Major impact of dust deposition on the productivity of the Arabian Sea

In the Arabian Sea (AS), spatiotemporal nutrient limitation patterns of primary production and the possible role of nutrient inputs from the atmosphere are still not well understood. Using a biogeochemical model forced by modeled aerosol deposition, we show that without high atmospheric iron inputs through dust deposition during the summer monsoon, primary production over the AS would be reduced by half. Atmospheric iron deposition also supports most of the nitrogen fixation over the AS. However, our ocean biogeochemistry modeling results suggest that dinitrogen fixation constitutes a negligible fraction of the primary production. Finally, we show that atmospheric inputs of nitrogen, mostly from anthropogenic activities in India, have a negligible impact on primary production

Simulated Arctic Ocean Response to Doubling of Riverine Carbon and Nutrient Delivery

International audienceThe Arctic Ocean, more than any other ocean, is influenced by riverine input of carbon and nutrients. That riverine delivery is likely to change with climate change as runoff increases, permafrost thaws, and tree lines advance. But it is unknown to what extent these changes in riverine delivery will affect Arctic Ocean primary production, air-to-sea CO 2 fluxes, and acidification. To test their sensitivity to changing riverine delivery, we made sensitivity tests using an ocean circulation model coupled to an ocean biogeochemical model. In separate idealized simulations, riverine inputs of dissolved inorganic carbon (C T), dissolved organic carbon (DOC), and nutrients were increased by 1%/year until doubling. Doubling riverine nutrient delivery increased primary production by 11% on average across the Arctic basin and by up to 34-35% locally. Doubling riverine DOC delivery resulted in 90% of that added carbon being lost to the atmosphere, partly because it was imposed that once delivered to the ocean, the riverine DOC is instantaneously remineralized to C T. That additional outgassing, when considered alone, reduced the net ingassing of natural CO 2 into the Arctic Ocean by 25% while converting the Siberian shelf seas and the Beaufort Sea from net sinks to net sources of carbon to the atmosphere. The remaining 10% of DOC remained in the Arctic Ocean, but having been converted to C T , it enhanced acidification. Conversely, doubling riverine C T increased the Arctic Ocean's average surface pH by 0.02 because riverine total alkalinity delivery increased at the same rate as riverine C T delivery

Simulated seasonal and interannual variability of the mixed layer heat budget in the northern Indian Ocean

A global ocean general circulation model (OGCM) is used to investigate the mixed layer heat budget of the northern Indian Ocean (NIO). The model is validated against observations and shows fairly good agreement with mixed layer depth data in the NIO. The NIO has been separated into three subbasins: the western Arabian Sea (AS), the eastern AS, and the Bay of Bengal (BoB). This study reveals strong differences between the western and eastern AS heat budget, while the latter basin has similarities with the BoB. Interesting new results on seasonal time scales are shown. The penetration of solar heat flux needs to be taken into account for two reasons. First, an average of 28 W m?2 is lost beneath the mixed layer over the year. Second, the penetration of solar heat flux tends to reduce the effect of solar heat flux on the SST seasonal cycle in the AS because the seasons of strongest flux are also seasons with a thin mixed layer. This enhances the control of SST seasonal variability by latent heat flux. The impact of salinity on SST variability is demonstrated. Salinity stratification plays a clear role in maintaining a high winter SST in the BoB and eastern AS while not in the western AS. The presence of freshwater near the surface allows heat storage below the surface layer that can later be recovered by entrainment warming during winter cooling (with a winter contribution of +2.1°C in the BoB). On an interannual time scale, the eastern AS and BoB are strongly controlled by the winds through the latent heat flux anomalies. In the western AS, vertical processes, as well as horizontal advection, contribute significantly to SST interannual variability, and the wind is not the only factor controlling the heat flux forcing.<br/

Physical control of interannual variations of the winter chlorophyll bloom in the northern Arabian Sea

International audienceThe northern Arabian Sea hosts a winter chlorophyll bloom, triggered by convective overturning in response to cold and dry northeasterly monsoon winds. Previous studies of interannual variations of this bloom only relied on a couple of years of data and reached no consensus on the associated processes. The current study aims at identifying these processes using both ∼ 10 years of observations (including remotely sensed chlorophyll data and physical parameters derived from Argo data) and a 20-year-long coupled biophysical ocean model simulation. Despite discrepancies in the estimated bloom amplitude, the six different remotely sensed chlorophyll products analysed in this study display a good phase agreement at seasonal and interannual timescales. The model and observations both indicate that the interannual winter bloom fluctuations are strongly tied to interannual mixed layer depth anomalies (∼ 0.6 to 0.7 correlation), which are themselves controlled by the net heat flux at the air–sea interface. Our modelling results suggest that the mixed layer depth control of the bloom amplitude ensues from the modulation of nutrient entrainment into the euphotic layer. In contrast, the model and observations both display insignificant correlations between the bloom amplitude and thermocline depth, which precludes a control of the bloom amplitude by daily dilution down to the thermocline depth, as suggested in a previous study

Climate change projections using the IPSL-CM5 Earth System Model: from CMIP3 to CMIP5

