Export 21 results:
Sort by: Author Title Type [ Year  (Desc)]
Le Fouest, V, Matsuoka A, Manizza M, Shernetsky M, Tremblay B, Babin M.  2018.  Towards an assessment of riverine dissolved organic carbon in surface waters of the western Arctic Ocean based on remote sensing and biogeochemical modeling. Biogeosciences. 15:1335-1346.   10.5194/bg-15-1335-2018   AbstractWebsite

Future climate warming of the Arctic could potentially enhance the load of terrigenous dissolved organic carbon (tDOC) of Arctic rivers due to increased carbon mobilization within watersheds. A greater flux of tDOC might impact the biogeochemical processes of the coastal Arctic Ocean (AO) and ultimately its capacity to absorb atmospheric CO2. In this study, we show that sea-surface tDOC concentrations simulated by a physical-biogeochemical coupled model in the Canadian Beaufort Sea for 2003-2011 compare favorably with estimates retrieved by satellite imagery. Our results suggest that, over spring-summer, tDOC of riverine origin contributes to 35% of primary production and that an equivalent of similar to 10% of tDOC is exported westwards with the potential of fueling the biological production of the eastern Alaskan nearshore waters. The combination of model and satellite data provides promising results to extend this work to the entire AO so as to quantify, in conjunction with in situ data, the expected changes in tDOC fluxes and their potential impact on the AO biogeochemistry at basin scale.

Eddebbar, YA, Long MC, Resplandy L, Rödenbeck C, Rodgers KB, Manizza M, Keeling RF.  2017.  Impacts of ENSO on air-sea oxygen exchange: Observations and mechanisms. Global Biogeochemical Cycles.   10.1002/2017GB005630   Abstract

Models and observations of atmospheric potential oxygen (APO ≃ O2 + 1.1 * CO2) are used to investigate the influence of El Niño–Southern Oscillation (ENSO) on air-sea O2 exchange. An atmospheric transport inversion of APO data from the Scripps flask network shows significant interannual variability in tropical APO fluxes that is positively correlated with the Niño3.4 index, indicating anomalous ocean outgassing of APO during El Niño. Hindcast simulations of the Community Earth System Model (CESM) and the Institut Pierre-Simon Laplace model show similar APO sensitivity to ENSO, differing from the Geophysical Fluid Dynamics Laboratory model, which shows an opposite APO response. In all models, O2 accounts for most APO flux variations. Detailed analysis in CESM shows that the O2 response is driven primarily by ENSO modulation of the source and rate of equatorial upwelling, which moderates the intensity of O2 uptake due to vertical transport of low-O2 waters. These upwelling changes dominate over counteracting effects of biological productivity and thermally driven O2 exchange. During El Niño, shallower and weaker upwelling leads to anomalous O2 outgassing, whereas deeper and intensified upwelling during La Niña drives enhanced O2 uptake. This response is strongly localized along the central and eastern equatorial Pacific, leading to an equatorial zonal dipole in atmospheric anomalies of APO. This dipole is further intensified by ENSO-related changes in winds, reconciling apparently conflicting APO observations in the tropical Pacific. These findings suggest a substantial and complex response of the oceanic O2 cycle to climate variability that is significantly (>50%) underestimated in magnitude by ocean models.

Xu, L, Cameron-Smith P, Russell LM, Ghan SJ, Liu Y, Elliott S, Yang Y, Lou S, Lamjiri MA, Manizza M.  2016.  DMS role in ENSO cycle in the tropics. Journal of Geophysical Research: Atmospheres. 121:13,537-13,558.   10.1002/2016JD025333   AbstractWebsite

