Bruno Ferron

Mixing in the Romanche Fracture Zone

The Romanche Fracture Zone is a major gap in the Mid-Atlantic Ridge at the equator, which is deep enough to allow significant eastward flows of Antarctic Bottom Water from the Brazil Basin to the Sierra Leone and Guinea Abyssal Plains. While flowing through the Romanche Fracture Zone, bottom-water properties are strongly modified due to intense vertical mixing. The diapycnal mixing coefficient in the bottom water of the Romanche Fracture Zone is estimated by using the finestructure of CTD profiles, the microstructure of high-resolution profiler data, and by constructing a heat budget from current meter data. The finestructure of density profiles is described using the Thorpe scalesLT. It is shown from microstructure data taken in the bottom water that the Ozmidov scale LO is related to LT by the linear relationship LO 5 0.95LT, similar to other studies, which allows an estimate of the diapycnal mixing coefficient using the Osborn relation. The Thorpe scale and the diapycnal mixing coefficient estimates show enhanced mixing downstream (eastward) of the main sill of the Romanche Fracture Zone. In this region, a mean diapycnal mixing coefficient of about 1000 3 1024 m2 s21 is found for the bottom water. Estimates of cross-isothermal mixing coefficient derived from the heat budgets constructed downstream of the current meter arrays deployed in the Romanche Fracture Zone and the nearby Chain Fracture Zone are in agreement with the finestructure estimates of the diapycnal mixing coefficient within the Romanche Fracture Zone. Although the two fracture zones occupy only 0.4% of the area covered by the Sierra Leone and Guinea Abyssal Plains, the diffusive heat fluxes across the 1.4 8C isotherm in the Romanche and Chain Fracture Zones are half that found over the abyssal plains across the 1.88C isotherm, emphasizing the role of these passages for bottom-water property modifications.

The effects of differential vertical diffusion of T and S in a box model of thermohaline circulation

A diffusive box model, consistent with geostrophy, is proposed as an alternative to more usual advective box models of the ocean thermohaline circulation. When vertical diffusion coefficients for T and S are taken as identically equal (the normal assumption in all numerical ocean models to date), the diffusive box model exhibits both steady-state modes and time-dependent behaviors which are essentially indistinguishable from those of an advective model, under both fixed flux and mixed (T restoring) boundary conditions. The thermohaline “circulation” of the diffusive box model, however, is a combination of a convective branch and a vertical diffusive branch, involving zero volume flux. Modifications in behavior of the diffusive box model are investigated for a plausible range of values for the ratio d equiv Ks/KT of the vertical turbulent diffusivities of S and T. When surface fluxes of heat and freshwater are constant, the model with d ne 1 exhibits additional steady-state modes in which convection is absent from the system, as well as a periodic oscillatory mode. Compared to results with d equiv 1 under mixed surface boundary conditions, the model with d ne 1 exhibits extended ranges of multiple equilibria, a different mode transition near present-day values of freshwater forcing magnitude, and the possibility of quasi-periodic oscillatory states. The sensitivity of the present box model, coupled with that previously observed in a primitive equation model (Gargett and Holloway, 1992), raises serious questions about the ability of numerical models to predict the evolution of the ocean thermohaline circulation under changing atmospheric forcing, even if other problems with such prediction were resolved.

Impact of 4D-variational assimilation of WOCE hydrography on the meridional circulation of the Indian Ocean