We present the global general circulation model IPSL-CM5 developed to study the long-term response of the climate system to natural and anthropogenic forcings as part of the 5th Phase of the Coupled Model Intercomparison Project (CMIP5). This model includes an interactive carbon cycle, a representation of tropospheric and stratospheric chemistry, and a comprehensive representation of aerosols. As it represents the principal dynamical, physical, and bio-geochemical processes relevant to the climate system, it may be referred to as an Earth System Model. However, the IPSL-CM5 model may be used in a multitude of configurations associated with different boundary conditions and with a range of complexities in terms of processes and interactions. This paper presents an overview of the different model components and explains how they were coupled and used to simulate historical climate changes over the past 150 years and different scenarios of future climate change. A single version of the IPSL-CM5 model (IPSL-CM5A-LR) was used to provide climate projections associated with different socio-economic scenarios, including the different Representative Concentration Pathways considered by CMIP5 and several scenarios from the Special Report on Emission Scenarios considered by CMIP3. Results suggest that the magnitude of global warming projections primarily depends on the socio-economic scenario considered, that there is potential for an aggressive mitigation policy to limit global warming to about two degrees, and that the behavior of some components of the climate system such as the Arctic sea ice and the Atlantic Meridional Overturning Circulation may change drastically by the end of the twenty-first century in the case of a no climate policy scenario. Although the magnitude of regional temperature and precipitation changes depends fairly linearly on the magnitude of the projected global warming (and thus on the scenario considered), the geographical pattern of these changes is strikingly similar for the different scenarios. The representation of atmospheric physical processes in the model is shown to strongly influence the simulated climate variability and both the magnitude and pattern of the projected climate changes

Evaluation of CNRM earth system model, CNRM-ESM2-1 : role of earth system processes in present-day and future climate

This study introduces CNRM-ESM2-1, the Earth system (ES) model of second generation developed by CNRM-CERFACS for the sixth phase of the Coupled Model Intercomparison Project (CMIP6). CNRM-ESM2-1 offers a higher model complexity than the Atmosphere-Ocean General Circulation Model CNRM-CM6-1 by adding interactive ES components such as carbon cycle, aerosols, and atmospheric chemistry. As both models share the same code, physical parameterizations, and grid resolution, they offer a fully traceable framework to investigate how far the represented ES processes impact the model performance over present-day, response to external forcing and future climate projections. Using a large variety of CMIP6 experiments, we show that represented ES processes impact more prominently the model response to external forcing than the model performance over present-day. Both models display comparable performance at replicating modern observations although the mean climate of CNRM-ESM2-1 is slightly warmer than that of CNRM-CM6-1. This difference arises from land cover-aerosol interactions where the use of different soil vegetation distributions between both models impacts the rate of dust emissions. This interaction results in a smaller aerosol burden in CNRM-ESM2-1 than in CNRM-CM6-1, leading to a different surface radiative budget and climate. Greater differences are found when comparing the model response to external forcing and future climate projections. Represented ES processes damp future warming by up to 10% in CNRM-ESM2-1 with respect to CNRM-CM6-1. The representation of land vegetation and the CO2-water-stomatal feedback between both models explain about 60% of this difference. The remainder is driven by other ES feedbacks such as the natural aerosol feedback

Contribution of tropical cyclones to the air-sea CO2 flux: A global view

Previous case studies have illustrated the strong local influence of tropical cyclones (TCs) on CO2 air-sea flux (FCO2), suggesting that they can significantly contribute to the global FCO2. In this study, we use a state-of-the art global ocean biochemical model driven by TCs wind forcing derived from a historical TCs database, allowing to sample the FCO2 response under 1663 TCs. Our results evidence a very weak contribution of TCs to global FCO2, one or two order of magnitude smaller than previous estimates extrapolated from case studies. This result arises from several competing effects involved in the FCO2 response to TCs, not accounted for in previous studies. While previous estimates have hypothesized the ocean to be systematically oversaturated in CO2 under TCs, our results reveal that a similar proportion of TCs occur over oversaturated regions (i.e. the North Atlantic, Northeast Pacific and the Arabian Sea) and undersaturated regions (i.e. Westernmost North Pacific, South Indian and Pacific Ocean). Consequently, by increasing the gas exchange coefficient, TCs can generate either instantaneous CO2 flux directed from the ocean to the atmosphere (efflux) or the opposite (influx), depending on the CO2 conditions at the time of the TC passage. A large portion of TCs also occurs over regions where the ocean and the atmosphere are in near equilibrium, resulting in very weak instantaneous fluxes. Previous estimates did also not account for any asynchronous effect of TCs on FCO2: during several weeks after the storm, oceanic pCO2 is reduced in response to vertical mixing, which systematically causes an influx anomaly. This implies that, contrary to previous estimates, TCs weakly affect the CO2 efflux when they blow over supersaturated areas because the instantaneous storm wind effect and post-storm mixing effect oppose with each other. In contrast, TCs increase the CO2 influx in undersaturated conditions because the two effects add up. These compensating effects result in a very weak contribution to global FCO2 and a very modest contribution to regional interannual variations (up to 10%)

Evaluation of CMIP6 DECK Experiments With CNRM‐CM6‐1

International audienceThis paper describes the main characteristics of CNRM-CM6-1, the fully coupled atmosphere-ocean general circulation model of sixth generation jointly developed by Centre National de Recherches Météorologiques (CNRM) and Cerfacs for the sixth phase of the Coupled Model Intercomparison Project 6 (CMIP6). The paper provides a description of each component of CNRM-CM6-1, including the coupling method and the new online output software. We emphasize where model's components have been updated with respect to the former model version, CNRM-CM5.1. In particular, we highlight major improvements in the representation of atmospheric and land processes. A particular attention has also been devoted to mass and energy conservation in the simulated climate system to limit long-term drifts. The climate simulated by CNRM-CM6-1 is then evaluated using CMIP6 historical and Diagnostic, Evaluation and Characterization of Klima (DECK) experiments in comparison with CMIP5 CNRM-CM5.1 equivalent experiments. Overall, the mean surface biases are of similar magnitude but with different spatial patterns. Deep ocean biases are generally reduced, whereas sea ice is too thin in the Arctic. Although the simulated climate variability remains roughly consistent with CNRM-CM5.1, its sensitivity to rising CO 2 has increased: the equilibrium climate sensitivity is 4.9 K, which is now close to the upper bound of the range estimated from CMIP5 models