We examined the multiyear mean and variability of dimethyl sulfide (DMS) and its relationship to sulfate aerosols, as well as cloud microphysical and radiative properties. We conducted a 150 year simulation using preindustrial conditions produced by the Community Earth System Model embedded with a dynamic DMS module. The model simulated the mean spatial distribution of DMS emissions and burden, as well as sulfur budgets associated with DMS, SO2, H2SO4, and sulfate that were generally similar to available observations and inventories for a variety of regions. Changes in simulated sea-to-air DMS emissions and associated atmospheric abundance, along with associated aerosols and cloud and radiative properties, were consistently dominated by El Niño–Southern Oscillation (ENSO) cycle in the tropical Pacific region. Simulated DMS, aerosols, and clouds showed a weak positive feedback on sea surface temperature. This feedback suggests a link among DMS, aerosols, clouds, and climate on interannual timescales. The variability of DMS emissions associated with ENSO was primarily caused by a higher variation in wind speed during La Niña events. The simulation results also suggest that variations in DMS emissions increase the frequency of La Niña events but do not alter ENSO variability in terms of the standard deviation of the Niño 3 sea surface temperature anomalies.

Lee, YJ, Matrai PA, Friedrichs MAM, Saba VS, Aumont O, Babin M, Buitenhuis ET, Chevallier M, de Mora L, Dessert M, Dunne JP, Ellingsen IH, Feldman D, Frouin R, Gehlen M, Gorgues T, Ilyina T, Jin MB, John JG, Lawrence J, Manizza M, Menkes CE, Perruche C, Le Fouest V, Popova EE, Romanou A, Samuelsen A, Schwinger J, Seferian R, Stock CA, Tjiputra J, Tremblay B, Ueyoshi K, Vichi M, Yool A, Zhang JL.  2016.  Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models. Journal of Geophysical Research-Oceans. 121:8635-8669.   10.1002/2016jc011993   AbstractWebsite

The relative skill of 21 regional and global biogeochemical models was assessed in terms of how well the models reproduced observed net primary productivity (NPP) and environmental variables such as nitrate concentration (NO3), mixed layer depth (MLD), euphotic layer depth (Z(eu)), and sea ice concentration, by comparing results against a newly updated, quality-controlled in situ NPP database for the Arctic Ocean (1959-2011). The models broadly captured the spatial features of integrated NPP (iNPP) on a pan-Arctic scale. Most models underestimated iNPP by varying degrees in spite of overestimating surface NO3, MLD, and Z(eu) throughout the regions. Among the models, iNPP exhibited little difference over sea ice condition (ice-free versus ice-influenced) and bottom depth (shelf versus deep ocean). The models performed relatively well for the most recent decade and toward the end of Arctic summer. In the Barents and Greenland Seas, regional model skill of surface NO3 was best associated with how well MLD was reproduced. Regionally, iNPP was relatively well simulated in the Beaufort Sea and the central Arctic Basin, where in situ NPP is low and nutrients are mostly depleted. Models performed less well at simulating iNPP in the Greenland and Chukchi Seas, despite the higher model skill in MLD and sea ice concentration, respectively. iNPP model skill was constrained by different factors in different Arctic Ocean regions. Our study suggests that better parameterization of biological and ecological microbial rates (phytoplankton growth and zooplankton grazing) are needed for improved Arctic Ocean biogeochemical modeling.

Nevison, CD, Manizza M, Keeling RF, Stephens BB, Bent JD, Dunne J, Ilyina T, Long M, Resplandy L, Tjiputra J, Yukimoto S.  2016.  Evaluating CMIP5 ocean biogeochemistry and Southern Ocean carbon uptake using atmospheric potential oxygen: Present-day performance and future projection. Geophysical Research Letters. 43:2077-2085.   10.1002/2015gl067584   AbstractWebsite

Observed seasonal cycles in atmospheric potential oxygen (APO similar to O-2+1.1 CO2) were used to evaluate eight ocean biogeochemistry models from the Coupled Model Intercomparison Project (CMIP5). Model APO seasonal cycles were computed from the CMIP5 air-sea O-2 and CO2 fluxes and compared to observations at three Southern Hemisphere monitoring sites. Four of the models captured either the observed APO seasonal amplitude or phasing relatively well, while the other four did not. Many models had an unrealistic seasonal phasing or amplitude of the CO2 flux, which in turn influenced APO. By 2100 under RCP8.5, the models projected little change in the O-2 component of APO but large changes in the seasonality of the CO2 component associated with ocean acidification. The models with poorer performance on present-day APO tended to project larger net carbon uptake in the Southern Ocean, both today and in 2100.

