Nicolas BarrierInstitute of Research for Development (IRD) · UMR Marbec
Institute of Research for Development (IRD)
- UMR Marbec
- Marseille, France
- Research engineer in scientific calculation
- Marseile, France
- Research Engineer in numerical modelling and scientific calculation
Research Items (19)
- Mar 2019
The Mediterranean Sea is among the main hotspots of marine biodiversity in the world. Under combined pressures of fishing activities and climate change it has also become a hotspot of global change, with increased concern about the worsening status of marine exploited species. More integrated modelling approaches are needed to anticipate global change impacts in the Mediterranean Sea, in order to help decision makers prioritizing management actions and strategies, mitigating impacts and adapting to changes. Our challenge was to develop a holistic model of the marine biodiversity in the Mediterranean Sea with an explicit representation of the spatial multispecies dynamics of exploited resources under the combined influence of climate variability and fishing pressure. An individual-based ecosystem model OSMOSE (Object-oriented Simulator of Marine ecOSystEms), including 100 marine species (fish, cephalopods and crustaceans) and representing about 95 % of the total declared catches, has been implemented for the first time at a high spatial resolution (400 square km) and at a large spatial scale (whole Mediterranean basin). The coupling of OSMOSE to the NEMOMED 12 physical model, and to the Eco3M-S biogeochemical and low trophic level model has been achieved to build the OSMOSE-MED end-to-end model. We fitted OSMOSE-MED to observed and estimated data of biomass and commercial catches using a likelihood approach and an evolutionary optimization algorithm. The outputs of OSMOSE-MED were then verified against observed biomass and catches, and confronted to independent datasets (MEDITS data, diet compositions and trophic levels). Although some improvements are suggested for future developments, the model results at different hierarchical levels, from individuals up to the ecosystem scale, were consistent with current knowledge and observations on the structure, the functioning and the dynamics of the ecosystems in the Mediterranean Sea. All the modelling steps, from the comprehensive representation of key ecological processes and feedbacks, the careful parameterization of the model, the confrontation to observed data, and the positive outcome from the validation process, allowed to strengthen the degree of realism of OSMOSE-MED and its relevance as an impact model to explore the futures of marine biodiversity under scenarios of global change, and as a tool to support the implementation of ecosystem-based fisheries management in the Mediterranean Sea.
To facilitate the wider implementation of ecosystem modeling platforms and, thereby, to help advance ecosystem based fisheries management (EBFM) worldwide, tools delivering a large quantity of inputs to ecosystem models are needed. We developed a web application providing OSMOSE ecosystem models with values for trophic, growth and reproduction parameters derived from data from two global information systems (FishBase and SeaLifeBase). Our web application guides the user through simple queries to extract information from FishBase and SeaLifeBase data archives, and it delivers all the configuration files necessary for running an OSMOSE model. Here, we present our web application and demonstrate it for the West Florida Shelf ecosystem. Our software architecture can serve as a basis for designing other advanced web applications using FishBase and SeaLifeBase data in support of EBFM.
Climate change is shifting the abundance and distribution of marine species with consequences for ecosystem functioning, seafood supply, management and conservation. Several approaches for future projection exist but these have never been compared systematically to assess their variability. We conducted standardized ensemble projections including 6 global fisheries and marine ecosystem models, forced with 2 Earth-system models and 4 emission scenarios in a fished and unfished ocean, to derive average trends and associated uncertainties. Without fishing, mean global animal biomass decreased by 5% (standard deviation 4%) under low and 17% (standard deviation 11%) under high emissions by 2100, primarily driven by increasing temperature and decreasing primary production. These climate-change effects were slightly weaker for larger animals and in a fished ocean. Considerable regional variation ranged from strong biomass increases in high latitudes to strong decreases in mid-low latitudes, with good model agreement on the direction of change but variable magnitude. Uncertainties due to differences among ecosystem or Earth-system models were similar, suggesting equal need for model improvement. Our ensemble projections provide the most comprehensive outlook on potential climate-driven ecological changes in the ocean to date. Realized future trends will largely depend on how fisheries and management adapt to these changes in a changing climate.
