Can PIES (Pressure Inverted Echo Sounders) replace tall moorings to monitor the AMOC?

Pressure Inverted Echo Sounders, sited on the seabed, indirectly measure the density of the water above them by combining pressure and travel time of an echo-sound pulse to the surface. Where the approximate structure of the water column is locally known, they can be used to select between a number of typical TS profiles (a gravest empirical mode or GEM field), providing temperature and salinity. But how accurate is this profile, and can such an instrument replace the expensive tall moorings currently used to monitor the MOC? We evaluate PIES deployments at 26N on the western boundary of the Atlantic between 2006 and 2018. We find that high-frequency (around weekly) variations in temperature are well captured by this technique, and the geostrophic part of the AMOC could be estimated in this way. However the GEM databases don't account for all low frequency variations in temperature and salinity profiles. At 26N we see for example, the results from PIES with cold bias above the thermocline and with a compensatory warm bias below it, and these biases lasting months or years. The profiles are also inaccurate at the surface, although seasonally-varying GEM fields may be helpful here. However the technique shows promise, and if it is developed further incorporating additional data sources such ARGO or as sea-surface temperature it may be possible to use it for long term monitoring of the Atlantic at 26N.