Lia Siegelman is a graduate student at the University of Western Brittany, France, and currently a visiting student at Caltech/Jet Propulsion Laboratory in the US, studying fine ocean fronts (<50 km), known as submesoscale fronts. Here, she describes recent research that seeks to understand the distribution of these fronts in the under-sampled area of the Southern Ocean as well as their influence for higher trophic levels. 

Advances in theory and modeling of the last two decades suggest that submesoscale fronts are a conspicuous feature of the ocean. In particular, they are thought to play a key role in the transport of heat, carbon and nutrients between the atmosphere and the ocean. Yet, little is known about them in the real ocean as their short lifetime (< week) and small size (<50 km) make them hard to observe directly.

In a recently published study (Siegelman et al. 2019a), we used newly available CTD data collected by a single female southern elephant seal (Mirounga leonina) during its 3-month journey west of the Kerguelen Islands to study submesoscale fronts. The novelty of the dataset resides in its unprecedented high-resolution. Indeed, the tag was set in continuous recording mode, which means that it recorded every single dive realized by the seal – or in our case 6942 dives (see the newly released MEOP-SMS dataset and Siegelman et al. 2019b for more information), allowing us to study submesoscale fronts in this part of the ocean for the first time.

These high-resolution observations provided evidence that submesoscale features are active in the Southern Ocean during summertime. Submeoscale fronts were principally observed on the edge of a standing meander of the Antarctic Circumpolar Current, accompanied by numerous temperature and salinity intrusions. The data also confirmed theoretical studies in showing that submesosale fronts are mainly generated by the background mesoscale strain field.

Strikingly, the seal increased its foraging activity at the standing meander site by up to 5 times. In particular, the seal spent significantly more time foraging in the vicinity of strong submesoscale features located on the edge of the meander where it also decreased its speed. Despite that most elephant seals target foraging grounds east of the Kerguelen Plateau, our findings suggest that excursions to the west are not accidental, and may be explained by the recurrently elevated physical and biological activity of the site. As such, other standing meanders of the ACC may also act as biological hotspots where trophic interactions are stimulated by submesoscale turbulence.

Reference:

• Siegelman, L., O’Toole, M., Flexas, M., Rivière, P., & Klein, P. (2019). Submesoscale ocean fronts act as biological hotspot for southern elephant seal. Scientific reports, 9(1), 5588.
• Siegelman, L., Roquet, F., Mensah, V., Rivière, P., Pauthenet, E., Picard, B., & Guinet, C. (2019). Correction and accuracy of high-and low-resolution ctd data from animal-borne instruments. Journal of Atmospheric and Oceanic Technology, 36(5), 745-760.

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