Ground-Truthed Fiber-Optic Sensing Reveals Internal Tide Bores Over Madeira's Steep Slope

We demonstrate the combined utility of distributed acoustic sensing (DAS) and repeat conductivity-temperature-depth (CTD) profiling for observing internal tide dynamics over a sloping seafloor. While DAS has been widely proposed as a method to infer ocean temperature variability from seafloor cables, quantitative in situ validation has been limited. Here we present the first published detailed comparison between DAS-inferred bottom temperature fluctuations and near-bottom temperature measured from high-resolution yoyo-CTD time series reaching 5 m above the seafloor, conducted over the EllaLink/GeoLab cable on the slope of Madeira. Linear regressions yield an empirical DAS temperature sensitivity of 21–37 µstrain (Formula presented.), a factor 2.5–4 greater than commonly used theoretical estimates. The calibrated DAS record reveals semidiurnal internal tide bores propagating upslope to (Formula presented.) 1,300 m depth, where propagation halts and transitions to a complex interference pattern. These observations highlight the potential of combining seafloor fiber sensing with targeted in situ profiling to observe near-bottom internal tide breaking processes.