Le Fouest, V, Manizza M, Tremblay B, Babin M.  2015.  Modelling the impact of riverine DON removal by marine bacterioplankton on primary production in the Arctic Ocean. Biogeosciences. 12:3385-3402.   10.5194/bg-12-3385-2015   AbstractWebsite

The planktonic and biogeochemical dynamics of the Arctic shelves exhibit a strong variability in response to Arctic warming. In this study, we employ a biogeochemical model coupled to a pan-Arctic ocean-sea ice model (MIT- gcm) to elucidate the processes regulating the primary production (PP) of phytoplankton, bacterioplankton (BP), and their interactions. The model explicitly simulates and quantifies the contribution of usable dissolved organic nitrogen (DON) drained by the major circum-Arctic rivers to PP and BP in a scenario of melting sea ice (1998-2011). Model simulations suggest that, on average between 1998 and 2011, the removal of usable riverine dissolved organic nitrogen (RDON) by bacterioplankton is responsible for a similar to 26% increase in the annual BP for the whole Arctic Ocean. With respect to total PP, the model simulates an increase of similar to 8% on an annual basis and of similar to 18% in summer. Recycled ammonium is responsible for the PP increase. The recycling of RDON by bacterioplankton promotes higher BP and PP, but there is no significant temporal trend in the BP : PP ratio within the ice-free shelves over the 1998-2011 period. This suggests no significant evolution in the balance between autotrophy and heterotrophy in the last decade, with a constant annual flux of RDON into the coastal ocean, although changes in RDON supply and further reduction in sea-ice cover could potentially alter this delicate balance.

Cassar, N, Nevison CD, Manizza M.  2014.  Correcting oceanic O-2/Ar-net community production estimates for vertical mixing using N2O observations. Geophysical Research Letters. 41:8961-8970.   10.1002/2014gl062040   AbstractWebsite

The O-2/Ar approach has become a key method to estimate oceanic net community production (NCP). However, in some seasons and regions of the ocean, strong vertical mixing of O-2-depleted deepwater introduces a large error into O-2/Ar-derived NCP estimates. In these cases, undersaturated-O-2/Ar observations have for all intents and purposes been ignored. We propose to combine underway O-2/Ar and N2O observations into a composite tracer that is conservative with respect to the influence of vertical mixing on the surface biological O-2 inventory. We test the proposed method with an ocean observing system simulation experiment (OSSE) in which we compare N2O-O-2/Ar and O-2/Ar-only gas flux estimates of NCP to the model-simulated true NCP in the Southern Ocean. Our proof-of-concept simulations show that the N2O-O-2/Ar tracer significantly improves NCP estimates when/where vertical mixing is important.

Manizza, M, Follows MJ, Dutkiewicz S, Menemenlis D, Hill CN, Key RM.  2013.  Changes in the Arctic Ocean CO2 sink (1996-2007): A regional model analysis. Global Biogeochemical Cycles. 27:1108-1118.   10.1002/2012gb004491   AbstractWebsite

The rapid recent decline of Arctic Ocean sea ice area increases the flux of solar radiation available for primary production and the area of open water for air-sea gas exchange. We use a regional physical-biogeochemical model of the Arctic Ocean, forced by the National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis, to evaluate the mean present-day CO2 sink and its temporal evolution. During the 1996-2007 period, the model suggests that the Arctic average sea surface temperature warmed by 0.04 degrees Ca-1, that sea ice area decreased by approximate to 0.1 x 10(6)km(2)a(-1), and that the biological drawdown of dissolved inorganic carbon increased. The simulated 1996-2007 time-mean Arctic Ocean CO2 sink is 586TgCa(-1). The increase in ice-free ocean area and consequent carbon drawdown during this period enhances the CO2 sink by approximate to 1.4TgCa(-1), consistent with estimates based on extrapolations of sparse data. A regional analysis suggests that during the 1996-2007 period, the shelf regions of the Laptev, East Siberian, Chukchi, and Beaufort Seas experienced an increase in the efficiency of their biological pump due to decreased sea ice area, especially during the 2004-2007 period, consistent with independently published estimates of primary production. In contrast, the CO2 sink in the Barents Sea is reduced during the 2004-2007 period due to a dominant control by warming and decreasing solubility. Thus, the effect of decreasing sea ice area and increasing sea surface temperature partially cancel, though the former is dominant.

