Mixing dynamics on the inner shelf of the Ebro Delta

Abstract

Turbidity, CTD, and temperature and shear microstructure data recorded along a 25 km transect from 8 to 69 m depth across the Ebro Delta shelf (northwestern Mediterranean) are presented. The water column was stratified and prior to the campaign the regional southwest flow was measured to have superimposed diurnal and sub-inertial components. The bottom boundary layer (BBL) at all stations deeper than 20 m has a vertical extension of 3 to 9 m, increasing to 12 m at the deeper stations and even to 14 m at a station at 45 m depth outside the studied transect. The law of the wall was identified at almost all stations with an extension ranging from 40% to 100% of that of the BBL. At some stations high dissipation rates were found in the interior of the bottom layer. The study shows the correspondence between the turbidity contents and the local mixing processes at the BBL and suggests that sub-critical reflection of internal waves could play an important role in boundary mixing. Our data also suggest that during the campaign, after two consecutive short episodes of high winds, the first baroclinic mode shifted to the second one ; We are grateful to M. Demestre for allowing us to be on board the Garcia de Cid within the context of the VEDA Project and the EU RESPONSE (Q5RS-2002- 00787) project and for facilitating the measurements. We are also very grateful to the editor and the anonymous reviewers, whose comments have helped to improve the first version of the paper. I. Lozovatsky and M Muñiz collaborated on the data analysis at an initial stage of this work. This paper was finished in accordance with the objectives of the project CGL2008-03608 funded by the Ministry of Science and Innovation of the Spanish Government and within the framework of the CLIMSEAS Project (FP7-PEOPLE-2009-IRSES N. 247512)