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Jullien S.,French National Center for Space Studies | Menkes C.E.,Institute Of Recherche Pour Le Developpement | Menkes C.E.,University Pierre and Marie Curie | Marchesiello P.,French National Center for Space Studies | And 8 more authors.
Journal of Physical Oceanography | Year: 2012

The present study investigates the integrated ocean response to tropical cyclones (TCs) in the South Pacific convergence zone through a complete ocean heat budget. The TC impact analysis is based on the comparison between two long-term (1979-2003) oceanic simulations forced by a mesoscale atmospheric model solution in which extreme winds associated with cyclones are either maintained or filtered. The simulations provide a statistically robust experiment that fills a gap in the current modeling literature between coarse-resolution and short-term studies. The authors' results show a significant thermal response of the ocean to at least 500-m depth, driven by competing mixing and upwelling mechanisms. As suggested in previous studies, vertical mixing largely explains surface cooling induced by TCs. However, TC-induced upwelling of deeper waters plays an unexpected role as it partly balances the warming of subsurface waters induced by vertical mixing. Below 100 m, vertical advection results in cooling that persists long after the storm passes and has a signature in the ocean climatology. The heat lost through TC-induced vertical advection is exported outside the cyclogenesis area with strong interannual variability. In addition, 60% of the heat input below the surface during the cyclone season is released back to the oceanic mixed layer through winter entrainment and then to the atmosphere. Therefore, seasonal modulation reduces the mean surface heat flux due to TCs to about 331023 PW in this region exposed to 10%-15% of the world's cyclones. The resulting climatological anomaly is a warming of about 0.18°C in the subsurface layer and cooling below the thermocline (less than 0.18°C). © 2012 American Meteorological Society.

Jouandet M.P.,Laboratoire dOceanographie Physique et Biogeochimique | Blain S.,French National Center for Scientific Research | Blain S.,CNRS Microbial Oceanography Laboratory | Metzl N.,University of Paris Descartes | Mongin M.,CSIRO
Antarctic Science | Year: 2011

The interannual variability of net community production (NCP) and air-sea CO 2 flux in a naturally iron fertilized and productive area of the Southern Ocean (Kerguelen plateau) was investigated using a 1D biogeochemical model driven by satellite chlorophyll, sea surface temperature and wind speed data for the 1997-2007 period. The model simulates the low fCO 2 and dissolved inorganic carbon (DIC) measured during summers 2004-05, 2005-06, 2006-07 and the high NCP derived from a seasonal carbon budget in the surface waters of these blooms. Although satellite data show high interannual variability in the dynamics and magnitude of the bloom during the 1997-2007 decade, the simulated interannual variability of the NCP was only ± 14%. This unexpected result could be due to the combined effect of both the duration and the start date of the bloom, the latter determining the depth of the mixed layer used to compute the NCP. In the productive area, the interannual variability of air-sea CO 2 flux (± 13%) was not only driven by the biological effect but also by the solubility effect. Our results contrast with previous studies in the high nutrient, low chlorophyll regions of the Southern Ocean. © Copyright Antarctic Science Ltd 2011.

King A.L.,University of Southern California | Saudo-Wilhelmy S.A.,University of Southern California | Leblanc K.,French National Center for Scientific Research | Leblanc K.,Aix - Marseille University | And 3 more authors.
ISME Journal | Year: 2011

Phytoplankton growth can be limited by numerous inorganic nutrients and organic growth factors. Using the subarctic diatom Attheya sp. in culture studies, we examined how the availability of vitamin B12 and carbon dioxide partial pressure (pCO2) influences growth rate, primary productivity, cellular iron (Fe), cobalt (Co), zinc (Zn) and cadmium (Cd) quotas, and the net use efficiencies (NUEs) of these bioactive trace metals (mol C fixed per mol cellular trace metal per day). Under B 12-replete conditions, cells grown at high pCO2 had lower Fe, Zn and Cd quotas, and used those trace metals more efficiently in comparison with cells grown at low pCO2. At high pCO2, B12-limited cells had 50% lower specific growth and carbon fixation rates, and used Fe 15-fold less efficiently, and Zn and Cd 3-fold less efficiently, in comparison with B 12-replete cells. The observed higher Fe, Zn and Cd NUE under high pCO2/B12-replete conditions are consistent with predicted downregulation of carbon-concentrating mechanisms. Co quotas of B 12-replete cells were 5-to 14-fold higher in comparison with B 12-limited cells, suggesting that 80% of cellular Co of B12-limited cells was likely from B12. Our results demonstrate that CO 2 and vitamin B12 interactively influence growth, carbon fixation, trace metal requirements and trace metal NUE of this diatom. This suggests the need to consider complex feedback interactions between multiple environmental factors for this biogeochemically critical group of phytoplankton in the last glacial maximum as well as the current and future changing ocean. © 2011 International Society for Microbial Ecology All rights reserved.

Dekaezemacker J.,Aix - Marseille University | Bonnet S.,Laboratoire dOceanographie Physique et Biogeochimique
Marine Ecology Progress Series | Year: 2011

Despite the increasing recognition of the biogeochemical importance of marine unicellular diazotrophic cyanobacteria, the environmental factors controlling their distribution and activity in the ocean are still very poorly understood. In particular, very few studies have considered the effect of combined nitrogen (N) on unicellular diazotroph activity, although their presence has been reported in surface waters exhibiting micromolar concentrations of dissolved in - organic nitrogen (DIN). Here, we studied the effect of nitrate (NO - 3) and ammonium (NH + 4) ad ditions (0.2 to 10 μmol l?1) on N 2 fixation and N uptake in 2 strains of Crocosphaera watsonii (WH8501 and WH0003) maintained in batch cultures in an N-free medium (YBC II) and a 12 h light:12 h dark cycle. Compared with the untreated control, increasing concentrations of NH + 4 to 10 μmol l?1 rapidly inhibited night-integrated N 2 fixation by up to 36 and 83% for WH8501 and WH0003, respectively. In contrast, night-integrated N 2 fixation was not significantly inhibited at any NO - 3 concentration. These results contrast with data obtained on Trichodesmium spp. (Cyano - bacteria) in which night-integrated N 2 fixation is inhibited up to 50% at the same NO - 3 concentrations. This study indicates that C. watsonii is able to take up combined nitrogen while keeping its ability to fix dinitrogen at high rates. © Inter-Research 2011.

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