Circulation and oxygen cycling in the Mediterranean Sea: Sensitivity to future climate change

Climate change is expected to increase temperatures and decrease precipitation in the Mediterranean Sea (MS) basin, causing substantial changes in the thermohaline circulation (THC) of both the Western Mediterranean Sea (WMS) and Eastern Mediterranean Sea (EMS). The exact nature of future circulation changes remains highly uncertain, however, with forecasts varying from a weakening to a strengthening of the THC. Here we assess the sensitivity of dissolved oxygen (O2) distributions in the WMS and EMS to THC changes using a mass balance model, which represents the exchanges of O2 between surface, intermediate, and deep water reservoirs, and through the Straits of Sicily and Gibraltar. Perturbations spanning the ranges in O2 solubility, aerobic respiration kinetics, and THC changes projected for the year 2100 are imposed to the O2 model. In all scenarios tested, the entire MS remains fully oxygenated after 100 years; depending on the THC regime, average deep water O2 concentrations fall in the ranges 151–205 and 160–219 µM in the WMS and EMS, respectively. On longer timescales (>1000 years), the scenario with the largest (>74%) decline in deep water formation rate leads to deep water hypoxia in the EMS but, even then, the WMS deep water remains oxygenated. In addition, a weakening of THC may result in a negative feedback on O2 consumption as supply of labile dissolved organic carbon to deep water decreases. Thus, it appears unlikely that climate-driven changes in THC will cause severe O2 depletion of the deep water masses of the MS in the foreseeable future

Impact of anomalous northward oceanic heat transport on global climate in a slab ocean setting

This paper explores the impact of anomalous northward oceanic heat transport on global climate in a slab ocean setting. To that end, the GCM LMDZ5A of the Laboratoire de Météorologie Dynamique is coupled to a slab ocean, with realistic zonal asymmetries and seasonal cycle. Two simulations with different anomalous surface heating are imposed: 1) uniform heating over the North Atlantic basin and 2) concentrated heating in the Gulf Stream region, with a compensating uniform cooling in the Southern Ocean in both cases. The magnitudes of the heating and of the implied northward interhemispheric heat transport are within the range of current natural variability. Both simulations show global effects that are particularly strong in the tropics, with a northward shift of the intertropical convergence zone (ITCZ) toward the heating anomalies. This shift is accompanied by a northward shift of the storm tracks in both hemispheres. From the comparison between the two simulations with different anomalous surface heating in the North Atlantic, it emerges that the global climate response is nearly insensitive to the spatial distribution of the heating. The cloud response acts as a large positive feedback on the oceanic forcing, mainly because of the low-cloud-induced shortwave anomalies in the extratropics. While previous literature has speculated that the extratropical Q flux may impact the tropics by the way of the transient eddy fluxes, it is explicitly demonstrated here. In the midlatitudes, the authors find a systematic northward shift of the jets, as well as of the associated Ferrel cells, storm tracks, and precipitation bands

A Glider Network Design Study for a Synoptic View of the Oceanic Mesoscale Variability

International audienceThis study presents an Observing System Simulation Experiment (OSSE) with a network of gliders in a realistic mesoscale field of eddies and filaments. The main objective is to demonstrate that the analysis skill evaluation, performed with different statistics, determines the optimal number of gliders needed to survey a ''glider observatory'' with a given simple topology of the glider array, in the shape of a ''double comb.'' Metrics, based on a spatial interpolation of the sampled data with a multiscale objective analysis method, are elaborated to evaluate the reconstruction of the three-dimensional temperature field with several glider networks, at a weekly time scale. The mesoscale structures obtained by the optimal network (front, eddies, eddies detachment) are also compared with the structures of the original simulation. This comparison demonstrates the efficiency of a glider fleet to sample a well-defined area at a given spatiotemporal scale. In this particular situation (midlatitude region, domain of 400 km 3 600 km, reconstruction of weekly snapshots), the optimum network is composed of 10 gliders. A relationship is highlighted between the spatial scales of the sampled area, the physical characteristics of the studied region, the reconstruction time scale, and the optimum number of gliders. The results presented here can be applied to design an actual in situ experiment

Interannual variability of deep convection in the Northwestern Mediterranean simulated with a coupled AORCM

