Olivier Aumont

Response of diatoms distribution to global warming and potential implications: A global model study

[1]xa0Using a global model of ocean biogeochemistry coupled to a climate model, we explore the effect of climate change on the distribution of diatoms, a key phytoplankton functional group. Our model results suggest that climate change leads to more nutrient-depleted conditions in the surface ocean and that it favors small phytoplankton at the expense of diatoms. At 4xCO2, diatoms relative abundance is reduced by more than 10% at the global scale and by up to 60% in the North Atlantic and in the subantarctic Pacific. This simulated change in the ecosystem structure impacts oceanic carbon uptake by reducing the efficiency of the biological pump, thus contributing to the positive feedback between climate change and the ocean carbon cycle. However, our model simulations do not identify this biological mechanism as a first-order process in the response of ocean carbon uptake to climate change.

Extratropical sources of Equatorial Pacific upwelling in an OGCM

The extratropical sources of equatorial undercurrent (EUC) water have been identified for an ocean circulation model using Lagrangian trajectory analysis. It has been found that the EUC waters emenate from a wide range of latitudes in the Pacific basin, with its densest constituent watermass being Subantarctic Mode Water (SAMW) from 50°S. Further analysis of the basin-scale circulation fields has revealed significant advective diapycnal mass fluxes associated with intergyre exchange. As a result of these diapycnal mass fluxes, the EUC transport as a function of density at 151°W (an Eulerian diagnostic) looks quite different from the original subduction rate as a function of density for the same collection of water particles. This implicates diapycnal vertical mixing as an important player in determining the preferred density horizon of maximum EUC transport along the equator. In summary, these results illustrate an important interdependence between advective and diapycnal mixing processes associated with basin-scale inter-gyre and inter-basin exchange in determining the mean equatorial stratification and EUC structure.

Comparison of global climatological maps of sea surface dimethyl sulfide

We have examined differences in regional and seasonal variability among sevenglobal climatologies of sea-surface dimethyl sulfide (DMS) concentrations. We foundlarge differences between recent climatologies and that typically used by mostatmospheric sulfur models. The relative uncertainty (1s/mean) in the latitudinaldistribution of the annual mean DMS concentration increases from about 50% in tropicaland temperate regions to nearly 100% in the high latitudes. We also compared theseclimatologies to new measurements in the North Atlantic Ocean taken during the 2001Programme Oce´an Multidisciplinaire Me´so Echelle (POMME) expeditions.

Seasonal and intraseasonal biogeochemical variability in the thermocline ridge of the southern tropical Indian Ocean

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) time series shows high variability of surface chlorophyll at seasonal and intraseasonal time scales in the oligotrophic southern tropical Indian Ocean thermocline ridge called the Seychelles-Chagos thermocline ridge (SCTR). The SCTR is characterized by an open ocean upwelling due to local Ekman pumping, which annually maintains the mixed layer (ML) shallow and is responsive to atmospheric forcing and in particular to the Madden-Julian Oscillation (MJO) at an intraseasonal time scale. Here we present an overview of SCTR biogeochemistry and investigate the physical processes driving the response observed at seasonal and intraseasonal time scales. Using satellite observations and biophysical ocean simulations, we show that seasonal and intraseasonal SeaWiFS signals (in austral winter and during MJO events, respectively) correspond to wind-induced mixing episodes. During such episodes, entrainment fertilizes the ML and allows phytoplankton production. Increased surface production is compensated by a decrease in the subsurface due to light limitation, leading to no significant change in integrated biomass and carbon export. Satellite observations and model results support the conclusion that the biogeochemical response to MJO is highly dependent on interannual variability of thermocline depth. Following Indian Ocean Dipole events, deepened nutrient-rich waters prevent nutrient input into the ML, decreasing the biogeochemical response to MJO. These results shed light on the physical processes at work in the strong surface temperature response to MJO in this region and suggest that entrainment cooling can play a role in the temperature signature to the MJO but is highly modulated by basin-scale interannual variability.

Physical and Biogeochemical Controls of the Phytoplankton Seasonal Cycle in the Indian Ocean: A Modeling Study

A three-dimensional primitive equation model Ocean Parallelise (OPA) was coupled to the biogeochemical model Pelagic Interaction Scheme for Carbon and Ecosystem Studies to simulate the ocean circulation and the marine biological productivity through the biogeochemical cycles of carbon and the main nutrients (P, N, Si, Fe). We focus on surface phytoplankton dynamics in the Indian Ocean extending from 30°S to 30°N and from 30°E to 120°E. The seasonal cycle of phytoplankton over the Indian Ocean is generally characterized by two blooms, one during the summer monsoon, the other one during the winter monsoon. Based on the method proposed by Levy et al. (2007), different biogeochemical provinces can be defined during the summer and winter monsoons. The model performed relatively well by simulating the main features of the cumulated increase in chlorophyll, and the time of the bloom onsets is consistent with data. It also reproduced quite well the main biogeochemical provinces in good agreement with data in most of the Arabian Sea (except in the central part), the Bay of Bengal, and in the convergence zone south of the equator. The analysis of the modeled biogeochemical processes has shown that during the blooms onset periods, the most limiting nutrient was nitrogen except some areas around India and the eastern part of the Bay of Bengal where the ecosystem tends toward silicate limitation. These limitations change during the blooms development. The model also highlighted a variety of the critical physical processes (horizontal and vertical advection, turbulent diffusion, mixed layer depth) involved in each biogeochemical province bloom dynamics.