Project - Python Physical Analyis Of the Gridded Ocean (PyPAGO)
Module containing multiple functions dedicated to Earth Scientits. - Calculation of monthly/daily climatologies/anomalies - Multi-taper spectral analysis - Masked Lambert Projection - Generation of fancy colormaps - NetCDF file manipulation - Lanczos and FFT filtering - And probably more to come
- Apr 2016
- European Geosciences Union
In order to understand the paleo-variability of Saharo-Sahelian paleoprecipitation, which is recorded in the sedi- ments of Lake Chad situated in central Sahel, we use a modelling chain going from global climate to basin-scale hydrological model. Namely, climate model outputs for the Holocene, starting with the mid-Holocene (6ka) avail- able from the IPSL-CM5 global climate model are statistically downscaled with the General Additive Model approach (Levavasseur et al., 2011), then used to feed the LPJmL model (Bondeau et al., 2007) which calculates the equilibrium vegetation and runoff. Climate and runoff are then given to the dynamic routing scheme HYDRA (Coe et al., 2000) in order to calculate the paleo river network and paleo extent of Lake Chad. The results at each step are compared with reconstructions derived from continental proxies on the regional scale in order to assess the robustness of the results. For the mid-Holocene, the downscaled precipitation matches very well precipitation estimations derived from lacustrine pollen data. For the historical period, the LPJmL simulated runoff averaged over the Chad basin depicts the same trend than observations of Lake Chad water level, but the absolute water level is overestimated in HYDRA, which can be attributed to humid biases both in LPJmL and HYDRA. Finally, we will investigate the relative changes in river network and Lake Chad extent between the present and the mid-Holocene.
- Feb 2016
The Northern Mediterranean Current is the return branch of the cyclonic circulation of the northwestern Mediterranean Sea. Because of geostrophic constraints, this warm and oligotrophic current is forced to flow westward along the continental slope of the Gulf of Lion. But, occasionally, it penetrates on the shelf and strongly impacts the local biogeochemistry and in turn the primary production. By combining in situ observations and high-resolution modelling, it is shown that intrusions on the eastern part of the gulf are mainly forced by easterly or northwesterly wind events, through physical mechanisms that are very different in nature. Easterlies induce a piling of water along the Gulf of Lion coast that drives, through geostrophy, an alongshore shelf-intruding current. This intrusive current occurs independently of the stratification and is concomitant with the wind forcing. On the other hand, intrusions due to northwesterlies only occur during stratified conditions and are related to the development of upwellings along the Gulf of Lion coasts. When the upwelling develops, a northwestward alongshore pressure force balances the Coriolis force associated with the onshore flow at depth. When the winds drop, the upwelling relaxes and the onshore flow weakens. Consequently, the Coriolis force no longer counterbalances the pressure force that ultimately dominates the momentum balance, causing the displacement of the Northern Current on the Gulf of Lion shelf approximately 1 day after the wind relaxation. This time lag between the northwesterlies decrease and the intrusions permits to anticipate possible changes in the biogeochemistry of the Gulf of Lion.
- Jan 2015
In the mid 1990s, the North Atlantic subpolar gyre has shown a dramatic warming event that has been thoroughly investigated from observations and numerical simulations. Some studies suggest that it was due to an interannual, wind-driven weakening and shrinking of the gyre that facilitated the penetration of warm Atlantic Water, the weakening of the gyre being attributed to changes in the North Atlantic Oscillation (NAO) and in the East Atlantic Pattern, which are the two dominant modes of atmospheric variability in the North Atlantic. However, other studies suggest that the warming event was due to a decadal, buoyancy-driven strengthening of the meridional overturning circulation and subsequent intensification of the poleward heat transport, in response to the positive NAO conditions of 1988-1995. To reconcile this discrepancy, the heat budget in the North Atlantic subpolar gyre is reconstructed from four ocean hindcast simulations sharing the same modelling platform but using different settings. The novelty of this work is the decomposition of the subpolar gyre into a western and an eastern subregion, which is motivated by water mass distribution around Reykjanes Ridge and by the fact that deep convection only occurs in the western subpolar gyre.