International audienceA hindcast experiment of the Mediterranean present-day climate is performed using a fully-coupled Atmosphere-Ocean Regional Climate Model (AORCM) for the Mediterranean basin. The new model, called LMDz-NEMO-Med, is composed of LMDz4-regional as atmospheric component and of NEMOMED8 as oceanic component. This AORCM equilibrates freely, without any flux adjustment, neither in fresh water nor in heat. At its atmospheric lateral boundary conditions, it is driven by ERA-40 data from 1958 to 2001, after a spin-up of 40 years in coupled configuration. The model performance is assessed and compared with available observational datasets. The model skill in reproducing mean state and inter-annual variability of main atmospheric and oceanic surface fields is in line with that of state-of-the-art AORCMs. Considering the ocean behaviour, the inter-annual variations of the basin-scale heat content are in very good agreement with the observations. The model results concerning salt content could not be adequately validated. High inter-annual variability of deep convection in the Gulf of Lion is simulated, with 53 % of convective winters, representative of the present climate state. The role of different factors influencing the deep convection and its inter-annual variability is examined, including dynamic and hydrostatic ocean preconditioning and atmospheric surface forcing. A conceptual framework is outlined and validated in linking the occurrence of deep convection to the efficiency of the integrated surface buoyancy fluxes along the winter season to mix the initially stratified averaged water column down to the convective threshold depth. This simple framework (based only on 2 independent variables) is able to explain 60 % (resp. 69 %) of inter-annual variability of the deep water formation rate (resp. maximum mixed layer depth) for the West Mediterranean Deep Water (WMDW) formation process. © 2012 Springer-Verlag

Characterization of Convective Plumes Associated With Oceanic Deep Convection in the Northwestern Mediterranean From High Resolution In-Situ Data Collected by Gliders

International audienc

The CIRCE simulations: Regional Climate Change Projections with Realistic Representation of the Mediterranean Sea

In this article we describe an innovative multi-model system developed within the CIRCE EU-FP6 Project and used to produce simulations of the Mediterranean Sea regional climate. The models include high-resolution Mediterranean Sea components, which allow to assess the role of the basin, and in particular of the air-sea feedbacks in the climate of the region. The models have been integrated from 1951 to 2050, using observed radiative forcings during the first half of the simulation period and the IPCC SRES A1B scenario during the second half. The projections show a substantial warming (about 1.5°-2°C) and a significant decrease of precipitation (about 5%) in the region for the scenario period. However, locally the changes might be even larger. In the same period, the projected surface net heat loss decreases, leading to a weaker cooling of the Mediterranean Sea by the atmosphere, whereas the water budget appears to increase, leading the basin to loose more water through its surface than in the past. These results are overall consistent with the findings of previous scenario simulations, such as PRUDENCE, ENSEMBLES and CMIP3. The agreement suggests that these findings are robust to substantial changes in the configuration of the models used to make the simulations. Finally, the models produce a 2021-2050 mean steric sea-level rise that ranges between +7 cm and +12 cm, with respect to the period of reference

Cyclic fluctuations of blue whiting (Micromesistius poutassou) linked to open-sea convection processes in the northwestern Mediterranean

12 pages, 7 figures, 1 tableIn the Mediterranean, blue whiting, Micromesistius poutassou, constitutes a traditional fisheries resource. Over several decades, blue whiting landings in the Catalan coast (northwestern Mediterranean) have displayed cyclical variations, of c. 6 yrs, slightly decreased to five in the last two decades, as shown through wavelet analysis. These fluctuations have persisted under very different levels of fishing effort. This study evaluates the hypothesis that deep-water formation in the adjacent Gulf of Lions, and the enhanced primary productivity related to it, determines recruitment strength in blue whiting that results ultimately in the observed periodicity of the blue whiting landings. The link between landings and environmental drivers was explored using lagged cross-correlations, with 0- and 1-yr lag. The variables considered included large-scale indices [North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO)], Mediterranean climate indices [MO and Western Mediterranean Oscillation (WeMO)], and variables defining the local environmental conditions in the northwestern Mediterranean (sea-air heat flux, winter air temperature anomaly and Rhône river runoff). Significant correlations were only found between landings (1961-2011) and sea-air heat flux, which is generally taken as an indicator of processes of deep water convection, at 0 and 1-yr lag. These results suggest that the observed fluctuations in blue whiting landings respond to oceanographic processes taking place in the Gulf of LionsThis study has been partially supported by the project MAR-CTM2010-18874Peer Reviewe