Abstract World Ocean Circulation Experiment (WOCE) hydrographic sections and a sea-surface climatology are combined with a ocean general circulation model through a 4D-variational method to analyze the meridional overturning of the Indian Ocean. The regional model is run with realistic surface forcings over year 1995 for which most of WOCE Indian Ocean sections were made. The assimilation controls the initial temperature and salinity fields, surface forcings and open-boundary velocities, temperature and salinity. When no observations are assimilated, the model shows that the deep (below 1000 m ) meridional overturning is weak compared to observation-based estimates. This is a common feature of general circulation models. In contrast, after the assimilation, the model develops a deep overturning of 17×10 6 m 3 s −1 at 32°S when a 10×10 6 m 3 s −1 Indonesian Throughflow (ITF) is prescribed. The mass flux of bottom waters that moves northward below 3200 m is balanced by a southward mass flux of deep waters between 1000 and 3200 m . The deep overturning carries 10% of the total southward energy flux of 1.2 PW at 32°S. The intensity of this deep overturning changes only by ±2×10 6 m 3 s −1 when the annual mean ITF is zero or 30×10 6 m 3 s −1 . The upper circulation is less constrained by the assimilation because of the large temporal and spatial variability of this ocean and also because of limitations in the representation of the mixed layer physics during the assimilation process. Limitations in the physics of the model also are thought to be the source of the slow erosion of the deep overturning when the model is run for several years from its optimal state.

Nd isotopic composition of water masses and dilution of the Mediterranean outflow along the southwest European margin

Nd isotopic compositions (epsilon Nd) of seawater profiles and deep-sea corals collected off the coast of Iberia and from the Bay of Biscay were measured (1) to constrain the Nd isotopic composition of water masses along the southwest European margin, (2) to track the Mediterranean Outflow Water (MOW) during its northward propagation, and (3) to establish hydrological changes during the last 1500 years. The Eastern North Atlantic Central Water (ENACW) is characterized by Nd isotopic composition of around -12.0. Mediterranean Sea Water (MSW) is collected from 800 and 1200 m depth and is characterized by epsilon Nd values ranging from -10.9, off the coast of Iberia, to -11.6 in the Bay of Biscay. These epsilon Nd results suggest a strong dilution of the pure MOW at the Strait of Gibraltar (epsilon Nd -9.4) of approximately 40% and 30% along its northward circulation pathway essentially with a contribution from ENACW. At around 2000 m depth, epsilon Nd water profiles display the occurrence of a nonradiogenic water mass (epsilon Nd -13), originating from the Labrador Sea (Labrador Sea Water). Fossil deep-sea corals, dated between 84 and 1500 years, display Nd isotopic compositions that vary moderately from present-day seawater values, suggesting a weaker influence of MOW in the formation of MSW during the Dark Ages and the Little Ice Age. These recent cold events seem to be associated with a reduction in the northward penetration of MSW, which may result from a greater eastward extension of the middepth subpolar gyre and/or a reduction of MSW formation, likely tied to a variation in deep Mediterranean water production.

Dissipation Rate Estimates from Microstructure and Finescale Internal Wave Observations along the A25 Greenland–Portugal OVIDE Line

AbstractA total of 96 finestructure and 30 microstructure full-depth vertical profiles were collected along the A25 Greenland–Portugal Observatoire de la Variabilite Interannuelle et Decennale en Atlantique Nord (OVIDE) hydrographic line in 2008. The microstructure of the horizontal velocity was used to calculate turbulent kinetic energy dissipation rates evmp, where vmp refers to the vertical microstructure profiler. The lowest dissipation values (evmp 5 × 10−10 W kg−1) are found in the main thermocline, around the Reykjanes Ridge, and in a 1000-m-thick layer above the bottom near 48°N. The finestructure of density was used to estimate isopycnal strain and that of the lowered acoustic Doppler current profiler to estimate the vertical shear of horizontal velocities. Strain and shear were used to estimate dissipation rates eG03 (Gregg et al.) associated with the internal w...

Biophysical interactions control the size and abundance of large phytoplankton chains at the Ushant tidal front.