There is now a strong scientific consensus that coastal marine systems of Western Europe are highly sensitive to the combined effects of natural climate variability and anthropogenic climate change. However, it still remains challenging to assess the spatial and temporal scales at which climate influence operates. While large-scale hydro-climatic indices, such as the North Atlantic Oscillation (NAO) or the East Atlantic Pattern (EAP) and the weather regimes such as the Atlantic Ridge (AR), are known to be relevant predictors of physical processes, changes in coastal waters can also be related to local hydro-meteorological and geochemical forcing. Here, we study the temporal variability of physical and chemical characteristics of coastal waters located at about 48°N over the period 1998-2013 using (1) sea surface temperature, (2) sea surface salinity and (3) nutrient concentration observations for two coastal sites located at the outlet of the Bay of Brest and off Roscoff, (4) river discharges of the major tributaries close to these two sites and (5) regional and local precipitation data over the region of interest. Focusing on the winter months, we characterize the physical and chemical variability of these coastal waters and document changes in both precipitation and river runoffs. Our study reveals that variability in coastal waters is connected to the large-scale North Atlantic atmospheric circulation but is also partly explained by local river influences. Indeed, while the NAO is strongly related to changes in sea surface temperature at the Brest and Roscoff sites, the EAP and the AR have a major influence on precipitations, which in turn modulate river discharges that impact sea surface salinity at the scale of the two coastal stations.
Atmospheric weather regimes are a promising alternative to the modes of variability traditionally used to assess the impacts of atmospheric variability on the oceanic circulation in the North Atlantic. Indeed, they preserve the spatial asymmetry of the dominant mode of variability, the North Atlantic Oscillation. Using numerical simulations and tide-gauge observations of dynamical sea-surface height, the impacts of the four winter weather regimes on the horizontal and meridional oceanic circulations are analysed.
In the mid-90s, the North-Atlantic subpolar gyre has shown a dramatic warming that has been attributed to changes in the large-scale atmospheric variability. This warming has often been attributed to an abrupt change in the North-Atlantic Oscillation, from highly positive in 1995 to highly negative in 1996. However, decadal prediction experiments suggest that this warming is the signature of a delayed ocean response to the positive NAO conditions of 1988-1995. Using four ocean hindcasts sharing the same modelling platform but using different forcings, resolutions, and parameterizations, the causes of the 1995 warming of the subpolar gyre are addressed. Heat budget cal- culations are performed in closed domains and the respective influences of surface heat fluxes and ocean heat convergence are separated. The novelty of this study is the further decomposition of the gyre into a western and an eastern part, the separation being provided by the Reykjanes and Mid-Atlantic Ridges. Our results suggest that in the western subpolar gyre, which contains the Labrador and Irminger seas, the warming is due to a delayed spin-up of the meridional overturning circulation to the strong NAO+ conditions of 1988-1995, consistent with the decadal prediction experiments. In the eastern subpolar gyre, the warming is due to a fast, barotropic wind-driven change in ocean convergence due to the switch in the NAO of 1995. Hence, the separation of the subpolar gyre reconciles the literature about the 1995 warming of the subpolar gyre.
- Jan 2014
A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO(-)), and positive NAO (NAO(+)) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO(+) causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO(-) induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO(+) and to spin down following persistent AR and NAO(-) conditions. These responses are driven by changes in deep water formation in the Labrador Sea.
