LAUSR

Motivated by potential effects of the Earth's rotation on the Deepwater Horizon oil plume, we conducted laboratory experiments on salt-water point plumes in a homogeneous rotating environment across a wide range of Rossby numbers 0.02 < Ro < 1.3. We report a striking physical instability in the plume dynamics near the source: after approximately one rotation period, the plume tilts laterally and starts to precess anticyclonically. The mean precession frequency ω scales linearly with the rotation rate Ω as ω≈0.4Ω. We find no evidence of a critical Rossby number above which precession ceases. We infer that a conventionally defined Rossby number is not an appropriate parameter when the plume is maintained over a long time: provided Ω ≠ 0, rotation is always important to the dynamics. This indicates that precession may occur in persistent oceanic or atmospheric plumes even at low latitudes.