Raffaele Bernardello

Response of the Ocean Natural Carbon Storage to Projected Twenty-First-Century Climate Change

The separate impacts of wind stress, buoyancy fluxes, and CO2 solubility on the oceanic storage of natural carbon are assessed in an ensemble of twentieth- to twenty-first-century simulations, using a coupled atmosphere‐ocean‐carbon cycle model. Time-varying perturbations for surface wind stress, temperature, and salinity are calculated from the difference between climate change and preindustrial control simulations, and are imposed on the ocean in separate simulations. The response of the natural carbon storage to each perturbation is assessed with novel prognostic biogeochemical tracers, which can explicitly decompose dissolved inorganic carbon into biological, preformed, equilibrium, and disequilibrium components. Strong responses of these components to changes in buoyancy and winds are seen at high latitudes, reflecting the critical role of intermediate and deep waters. Overall, circulation-driven changes in carbon storage are mainly due to changes in buoyancy fluxes, with wind-driven changes playing an opposite but smaller role. Results suggest that climate-driven perturbations to the ocean natural carbon cycle will contribute 20PgC to the reduction of the ocean accumulated total carbon uptake over the period 1860‐2100. This reflects a strong compensation between a buildup of remineralized organic matter associated with reduced deep-water formation (196PgC) and a decrease of preformed carbon (2116PgC). The latter is due to a warming-induced decrease in CO2 solubility (252PgC) and a circulation-induced decrease in disequilibrium carbon storage (264PgC). Climate change gives rise to a large spatial redistribution of ocean carbon, with increasing concentrations at high latitudes and stronger vertical gradients at low latitudes.

Factors controlling interannual variability of vertical organic matter export and phytoplankton bloom dynamics – a numerical case-study for the NW Mediterranean Sea

Mid-latitude spring blooms of phytoplankton show considerable year-to-year variability in timing, spatial extent and intensity. It is still unclear to what degree the bloom variability is connected to the magnitude of the vertical flux of organic matter. A coupled three-dimensional hydrodynamic-biogeochemical model is used to relate interannual variability in phytoplankton spring-bloom dynamics to variability in the vertical export of organic matter in the NW Mediterranean Sea. Simulation results from 2001 to 2010, validated against remote-sensing chlorophyll, show marked interannual variability in both timing and shape of the bloom. Model results show a tendency for the bloom to start later after cold and windy winters. However, the onset of the bloom occurs often when the mixed layer is still several hundred metres deep while the heat flux is already approaching zero and turbulent mixing is low. Frequency and intensity of wind episodes control both the timing and development of the bloom and the consequent export flux of organic matter. The wintertime flux is greater than zero and shows relatively low interannual variability. The magnitude of the interannual variability is mainly determined in March when the frequency of windy days positively correlates with the export flux. Frequent wind-driven mixing episodes act to increase the export flux and, at the same time, to interrupt the bloom. Perhaps counterintuitively, our analysis shows that years with discontinuous, low-chlorophyll blooms are likely to have higher export flux than years with intense uninterrupted blooms. The NW Mediterranean shows strong analogy with the North Atlantic section within the same latitude range. Hence, our results may also be applicable to this quantitatively more important area of the world ocean.

Impact of Weddell Sea deep convection on natural and anthropogenic carbon in a climate model

A climate model is used to investigate the influence of Weddell Sea open ocean deep convection on anthropogenic and natural carbon uptake for the period 1860–2100. In a three-member ensemble climate change simulation, convection ceases on average by year 1981, weakening the net oceanic cumulative uptake of atmospheric CO2 by year 2100 (−4.3 Pg C) relative to an ocean that has continued convection. This net weakening results from a decrease in anthropogenic carbon uptake (−10.1 Pg C), partly offset by an increase in natural carbon storage (+5.8 Pg C). Despite representing only 4% of its area, the Weddell Sea is responsible for 22% of the Southern Ocean decrease in total climate-driven carbon uptake and 52% of the decrease in the anthropogenic component of oceanic uptake. Although this is a model-specific result, it illustrates the potential of deep convection to produce an intermodel spread in future projections of ocean carbon uptake.

Modelling the Pelagic Ecosystem Dynamics: The NW Mediterranean

The word pelagic comes from the Greek ┨έ┣αγ┧ς meaning open sea and refers to the marine and oceanic domain away from the shore line and from surface to bottom (Wikipedia). The pelagic ecosystem includes the ever-moving and continuously changing waters, the habitat, and the diverse and inter-related groups of organisms or communities. Hydrodynamics, forced by external, mostly atmospheric processes set the very special physical conditions that, to a great extent, control the functioning of the biological processes. Currents, waves, mixing, turbulence, air/sea exchanges or fertilization are all mechanisms allowing planktonic communities, the most important in terms of biomass and fluxes of matter and energy, to develop and sustain other communities higher up in the trophic chain.