A TOPOGRAPHIC-ROSSBY MODE RESONANCE OVER THE ICELAND-FAEROE RIDGE

An array of current meter moorings along 12 degrees W on the southern side of the Iceland-Faeroe Ridge reveals a narrowband barotropic oscil lation with period 1.8 days in spectra of velocity. The signal is cohe rent over at least 55-km scales and propagates phase with shallow wate r on the right (toward the northwest). Velocity ellipses tend to be el ongated (crossing contours of f/H) and rotate anticyclonically. Soluti ons of the rigid-lid barotropic shallow-water equations predict the oc currence of a topographic-Rossby normal mode on the south side of the ridge with spatial Scales exceeding 250 km and with intrinsic period n ear 1.84 days. This fundamental mode of the south side of the ridge ha s predicted spatial structure, phase propagation, and velocity ellipse s consistent with the observed oscillation. The frictional amplitude e -folding decay time for this normal mode is estimated from the observa tions to be 13 days. The observed ocean currents are significantly coh erent with zonal wind stress fluctuations (but not with wind stress cu rl) in the relevant period band, which indicates the oscillation is wi nd forced. This appears to be the first clear evidence of a stochastic ally forced resonant barotropic topographic-Rossby normal mode in the ocean.