Manizza, M, Follows MJ, Dutkiewicz S, Menemenlis D, Hill CN, Key RM.  2013.  Changes in the Arctic Ocean CO2 sink (1996–2007): A regional model analysis. Global Biogeochemical Cycles. :n/a-n/a.   10.1002/2012GB004491   AbstractWebsite

The rapid recent decline of Arctic Ocean sea ice area increases the flux of solar radiation available for primary production and the area of open water for air-sea gas exchange. We use a regional physical-biogeochemical model of the Arctic Ocean, forced by the National Centers for Environmental Prediction/National Center for Atmospheric Research atmospheric reanalysis, to evaluate the mean present-day CO2 sink and its temporal evolution. During the 1996–2007 period, the model suggests that the Arctic average sea surface temperature warmed by 0.04°C a−1, that sea ice area decreased by ∼0.1 × 106 km2 a−1, and that the biological drawdown of dissolved inorganic carbon increased. The simulated 1996–2007 time-mean Arctic Ocean CO2 sink is 58 ± 6 Tg C a−1. The increase in ice-free ocean area and consequent carbon drawdown during this period enhances the CO2 sink by ∼1.4 Tg C a−1, consistent with estimates based on extrapolations of sparse data. A regional analysis suggests that during the 1996–2007 period, the shelf regions of the Laptev, East Siberian, Chukchi, and Beaufort Seas experienced an increase in the efficiency of their biological pump due to decreased sea ice area, especially during the 2004–2007 period, consistent with independently published estimates of primary production. In contrast, the CO2 sink in the Barents Sea is reduced during the 2004–2007 period due to a dominant control by warming and decreasing solubility. Thus, the effect of decreasing sea ice area and increasing sea surface temperature partially cancel, though the former is dominant.

Schuster, U, McKinley GA, Bates N, Chevallier F, Doney SC, Fay AR, Gonzalez-Davila M, Gruber N, Jones S, Krijnen J, Landschutzer P, Lefevre N, Manizza M, Mathis J, Metzl N, Olsen A, Rios AF, Rodenbeck C, Santana-Casiano JM, Takahashi T, Wanninkhof R, Watson AJ.  2013.  An assessment of the Atlantic and Arctic sea-air CO2 fluxes, 1990-2009. Biogeosciences. 10:607-627. AbstractWebsite

The Atlantic and Arctic Oceans are critical components of the global carbon cycle. Here we quantify the net sea-air CO2 flux, for the first time, across different methodologies for consistent time and space scales for the Atlantic and Arctic basins. We present the long-term mean, seasonal cycle, interannual variability and trends in sea-air CO2 flux for the period 1990 to 2009, and assign an uncertainty to each. We use regional cuts from global observations and modeling products, specifically a pCO(2)-based CO2 flux climatology, flux estimates from the inversion of oceanic and atmospheric data, and results from six ocean biogeochemical models. Additionally, we use basin-wide flux estimates from surface ocean pCO(2) observations based on two distinct methodologies. Our estimate of the contemporary sea-air flux of CO2 (sum of anthropogenic and natural components) by the Atlantic between 40 degrees S and 79 degrees N is -0.49 +/- 0.05 Pg C yr(-1), and by the Arctic it is -0.!

