Coherence of western boundary pressure at the RAPID WAVE array: boundary wave adjustments or deep western boundary current advection?

We investigate the coherence between ocean bottom pressure signals at the the RAPID WAVE array on the western North Atlantic continental slope, including the Woods Hole Oceanographic Institution Line W. Highly coherent pressure signals propagate southwestward along the slope, at speeds in excess of 128 m/s, consistent with expectations of barotropic Kelvin-like waves. We also see coherent signals in the smaller pressure differences relative to 1000 m depth, which are expected to be associated with depth-dependent basin-wide meridional transport variations, or an overturning circulation. These signals are coherent and almost in phase for all time scales from 3.6 years down to 3 months. Coherence is still seen at shorter time scales for which group delay estimates are consistent with a propagation speed of about 1 m/s over 990 km of continental slope, but with large error bounds on the speed. This is roughly consistent with expectations for propagation of coastally-trapped waves, though somewhat slower than expected. A comparison with both Eulerian currents and Lagrangian float measurements shows that the coherence is inconsistent with a propagation of signals by advection, except possibly on time scales longer than 6 months.