Microstructure observations of the summer-to-winter destratification at a coastal site in the Gulf of Naples

A dissection of the physics of the seasonal cycle of the oceanic upper layer stratification is necessary to improve climate predictions and to constrain the response of biogeochemical cycles to the climate change. Here we present a time series of vertical profiles of ε, the dissipation rate of turbulent kinetic energy, obtained from a microstructure profiler at a mid-latitude 75m-deep coastal site covering the destratification occurring during the the summer-to-winter. The main signature of the destratification is a progressive deepening of the mixed layer depth (MLD) from September to November, that finally extended to the bottom of the full water-column at the beginning of winter. By grouping the data into temporal and vertical bins we found that the statistics of ε depend upon the time of the year and the position with respect to the MLD. A seasonal increase in storminess is correlated with the increase in intermittency of the turbulence in the mixed layer. A co-location of patches of higher ε with the shear maxima of the two first baroclinic modes suggests internal waves activity plays a role in the setting the mixing intensity in the interior despite the lack of tidal forcing. The low-passed microstructure shear distribution seems to support this hypothesis despite possible signal contaminations. The actual origin of these energetic motions remains to be investigated. Overall, this study confirms that the variability of the stratification is ruled by several physical processes whose importance varies with the seasons. Predicting a change in stratification thus requires tackling the challenge of understanding and parameterising these processes.

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Kokoszka Florian, Conversano Fabio, Ludicone Daniele, Ferron Bruno, Bouruet-Aubertot Pascale, Mc Millan Justine (2020). Microstructure observations of the summer-to-winter destratification at a coastal site in the Gulf of Naples. Authorea. (July 21, 2). 37p.. https://doi.org/10.1002/essoar.10505074.3, https://archimer.ifremer.fr/doc/00662/77416/

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