Biweekly current oscillations on the continental slope of the Gulf of Guinea

Current meter measurements have been carried out for 3 years on the continental slope of the Gulf of Guinea, near 7.5∘S off the Angola coast. Currents in a water depth of 1300 m over the continental slope show a remarkable biweekly oscillation, bottom intensified, and with currents oriented nearly parallel to the isobaths. With a peak-to-peak amplitude reaching 20-30cms-1 at 30 m above the bottom, this signal is the most energetic at sub-inertial frequencies. Simultaneous measurements deeper on the continental rise (in a water depth of 4000 m) show a more complex signal dominated by lower frequencies, and with less clear polarization. Simple linear topographic wave theories are compared to the observations. A combination of coastal trapped waves with cross-slope mode 3-5 could be consistent with the observed currents. A three-dimensional 1/6∘ model suggests the existence of modes trapped to the slope, although with lower amplitude than observed.