We report the first clear experimental evidence of a stationary two-dimensional inverse energy cascade. The experiments are performed in electromagnetically driven flows, using thin, stably stratified layers; measurements of the instantaneous velocity fields show that the fluctuations are homogeneous and isotropic in the inertial range. The energy spectrum displays the expected $k^{-5\mathrm{}/3}$ law with a Kolmogorov constant lying in the range 5.5–7, which is consistent with the current numerical estimates.

• Received 18 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4162