Purpose of Activity
Here we study the mechanism by which the wind drives ocean circulation. We induce cyclonic and anticyclonic circulation by blowing air over the surface of a rotating tank of water using fans co-rotating with the turntable, as sketched in Figure 1. The Ekman layers and patterns of upwelling and downwelling can be visualized by using dye crystals and paper dots.
The flow within the surface Ekman layer associated with the action of the wind is convergent in anticyclonic flow and divergent in cyclonic flow if the apparatus is rotating cyclonically (Ω > 0, corresponding to the northern hemisphere), as sketched in Figure 2. The convergent flow drives downward vertical motion (called Ekman pumping, Figure 2a); the divergent flow drives upward vertical motion from beneath (called Ekman suction, Figure 2b).
The experiment demonstrates how divergence in Ekman transport leads to vertical motion. Students visualize how continuity of the flow leads to upwelling in a cyclone and downwelling in an anticyclone, which then helps make connections with gyre-scale ocean circulation. Students learn how to visualize circulation at the surface and at depth using paper dots and dye tracers.