- Dec 2012
A perceived limitation of z-coordinate models associated with spurious diapycnal mixing in eddying, frontal flow, can be readily addressed through appropriate attention to the tracer advection schemes employed. It is demonstrated that tracer advection schemes developed by Prather and collaborators for application in the stratosphere, greatly improve the fidelity of eddying flows, reducing levels of spurious diapycnal mixing to below those directly measured in field experiments, ∼1 × 10−5 m2 s−1. This approach yields a model in which geostrophic eddies are quasi-adiabatic in the ocean interior, so that the residual-mean overturning circulation aligns almost perfectly with density contours. A reentrant channel configuration of the MIT General Circulation Model, that approximates the Antarctic Circumpolar Current, is used to examine these issues. Virtual analogs of ocean deliberate tracer release field experiments reinforce our conclusion, producing passive tracer solutions that parallel field experiments remarkably well.
Interannual variability of subtropical sea-surface-height (SSH) anomalies, estimated by satellite and tide-gauge data, is investigated in relation to wintertime daily North-Atlantic weather regimes. Sea-level anomalies can be viewed as proxies for the subtropical gyre intensity because of the intrinsic baroclinic structure of the circulation. Our results show that the strongest correlation between SSH and weather regimes is found with the so-called Atlantic-Ridge (AR) while no significant values are obtained for the other regimes, including those related to the North Atlantic Oscillation (NAO), known as the primary actor of the Atlantic dynamics. Wintertime AR events are characterized by anticyclonic wind anomalies off Europe leading to a northward shift of the climatological wind-stress curl. The latter affects subtropical SSH annual variability by altered Sverdrup balance and ocean Rossby wave dynamics propagating westward from the African coast towards the Caribbean. The use of a simple linear planetary geostrophic model allows to quantify those effects and confirms the primary importance of the winter season to explain the largest part of SSH interannual variability in the Atlantic subtropical gyre. Our results open new perspectives in the comprehension of North-Atlantic Ocean variability emphasizing the role of AR as a driver of interannual variability at least of comparable importance to NAO.
The first mode of atmospheric variability in the North-Atlantic is the so-called North-Atlantic Oscillation (NAO), which, for its positive (negative) phase, corresponds to the simultaneous strengthening (weakening) of the Icelandic Low and Azores High. The NAO impact on ocean circulation has been thoroughly studied in literature; however, littlle is known about the role of the other modes of variability, such as the East-Atlantic Pattern (EAP) and the Scandinavian pattern. The impact of the North-Atlantic atmosphere on oceanic circulation is here investigated through the weather regime paradigm. Weather regimes (WR) defined as large-scale, recurrent, and quasi-stationary atmospheric patterns, are preferred to modes of variability because they account for non-symmetry in space and sign of the atmospheric variability. Ocean-only experiments (here NEMO-based North-Atlantic model model at 0.5 degrees of resolution), forced with daily surface fields reconstructed from one single WR among the four traditionally extracted in the North Atlantic, allow us to assess the sensitivity of the ocean circulation to the type of WR. Our results show that the North Atlantic circulation seems to be the most sensitive to the Atlantic-Ridge WR (AR, positive phase of the EAP), characterized by an anticyclonic wind-anomaly off Europe. Under Atlantic Ridge forcing, the subtropical and subpolar gyres are approximately 15 Sverdrups weaker, and the net mass, heat and freshwater transports are strongly impacted, especially at 42N. The response of the subtropical gyre is fast (in the order of 2-3 years) while it takes about approximately 10 years for the subpolar gyre to stabilize. We also show that the response of the ocean circulation to the NAO is not symmetrical between the positive and negative phase. Sea-Surface Height (SSH) at Bermuda (Esso-Pier) has been used as a proxy of subtropical gyre variability to assess the validity of the model results. Lead-lag correlations show that SSH anomalies are correlated with AR occurrences (AR leads by 0-2 years). We argue, by the use of a simple baroclinic and barotropic model that the impact of AR on SSH is mostly wind-driven.
