The Impact of Tides on the Bay of Biscay Dynamics

Abstract

The impact of tides on the Bay of Biscay dynamics is investigated by means of an ocean model twin-experiment, consisted of two simulations with and without tidal forcing. The study is based on a high-resolution (1/36∘) regional configuration of NEMO (Nucleus for European Modelling of the Ocean) performing one-year simulations. The results highlight the imprint of tides on the thermohaline properties and circulation patterns in three distinct dynamical areas in the model domain: the abyssal plain, the Armorican shelf and the English Channel. When tides are activated, the bottom stress is increased in the shelf areas by about two orders of magnitude with respect to the open ocean, subsequently enhancing vertical mixing and weakening stratification in the bottom boundary layer. The most prominent feature reproduced only when tides are modelled, is the Ushant front near the entrance of the English Channel. Tides appear also to constrain the freshwater transport of rivers from the continental shelf to the open ocean. The spectral analysis revealed that the tidal forcing contributes to the SSH variance at high frequencies near the semidiurnal band and to the open ocean mesoscale and small-scale features in the presence of summer stratification pattern.

Response of the sea surface temperature to heatwaves during the France 2022 meteorological summer

“…In such situation, a thermal stratification occurs, making impossible any exchange with the colder deep waters. This effect is clearly visible as the Mediterranean is not subject to tides, unlike the English Channel where tides impact the SSTs as seen in the Ushant Front (Chevallier et al, 2014;Karagiorgos et al, 2020). In such areas, the possibilities of mixing is limited and the SST warming is enhanced.…”

Assessment of a regional physical–biogeochemical stochastic ocean model. Part 1: Ensemble generation

“…In all cases, physical model errors are found to have a larger impact on chlorophyll spread than those of the biogeochemical perturbations. It is straightforward to see why: if the ocean physics are identical for all members (as is the case for the SB ensemble) it behaves as a strong dynamical attractor for the system; vertical nutrient fluxes are of course sensitive to the velocity field and in particular its divergence, the latter scaling over the continental shelf break (Karagiorgos et al, 2020), a process which might dominate other sources of uncertainty, such as those of the 𝑆𝑀𝑆 of the biogeochemical model. In general, the spread is largest in SPB where both physics and biogeochemical perturbations are applied simultaneously.…”

Assessment of a regional physical–biogeochemical stochastic ocean model. Part 2: Empirical consistency

“…Rank histograms for the ocean colour PFT showed persistent model underestimation and underspread for chlorophyll abundance, with some improvement for nanophytoplankton after a spin-up period on the order of 3 months during the winter low primary productivity. Rank histograms have been used successfully as a reliability tool identifying on several occasions consistent modeldata configurations and attributing this result to physical and biogeochemical processes, such as the spring shoaling of the thermocline, the frontal activity in the shelves explained by river plume migration, the tidal mixing in the English Channel (Karagiorgos et al, 2020), and to a lesser extent the chlorophyll abundance during spring, mainly on the onset and relaxation of bloom events.…”