Impact of the subtropical mode water biogeochemical properties on primary production in the North Atlantic: New insights from an idealized model study

[1]xa0An idealized biophysical model of the North Atlantic was designed to investigate the setting and variability of the subtropical mode water (STMW) biogeochemical properties and its impact on surface primary production in the North Atlantic. The model solution first emphasizes that the exact timing of STMW formation versus the timing of the spring bloom is of primary importance for setting the STMW biogeochemical properties. The surface primary production reaches its maximum in March in the STMW formation region just before its subduction. Thus the spring bloom consumes nitrate at the surface before STMW subducts, and STMW leaves the upper layers depleted in nutrients and fueled in organic matter. This spring consumption explains the low nutrient content of STMW observed near its source region by J. B. Palter et al. (2005). Furthermore, the model suggests that STMW plays a key role in exporting dissolved organic matter (DOM) at subsurface. The spring bloom produces a significant amount of DOM sequestrated in the mode waters after its subduction. This large pool of DOM is then remineralized with time along the transit of STMW through the subtropical gyre. Consequently, the nutrient content of STMW increases as it moves away from its source region. Finally, the model shows also that STMW is very important in controlling primary production in the western boundary current (WBC) region. Indeed, STMW remains isolated from the surface along its trajectory within the subtropical gyre. It joins the mixed layer by obduction in the WBC region only. This nutrient-rich old STMW irrigates and fertilizes the euphotic zone primarily in the WBC and then spreads along the boundary between the two gyres by advection.

Using altimetry to help explain patchy changes in hydrographic carbon measurements

Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O-2) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O-2 inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short-timescale (6 months to 2 years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first-order contribution to DIC and O-2 variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O-2 column inventory variations and sea surface height (SSII) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSII and both DIC and O-2 column inventories for many regions suggests that SSII changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from Climate Variability and Predictabilitys CO2/Repeat Hydrography program).

On evaluating ocean models with atmospheric potential oxygen

Annual data on carbon emissions from fossil-fuel combustion and cement manufacture have been used in studies of the carbon cycle for the last few decades. However, annual data do not specify carbon emissions on the seasonal timescales relevant to biospheric uptake and other processes affecting the carbon cycle. Estimates of monthly emissions from fossil-fuel consumption in the United States (US) have shown that an increasing percentage of the annual emissions are occurring during the growing season; however, carbon emitted from flaring natural gas at well sites was not accounted for in those emissions estimates, nor was carbon emitted during cement manufacture. Here we show that emissions from flaring, which amount to around 0.1 % of all fossil-fuel carbon emissions in the US, have no clear and persistent annual pattern that can be detected in the data. In contrast, carbon emissions from cement manufacture, which add about 0.7% to carbon emissions from fossil fuels in the US, have a clear and persistent annual pattern including low values in late winter and early spring. In this paper, we provide a few remarks on carbon emissions from natural-gas flaring before presenting monthly emissions estimates. We then focus on the methodology for calculating carbon emissions from cement manufacture before presenting and discussing the monthly emissions estimates.

Indonesian throughflow control of the eastern equatorial Pacific biogeochemistry

Two model simulations were performed to address the influence of the Indonesian throughflow (ITF) on the biogeochemical state of the equatorial Pacific. A simulation where the ITF is open is compared with an experiment where it is closed, and it is first shown that the impacts on the physical circulation are consistent with what has been found in previous modelling studies. In terms of biochemistry, closing the ITF results in increased iron concentration at the origin of the Equatorial Undercurrent (EUC). But the 11Sv of water otherwise transferred to the Indian Ocean remain in the equatorial Pacific, which result in a 30 m deepening of the thermocline/ferricline in the eastern Pacific. This deepening decreases the iron concentration of the equatorial wind driven upwelled water and cancels the iron increase advected by the EUC. The iron decrease of the equatorial upwelled water leads to decrease primary production by 15% along the equator.

THE SEASONAL CYCLE OF SURFACE CHLOROPHYLL ALONG THE PERUVIAN COAST: COMPARISON BETWEEN SEAWIFS SATELLITE OBSERVATIONS AND DYNAMICAL/BIOGEOCHEMICAL COUPLED MODEL SIMULATIONS

The seasonal cycle of surface chlorophyll along the Peruvian coasts is studied with SeaWiFS satellite observations and coupled dynamical/biogeochemical ORCA/PISCES model simulations. The observed mean spatial structure of surface chlorophyll is shown to be in good agreement with that of the model. However the seasonal cycles contrast: whereas the model produces a spring bloom consistent with maximum upwelling and deepest mixed layer in the winter season, the observed seasonal cycle shows two distinct blooms one in late spring-early summer and other in fall. Several hypotheses are suggested to explain the fall bloom and its non-occurrence in the model