Subinertial variability in the deep ocean near the East Pacific Rise between 9° and 10°N

The subinertial variability of deep-ocean currents near the crest of the East Pacific Rise in the tropical East Pacific is examined using observations from a collection of moored instruments augmented with sea-surface height data. The moored velocity observations reveal low-frequency currents with characteristic time scales of 1–3 months and maximum speeds up to 10 cm/s. Directly over the ridge axis, the subinertial motions are dominated by the along-axial (meridional) velocities, which are coherent in phase and amplitude along the entire length of the ridge segment between 9 and 10N. With increasing distance from the ridge crest the low-frequency currents become weaker and less dominated by the along-axial flow component. Lag-correlations between the velocity records indicate westward signal propagation with a speed on the same order as the speed of westward-propagating sea-surface height anomalies, which are also associated with characteristic time scales of 1–3 months. As, furthermore, the subinertial velocities at depth are significantly correlated with geostrophic near-surface currents estimated from sea-surface height data, we conclude that the subinertial velocity field near the EPR crest is mainly a superposition of velocities associated with eddies propagating westward across the ridge and “topographic flows”, such as trapped waves and boundary currents.