame using computer cooling fans. The fans are inexpensive and of the right size for the experiments. The fans easily attach to the edges of the tank and blow air over the water's surface. Thus they create currents in the tank that can be studied in the rotating frame. This technique is used for simple classroom demonstrations to model cyclones, anti-cyclones, Ekman transport, and their effectiveness at driving western boundary currents.
To create the cyclone/anticyclone, two fans are attached to the edge of the tank. The wind driven circulation is observed using a combination of potassium permanganate crystals and floating particles at the surface. Depending on the direction of the fans and the direction of the tanks' rotation, either convergence or divergence occurs at the surface. The resulting circulation mimics that of an atmospheric cyclone. The anticyclone is created by reversing the direction of the fans. To create the western boundary current, the fans blow over the water's surface to simulate wind stress in the ocean. With a tilted base of the tank to mimic the beta effect, swift western boundary currents are clearly visible. The overall circulation in the tank can be made to display important features of general ocean circulation. The experiments were tried with a variety of fan sizes, fan positions, and fan speeds to determine the most effective way of demonstrating the wind driven phenomena.
Poster Session 1, Student Conference General Poster Session
Sunday, 20 January 2008, 5:30 PM-7:00 PM, Exhibit Hall B
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