Abstract
Abstract. In the north-east Atlantic Ocean, the Bay of Biscay is an intersection between a coastal constrained dynamics (wide continental shelf and shelf break regions) and an eastern boundary circulation system. In this framework, the eddy kinetic energy is 1 order of magnitude lower than in western boundary systems. To explore this coastal complex system, a high-resolution (1 km, 100 vertical sigma layers) model experiment including tidal dynamics over a period of 10 years (2001–2010) has been implemented. The ability of the numerical environment to reproduce main patterns over interannual scales is demonstrated. Based on this experiment, the features of the (sub)mesoscale processes are described in the deep part of the region (i.e. abyssal plain and continental slope). A system with the development of mixed layer instabilities at the end of winter is highlighted. Beyond confirming an observed behaviour of seasonal (sub)mesoscale activity in other regions, the simulated period allows exploring the interannual variability of these structures. A relationship between the winter maximum of mixed layer depth and the intensity of (sub)mesoscale related activity (vertical velocity, relative vorticity) is revealed and can be explained by large-scale atmospheric forcings (e.g. the cold winter in 2005). The first submesoscale-permitting exploration of this 3-D coastal system shows the importance of (sub)mesoscale activity in this region with its evolution implying a potentially significant impact on vertical and horizontal mixing.
Highlights
As a semi-enclosed region, the Bay of Biscay (Fig. 1) can be divided in three dynamical regimes: the circulation over the continental shelf, the transition region above the shelf break and the open ocean part
The present study aims to characterize the interannual variability of themesoscale dynamics and discuss the possible processes explaining this variability
With the rise of numerical capabilities, coastal dynamics can be explored at regional scale over pluri-annual periods keeping a high spatial resolution needed to solve at themesoscale
Summary
As a semi-enclosed region, the Bay of Biscay (Fig. 1) can be divided in three dynamical regimes: the circulation over the continental shelf, the transition region above the shelf break and the open ocean part. The most intense circulation patterns are today explained by intermittent coastal densitydriven jets disturbed by tidal dynamics over the continental shelf, a slope current with seasonal and interannual reversals meandering to generate eddies and an open ocean region with a weak average circulation but several eddies propagating. From this statement, the questions to be addressed to fulfil the scheme explaining the evolution of this coastal system concerns the mesoscale and submesoscale dynamics. After controlling the efficiency and accuracy of a coastal model with a 1 km spatial resolution to reproduce the observed processes in the Bay of Biscay, the (sub)mesoscale variability at annual and interannual scales is explored as a first step to define the role of related vertical motions at small scales on long-term evolution and associated biogeochemical production
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