Phytoplankton blooms are usually dominated by chain-forming diatom species that can alter food pathways from primary producers to predators by reducing the interactions between intermediate trophic levels. The food-web modifications are determined by the length of the chains; however, the estimation is biased because traditional sampling strategies damage the chains and, therefore, change the phytoplankton size structure. Sedimentological studies around oceanic fronts have shown high concentrations of giant diatom mats (>1 cm in length), suggesting that the size of diatom chains is underestimated in the pelagic realm. Here, we investigate the variability in size and abundance of phytoplankton chains at the Ushant tidal front (NW France) using the Video Fluorescence Analyzer (VFA), a novel and non-invasive system. CTD and Scanfish profiling characterized a strong temperature and chlorophyll front, separating mixed coastal waters from the oceanic-stratified domain. In order to elucidate spring-neap variations in the front, vertical microstructure profiler was used to estimate the turbulence and vertical nitrate flux. Key findings were: (1) the VFA system recorded large diatom chains up to 10.7 mm in length; (2) chains were mainly distributed in the frontal region, with maximum values above the pycnocline in coincidence with the maximum chlorophyll; (3) the diapycnal fluxes of nitrate enabled the maintenance of the bloom in the frontal area throughout the spring-neap tidal cycle; (4) from spring to neap tide the chains length was significantly reduced; (5) during neap tide, the less intense vertical diffusion of nutrients, as well as the lower turbulence around the chains, intensified nutrient-depleted conditions and, thus, very large chains became disadvantageous. To explain this pattern, we suggest that size plasticity is an important ecological trait driving phytoplankton species competition. Although this plasticity behavior is well known from experiments in the laboratory, it has never been reported from observations in the field.

Variability of the Turbulent Kinetic Energy Dissipation along the A25 Greenland–Portugal Transect Repeated from 2002 to 2012

AbstractThe variability of the turbulent kinetic energy dissipation due to internal waves is quantified using a finescale parameterization applied to the A25 Greenland–Portugal transect repeated every two years from 2002 to 2012. The internal wave velocity shear and strain are estimated for each cruise at 91 stations from full depth vertical profiles of density and velocity. The 2002–12 averaged dissipation rate 〈e2002–2012〉 in the upper ocean lays in the range 1–10 × 10−10 W kg−1. At depth, 〈e2002–2012〉 is smaller than 1 × 10−10 W kg−1 except over rough topography found at the continental slopes, the Reykjanes Ridge, and in a region delimited by the Azores–Biscay Rise and Eriador Seamount. There, the vertical energy flux of internal waves is preferentially oriented toward the surface and 〈e2002–2012〉 is in the range 1–20 × 10−10 W kg−1. The interannual variability in the dissipation rates is remarkably small over the whole transect. A few strong dissipation rate events exceeding the uncertainty of the fi...

The dissipation of kinetic energy in the Lofoten Basin Eddy

AbstractOcean microstructure, current, and hydrography observations from June 2016 are used to characterize the turbulence structure of the Lofoten Basin eddy (LBE), a long-lived anticyclone in the...

Modeling of the bottom water flow through the Romanche Fracture Zone with a primitive equation model - Part I: Dynamics

This paper investigates the dynamics of the Antarctic Bottom Water (AABW) e ow through the RomancheFractureZone(RFZ)inaprimitiveequationmodelwithahighhorizontalandverticalresolution. Two examples of e ows over simple bathymetries show that a reduced gravity model captures the essential dynamics ofthe primitiveequationmodel.Thereduced gravity model is then used asa toolto identify what arethe bathymetricstructures(sills,narrows)thatmostlyconstraintheAABWe owthrough the RFZ.When only these structures are represented in the primitive equation model, the AABW e ow is shown to be coherentwithobservations(transports,densityand velocitystructures).

Contrasted turbulence intensities in the Indonesian Throughflow: a challenge for parameterizing energy dissipation rate

Microstructure measurements were performed along two sections through the Halmahera Sea and the Ombai Strait and at a station in the deep Banda Sea. Contrasting dissipation rates (𝜖) and vertical eddy diffusivities (Kz) were obtained with depth-averaged ranges of ∼[9×10−10−10−5]