Planetary poles in the lab

Device and data: We performed a series of laboratory experiments in a rotating tank (Ω= 3.0 rad s−1) of the radius R = 29.7 cm. The mean water depth was 4 cm, the β-effect was stipulated by the parabolic curvature of the free surface. The small-scale forcing was generated by the electromagnetic force produced by running continuous electric current through a thin layer of saline solution and placing small permanent magnets on the bottom of the tank. The counterclockwise rotation of the tank emulated the planet's rotation. All magnets were mounted along an arc of 90◦ and the radius, a, of 17 cm. The magnets had the same orientation of polarity chosen such as to introduce a westward momentum and facilitate the formation of a westward zonal jet. Velocities were measured by analyzing images of passively advected styrene particles with mean size of about 5×10−5 m monitored by a video camera with a spatial resolution of 1023 × 1240 pixels at a frequency of 20 Hz. The particles were seeded at a fluid surface lit by two lateral lamps. The camera was mounted perpendicularly to the tank and corotated along with the system and the data recording computer. The acquired images were analyzed by a feature tracking algorithm [Lacorata and Espa, 2012] that reconstructs particle trajectories from displacements between subsequent frames over a fixed time step of 0.05 s and produces instantaneous Lagrangian velocities. The time history of the Eulerian velocity field is then obtained by interpolating the sparse data over a regular grid. The cylindrical coordinate grid employed in this study had Nr = 60 radial points (radial resolution of about 0.5 cm) and Nθ = 90 points in the angular direction (angular resolution of 1°).