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.

Understanding the governing mechanisms of atmosphere–wave–ocean​ interactions is critical for unravelling the formation and evolution mechanisms of severe weather phenomena. This study aims at investigating the effects of atmosphere–wave–ocean​ feedbacks on a Mediterranean tropical-like cyclone (medicane), occurred on 27–30 September 2018 at the central-eastern Mediterranean Sea and characterized by severe environmental and socioeconomic impact. To unveil the interactions across the air–sea interface, the medicane was simulated by an integrated modelling system consisting of the Chemical Hydrological Atmospheric Ocean wave System (CHAOS), upgraded by embedding to it the Nucleus for European Modelling of the Ocean (NEMO) as ocean circulation component. Coupled simulations revealed that air–seaheat transfer and Ekman pumping, bringing sub-surface cold waters in upper ocean layers (upwelling), caused SST cooling (∼2–3 °C). SST cooling triggered a negative feedback loop procedure tending to balance between atmospheric and ocean processes. It also attenuated the cyclone and, subsequently, reduced the atmospheric energy embedded in ocean through the upper ocean vertical stratification weakening, thus, upper ocean vertical mixing, upwelling and SST cooling. The waves adjusted this feedback loop making the system more resistant in air–sea flux variations. Waves additionally weakened the cyclone not only due to the kinetic energy loss in the lower-atmosphere but also due to the enhancement of SST cooling which is attributed to the strengthening of Ekman pumping and vertical mixing, forced by wind stress increase. Nevertheless, waves partially balanced the air–wave–sea exchanges through the slight enthalpy flux gain under high wind conditions which is explained by considering the increase of enthalpy transfer coefficient in rougher sea areas.

The variability of the water mass exchange between the Arabian Gulf and the Indian Ocean is investigated using a high-resolution (1/36°) ocean model. We focus on the period from December 1996 to March 1998, having as reference in situ measurements at the Strait of Hormuz. Previous studies, based on models and observations, suggested a perpetual deep outflow, mainly in the southern part of the Strait, and a variable flow in the upper layers. In the present study, we confirm that there is a permanent core of a deep outflow in the Strait at depths greater than 40 m, characterised by high-salinity waters. In addition, we show that there is a seasonal signal in the upper layers net flow in the southern part of the Strait, altering from net inflow during winter/spring to net outflow during summer/fall. The mean annual inflow through the Strait is estimated at 0.22 ± 0.01 Sv and the deep outflow at 0.147 ± 0.01 Sv. The water mass exchange through the Strait is controlled by synoptic processes with high variability net transport fields. These processes characterise the structure and the intensity of the transport patterns, exhibiting 2- to 5-day period. On synoptic time scales, winds drive an immediate baroclinic flow at the Strait of Hormuz, affecting mostly the upper layers, and a quasi-barotropic flow that peaks approximately 2 days later.

The interannual variability and the mechanisms controlling the dissolved oxygen concentration in the Mediterranean Sea were investigated through generating gridded fields of dissolved oxygen, salinity and potential temperature. The Data-Interpolating Variational Analysis (DIVA) software was used to produce a gridded dataset for the time period 1960–2011. High oxygen concentrations for the upper and bottom layers, separated by an oxygen minimum zone at intermediate layers, are a typical structure of the dissolved oxygen in the Eastern and the Western Mediterranean sub-basins. Although an oxygen minimum zone is observed in both sub-basins, its vertical positions are different; in the Eastern Mediterranean at between 600 and 1200 m depth and in the Western Mediterranean at between 400 and 600 m. The vertical distribution of dissolved oxygen shows significant differences between the two sub-basins and their temporal evolution reveals large interannual to decadal variability. A negative correlation was observed between dissolved oxygen and surface potential temperature due to solubility changes over the whole period. However, the positive correlation between the dissolved oxygen and potential temperature in the Eastern Mediterranean deep layers is an indication that the dynamical processes are dominant and are involved in the dissolved oxygen interannual variability. The dissolved oxygen variability presents shifts with a multi-decadal signal, rather than trends as observed in the global ocean, associated with mixing processes and decadal oscillations that influence the dense water formation or biological activity.

We downscaled a free ensemble of a regional, parent model to a high-resolution coastal, child ensemble in the Bay of Biscay. The child ensemble was forced at the open boundaries by the parent ensemble, and locally by perturbing the winds. By comparing ensembles generated by each of these forcing perturbations separately and combined we were able to consider the ensemble from either of two paradigms: (1) characterising high-resolution, coastal model errors using local and non-local forcing perturbations, or (2) downscaling regional model errors into the coastal domain. We found that most of the spread in the child ensembles was generated from the ensemble of open boundary conditions, with the local wind perturbations on their own generating substantially less ensemble spread. Together, the two sources of error increased the ensemble spread by only a small amount over the non-local perturbations alone. In general, the spread in sea surface height was greater in the child ensembles than in the parent ensemble, probably due to the more refined dynamics, while the spread in sea surface temperature was lower, likely due to the way the open boundary conditions were averaged. Deep below the surface, though, the child ensemble featured a large spread even where the parent model’s spread was very weak. This enhanced error response is a promising result for an ensemble data assimilation system, as it could be exploited to correct the model deep below the surface.