Observations of arrested internal lee waves at a tall, steep submarine ridge

Steady ocean currents flowing over topography produce arrested internal lee waves, which have phase speeds equal to the steady current speed. Models suggest these waves may be an energy sink for the general circulation. Recent interest in these lee waves has been focused on abyssal hill topography in the Southern Ocean, where half of the global energy flux from geostrophic flow into lee waves may occur according to models. Other areas of possible lee wave generation include the western Pacific, where strong, surface-intensified currents flow over tall, steep topography. As part of ONR’s Flow Encountering Abrupt Topography (FLEAT) program, spatial surveys with shipboard ADCP and a towed CTD (SeaSoar) were conducted over a submarine ridge near Merir Island in the midst of the North Equatorial Countercurrent (NECC). We observed a strong mean flow in the NECC ranging from 0.3 to 1.0 m/s in the upper 400 m upstream of Merir Island. Arrested internal lee waves were found. Oscillations in density and velocity had horizontal wavelengths of roughly 20 km, had intrinsic periods of O(0.1-1 days), and were persistent over the 2 days of the survey. The waves carried a westward and upward energy flux and exerted a drag on the mean current.