Nevison, CD, Keeling RF, Kahru M, Manizza M, Mitchell BG, Cassar N.  2012.  Estimating net community production in the Southern Ocean based on atmospheric potential oxygen and satellite ocean color data. Global Biogeochemical Cycles. 26   10.1029/2011gb004040   AbstractWebsite

The seasonal cycle of atmospheric potential oxygen (APO similar to O-2 + 1.1 CO2) reflects three seasonally varying ocean processes: 1) thermal in- and outgassing, 2) mixed layer net community production (NCP) and 3) deep water ventilation. Previous studies have isolated the net biological seasonal signal (i.e., the sum of NCP and ventilation), after using air-sea heat flux data to estimate the thermal signal. In this study, we resolve all three components of the APO seasonal cycle using a methodology in which the ventilation signal is estimated based on atmospheric N2O data, the thermal signal is estimated based on heat flux or atmospheric Ar/N-2 data, and the production signal is inferred as a residual. The isolation of the NCP signal in APO allows for direct comparison to estimates of NCP based on satellite ocean color data, after translating the latter into an atmospheric signal using an atmospheric transport model. When applied to ocean color data using algorithms specially adapted to the Southern Ocean and APO data at three southern monitoring sites, these two independent methods converge on a similar phase and amplitude of the seasonal NCP signal in APO and yield an estimate of annual mean NCP south of 50 degrees S of 0.8-1.2 Pg C/yr, with corresponding annual mean NPP of similar to 3 Pg C/yr and a mean growing season f ratio of similar to 0.33. These results are supported by ocean biogeochemistry model simulations, in which air-sea O-2 and N2O fluxes are resolved into component thermal, ventilation and (for O-2) NCP contributions.

Tank, SE, Manizza M, Holmes RM, McClelland JW, Peterson BJ.  2012.  The Processing and Impact of Dissolved Riverine Nitrogen in the Arctic Ocean. Estuaries and Coasts. 35:401-415.   10.1007/s12237-011-9417-3   AbstractWebsite

Although the Arctic Ocean is the most riverine-influenced of all of the world's oceans, the importance of terrigenous nutrients in this environment is poorly understood. This study couples estimates of circumpolar riverine nutrient fluxes from the PARTNERS (Pan-Arctic River Transport of Nutrients, Organic Matter, and Suspended Sediments) Project with a regionally configured version of the MIT general circulation model to develop estimates of the distribution and availability of dissolved riverine N in the Arctic Ocean, assess its importance for primary production, and compare these estimates to potential bacterial production fueled by riverine C. Because riverine dissolved organic nitrogen is remineralized slowly, riverine N is available for uptake well into the open ocean. Despite this, we estimate that even when recycling is considered, riverine N may support 0.5-1.5 Tmol C year(-1) of primary production, a small proportion of total Arctic Ocean photosynthesis. Rapid uptake of dissolved inorganic nitrogen coupled with relatively high rates of dissolved organic nitrogen regeneration in N-limited nearshore regions, however, leads to potential localized rates of riverine-supported photosynthesis that represent a substantial proportion of nearshore production.

Manizza, M, Keeling RF, Nevison CD.  2012.  On the processes controlling the seasonal cycles of the air-sea fluxes of O2 and N2O: A modelling study. Tellus Series B-Chemical and Physical Meteorology. 64   10.3402/tellusb.v64i0.18429   AbstractWebsite

