Seismic Oceanography in the Tyrrhenian Sea: Thermohaline Staircases, Eddies, and Internal Waves

Abstract

We use seismic oceanography to document and analyze oceanic thermohaline fine structure 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 fine structure 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 mesoscale 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 toward 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.

OceanGliders: A Component of the Integrated GOOS

“…can have a great impact on the ocean circulation/ecosystem state and evolution through their particular structures and transport mechanisms. Other fine scale processes are clearly involved in the ocean mixing, like microturbulence (Fer et al, 2014;Palmer et al, 2015;Schultze et al, 2017) or frontogenesis, filamentation due to stirring or symmetric instability (Figure 11 and Ruiz et al, 2012;Thompson et al, 2014Thompson et al, , 2016Thomsen et al, 2016;Pietri et al, 2013;Brannigan et al, 2017;Buffett et al, 2017;Du Plessis et al, 2017;Pascual et al, 2017;Kolodziejczyk et al, 2018) that can lead to significant vertical velocities and fluxes. However, the extent and variability of their impact over long periods of time still needs to be assessed.…”

Profiling float observation of thermohaline staircases in the western Mediterranean Sea and impact on nutrient fluxes

“…Sparnocchia et al (1999) confirmed such extension northwest of Sicily. More recently, high-resolution synoptic observations of staircases by seismic data have been reported along three zonal transects north of the 40 • N parallel (Buffett et al, 2017). The staircases are depicted as continuous stripes, well-ordered in the central part of the basin and weakening close to the continental slope, with remarkable spatial continuity.…”

“…Thanks to biannual surveys at a higher spatial resolution, the authors reported the different layering aspects: well-ordered vertical structures around 4 • E progressively eroded westwards and jumbled profiles in the eastern stations. More recently, glider surveys along 3 and 4 • E have provided high-resolution synoptic observations of thermohaline staircases with a remarkable structural spatial continuity between the two layering aspects (Cotroneo et al, 2019). These reports are in agreement with the regionalization by staircase regimes proposed by Krahmann (1997): a region of inverted layers close to Sardinia where LIW enter the basin and flow northwards along the continental slope, farther west a region of well-ordered layering and then a westernmost region with "diffusively reduced" layers.…”

“…Krahmann (1997) reported the first synoptic observations of thermohaline staircases in the Algerian Basin, confirmed by Bryden et al (2014) from biannual surveys across this region. More recently, Buffett et al (2017) revealed their remarkable spatial continuity from very high resolution synoptic observations.…”

“…In order to untangle the two processes, an accurate measurement of thermohaline changes requires temporal continuity of layers, which is the most challenging aspect for observations. Indeed, because thermohaline staircases occur far from the coastal zones and develop at depth, data acquisition is often limited to high-resolution but shortterm records (e.g., Buffett et al, 2017) or coarse temporal resolution but long-term records (e.g., Falco et al, 2016;Durante et al, 2019). Moreover, most of the existing studies have not considered the role of thermohaline staircases in nutrient distribution, because pertinent observations of biogeochemical parameters were hard to obtain in the water column at relevant temporal and spatial scales.…”