We investigate the climatic impact of opening the Central America Seaway (CAS) in a coupled atmosphere-ocean-sea-ice model. A highly idealized land distribution is employed in which two meridional barriers extend from the North Pole in to the southern hemisphere, thus dividing the ocean in to a large basin, a small basin and a circumpolar flow around the South Pole. Such a configuration captures the essential zonal and inter-hemispheric asymmetries of the current climate. These simple geometrical constraints are sufficient to localize the deep-reaching meridional overturning circulation (MOC) to the northern extremity of the small basin. Given this reference experiment, we open up an analogue of the Central America Seaway on the western margin of the small basin north of the equator. Both deep and shallow passageways are considered. We find that although a major reorganization of ocean circulation occurs, along with significant local water-mass changes, global heat and freshwater meridional transports are largely unchanged, as are temperatures over the North Pole. In particular we do not observe a weakening of the MOC in the small basin, with salinity exchange between the large basin playing only a minor role. The simplicity of the geometrical configuration used in our experiments enables us to tease apart exactly what is going on. Experiments in which the salinity and temperature states of the small and large basins are interchanged, for example, show that our solutions are robust, with deep convection returning to the small basin after 800 years or so. Our experiments suggest to us that the closing of the CAS alone is not sufficient to lead to the onset of northern hemisphere glaciations 2 Ma years or so ago.
The evolution of the Atlantic Meridional Overturning Circulation (AMOC) is crucial to be estimated in the context of climate change because of its central role in Earth energy equilibrium. Coupled atmosphere-ocean models used so far in CMIPs suffer from large uncertainties associated with, among others, the misrepresentation of oceanic processes (due to coarse resolution) and of air-sea interface processes. Erroneous wind patterns and buoyancy budgets especially alter the representation of the AMOC ; it is thus necessary to find a way to get around these obstacles. A statistical-dynamical downscaling scheme has been developed in Minvielle et al (2011) to generate unbiased sea-surface atmospheric variables from CMIP models used subsequently as forcing fields for a higher resolution ocean model. A transfer function is first built over a learning period (1958-2002) between air-sea variables and the occurrences of ”observed” (ERA40) atmospheric weather regimes (WR) extracted from daily large-scale pressure fields over the Atlantic. The statistical relationship is then used to reconstruct a modeled set of ocean forcing fields and is validated over the late XXth century before being applied to scenario experiments. Our study is based here on the outputs of the CNRM-CM3 model and on the use of the NEMO ocean model integrated at the 0.5 degree resolution (ORCA05). Because the transfer function is only based on atmospheric dynamics over 1958-2002, the surface temperature trend associated with greenhouse gazes (GHG) direct effect (that is so far weak but is expected to be very important in the more-or-less near future following scenarios of GHG emissions) is not captured. The respective role of the change in the atmospheric dynamics (through the modification of occurrences and properties of WR) and the increase of surface temperature due to GHGs radiative forcing is assessed here from two sensitivity ORCA05 experiments: one where the sole changes in WR given by CNRM-CM3 A2 scenario is applied (hereafter A2-WR), and a second one in which the surface temperature trend estimated from CNRM-CM3 is superimposed (A2-WR+T2). Those are compared to the original low-resolution CNRM-CM3 coupled model. We find that the AMOC response is opposite between the two sensitivity ocean-forced experiments and is also very different from the original coupled one. In CNRM-CM3, the AMOC collapses by the end of the XXIth century with a -14Sv decrease. In A2-WR+T2, the AMOC decreases by only -4 Sv ( 40N, 1500m) while it increases by + 2 Sv (40N, 2500m) in A2-WR. We show that such an increase is associated with greater occurrences of positive North Atlantic Oscillation WR at the end of the XXIth century, leading to enhanced deep convection and acceleration of the subpolar gyre. The latter is dominated in A2-WR+T2 by the overall stabilization of the ocean due to surface temperature warming leading to a clear slackening of the entire ocean dynamics in the Atlantic.