ALBERT

Description: We use seismic oceanography to document and analyze oceanic thermohaline finestructure across the Tyrrhenian Sea. Multichannel seismic (MCS) reflection data were acquired during the MEDiterranean OCcidental survey in April-May 2010. We deployed along-track expendable bathythermograph probes simultaneous with MCS acquisition. At nearby locations we gathered conductivity-temperature-depth data. An autonomous glider survey added in-situ measurements of oceanic properties. The seismic reflectivity clearly delineates thermohaline finestructure in the upper 2,000 m of the water column, indicating the interfaces between Atlantic Water/Winter Intermediate Water, Levantine Intermediate Water, and Tyrrhenian Deep Water. We observe the Northern Tyrrhenian Anticyclone, a near-surface meso-scale eddy, plus laterally and vertically extensive thermohaline staircases. Using MCS we are able to fully image the anticyclone to a depth of 800 m and to confirm the horizontal continuity of the thermohaline staircases of more than 200 km. The staircases show the clearest step-like gradients in the center of the basin while they become more diffuse towards the periphery and bottom, where impedance gradients become too small to be detected by MCS. We quantify the internal wave field and find it to be weak in the region of the eddy and in the center of the staircases, while it is stronger near the coastlines. Our results indicate this is because of the influence of the boundary currents, which disrupt the formation of staircases by preventing diffusive convection. In the interior of the basin the staircases are clearer and the internal wave field weaker, suggesting that other mixing processes such as double-diffusion prevail. Synopsis We studied the internal temperature and salinity structure of the Tyrrhenian Sea (Mediterranean) using the multichannel seismic reflection method (the same used in the hydrocarbon industry). Low frequency sound (seismic) waves are produced at the surface with an explosive air source and recorded by a towed cable containing hydrophones (underwater microphones). The data are processed to reveal 'stratigraphy' that result from contrasts in density that are themselves caused by changes in temperature and salinity. In this way we can map ocean circulation in two-dimensions. We also deployed in situ oceanographic probes to measure temperature and salinity in order to corroborate and optimize the processing of the seismic data. We then quantified the internal gravity wave field by tracking the peaks of seismic trace wavelets. Our results show that the interior of the Tyrrhenian Sea is largely isolated from internal waves that are generated by a large cyclonic boundary current that contains waters from the Atlantic ocean and other parts of the Mediterranean. This isolation allows the thermohaline finestructure to form, where small scale vertical mixing processes are at play. Understanding these mixing processes will aid researchers study global ocean circulation and to add constraints that can help improve climate models.