High-resolution diapycnal mixing map of the Alboran Sea thermocline from seismic reflection images

Abstract. The Alboran Sea is a dynamically active region where the salty and warm Mediterranean water first encounters the incoming milder and cooler Atlantic water. The interaction between these two water masses originates a set of sub-mesoscale structures and a complex sequence of processes that entail mixing close to the thermocline. Here we present a high-resolution map of the diapycnal diffusivity around the thermocline depth obtained using acoustic data recorded with a high-resolution multichannel seismic system. The map reveals a patchy thermocline, with areas of strong diapycnal mixing juxtaposed with others of weaker mixing. The patch size is of a few kms in the horizontal scale and of 10–15 m in the vertical one. The comparison of the obtained maps with the original acoustic images shows that vigorous mixing tends to occur in areas of internal wave instability, whereas mixing levels in more stable areas is lower. These results are also compared with others obtained using conventional probes. The values obtained using the two methods agree within uncertainty bounds, and they are also consistent with reference theoretical values. Overall, our results demonstrate that high-resolution seismic systems allow to remotely quantify mixing at the thermocline depth with a lateral resolution of O(101 m).