Semidiurnal tidal currents in the deep ocean near the East Pacific Rise between 9° and 10°N

Abstract

Spatial and temporal variations of the semidiurnal tidal currents, especially the baroclinic components, in the deep ocean near a segment of the East Pacific Rise (EPR) are examined using observations from a set of current meter moorings. Over the ridge crest, the baroclinic semidiurnal tidal current is dominated by the zonal component and is coherent along the observed ridge segment. In the direction across the ridge crest, the baroclinic semidiurnal tidal current is not coherent and its magnitude decreases away from the ridge crest. Vertically, the baroclinic semidiurnal tidal current covaries in a few hundred meters near the seafloor. Both the semidiurnal kinetic energy (KE) and shear show significant subinertial temporal variations. By comparing with background forcings, the author found that the subinertial variation of the semidiurnal tides over the ridge crest is related to both the spring-neap cycles and the eddy-induced low-frequency flows in the deep ocean, particularly their cross-ridge components. The observation that the baroclinic tidal currents at the axial stations flowing perpendicularly to the ridge crest suggests that the baroclinic semidiurnal tides at the axial stations are generated locally. It is therefore probable that the subinertial variation of the baroclinic semidiurnal tides near the EPR is due to the eddy-modulated internal tide generation. Since the mesoscale eddies in the eastern tropical Pacific vary with the state of the climate, the observed eddy-modulated internal tide generation connects climate change and variability to the physical and biogeochemical dynamics in the deep ocean and implies an unexplored feedback mechanism potentially affecting the climate system. Key Points Semidiurnal tidal currents vary differently along and across the ridge crest Semidiurnal internal tides show temporal variation on the subinertial timescale Subinertial flows modulate the semidiurnal internal tides over the ridge crest © 2014. American Geophysical Union. All Rights Reserved.