A Topographic-Rossby Mode Resonance over the Iceland-Faeroe Ridge

An array of current-meter moorings along 12W on the southern side of the IcelandFaeroe Ridge reveals a narrowband barotropic oscillation with period 1.8 days in spectra of velocity. The signal is coherent over at least 55km scales and propagates phase with shallow water on the right (towards the northwest). Velocity ellipses tend to be elongated (crossing contours of f=H) and rotate anticyclonically. Solutions of the rigid-lid barotropic shallowwater equations predict the occurrence of a topographic-Rossby normal mode on the south side of the ridge with spatial scales exceeding 250km and with intrinsic period near 1.84 days. This fundamental mode of the south side of the ridge has predicted spatial structure, phase propagation and velocity ellipses that are consistent with the observed oscillation. The frictional amplitude efolding decay time for this normal mode is estimated from the observations to be 13 days. The observed ocean currents are signi cantly coherent with zonal wind stress uctuations (but not with wind-stress curl) in the relevant period band which indicates the oscillation is wind-forced. This appears to be the rst clear evidence of a stochastically forced resonant barotropic topographic-Rossby normal mode in the ocean. 2 A Topographic-Rossby Mode Resonance over the Iceland-Faeroe Ridge