The seasonal dynamics of the air-sea gas flux of oxygen (O-2) are controlled by multiple processes occurring simultaneously. Previous studies showed how to separate the thermal component from the total O-2 flux to quantify the residual oxygen flux due to biological processes. However, this biological signal includes the effect of both net euphotic zone production (NEZP) and subsurface water ventilation. To help understand and separate these two components, we use a large-scale ocean general circulation model (OGCM), globally configured, and coupled to a biogeochemical model. The combined model implements not only the oceanic cycle of O-2 but also the cycles of nitrous oxide (N2O), argon (Ar) and nitrogen (N-2). For this study, we apply a technique to distinguish the fluxes of O-2 driven separately by thermal forcing, NEZP, and address the role of ocean ventilation by carrying separate O-2 components in the model driven by solubility, NEZP and ventilation. Model results show that the ventilation component can be neglected in summer compared to the production and thermal components polewards but not equatorward of 30 degrees in each hemisphere. This also implies that neglecting the role of ventilation in the subtropical areas would lead to overestimation of the component of O-2 flux due to NEZP by 20-30%. Model results also show that the ventilation components of air-sea O-2 and N2O fluxes are strongly anti-correlated in a ratio that reflects the subsurface tracer/tracer relationships (similar to 0.1 mmol N2O/mol O-2) as derived from observations. The results support the use of simple scaling relationships linking together the thermally driven fluxes of Ar, N-2 and O-2. Furthermore, our study also shows that for latitudes polewards of 30 degrees of both hemispheres, the Garcia and Keeling (2001) climatology, when compared to our model results, has a phasing error with the fluxes being too early by similar to 2-3 weeks.

Manizza, M, Follows MJ, Dutkiewicz S, Menemenlis D, McClelland JW, Hill CN, Peterson BJ, Key RM.  2011.  A model of the Arctic Ocean carbon cycle. Journal of Geophysical Research-Oceans. 116   10.1029/2011jc006998   AbstractWebsite

A three dimensional model of Arctic Ocean circulation and mixing, with a horizontal resolution of 18 km, is overlain by a biogeochemical model resolving the physical, chemical and biological transport and transformations of phosphorus, alkalinity, oxygen and carbon, including the air-sea exchange of dissolved gases and the riverine delivery of dissolved organic carbon. The model qualitatively captures the observed regional and seasonal trends in surface ocean PO(4), dissolved inorganic carbon, total alkalinity, and pCO(2). Integrated annually, over the basin, the model suggests a net annual uptake of 59 Tg C a(-1), within the range of published estimates based on the extrapolation of local observations (20-199 Tg C a(-1)). This flux is attributable to the cooling (increasing solubility) of waters moving into the basin, mainly from the subpolar North Atlantic. The air-sea flux is regulated seasonally and regionally by sea-ice cover, which modulates both air-sea gas transfer and the photosynthetic production of organic matter, and by the delivery of riverine dissolved organic carbon (RDOC), which drive the regional contrasts in pCO(2) between Eurasian and North American coastal waters. Integrated over the basin, the delivery and remineralization of RDOC reduces the net oceanic CO(2) uptake by similar to 10%.

McGuire, AD, Hayes DJ, Kicklighter DW, Manizza M, Zhuang Q, Chen M, Follows MJ, Gurney KR, McClelland JW, Melillo JM, Peterson BJ, Prinn RG.  2010.  An analysis of the carbon balance of the Arctic Basin from 1997 to 2006. Tellus Series B-Chemical and Physical Meteorology. 62:455-474.   10.1111/j.1600-0889.2010.00497.x   AbstractWebsite

This study used several model-based tools to analyse the dynamics of the Arctic Basin between 1997 and 2006 as a linked system of land-ocean-atmosphere C exchange. The analysis estimates that terrestrial areas of the Arctic Basin lost 62.9 Tg C yr-1 and that the Arctic Ocean gained 94.1 Tg C yr-1. Arctic lands and oceans were a net CO(2) sink of 108.9 Tg C yr-1, which is within the range of uncertainty in estimates from atmospheric inversions. Although both lands and oceans of the Arctic were estimated to be CO(2) sinks, the land sink diminished in strength because of increased fire disturbance compared to previous decades, while the ocean sink increased in strength because of increased biological pump activity associated with reduced sea ice cover. Terrestrial areas of the Arctic were a net source of 41.5 Tg CH(4) yr-1 that increased by 0.6 Tg CH(4) yr-1 during the decade of analysis, a magnitude that is comparable with an atmospheric inversion of CH(4). Because the radiative forcing of the estimated CH(4) emissions is much greater than the CO(2) sink, the analysis suggests that the Arctic Basin is a substantial net source of green house gas forcing to the climate system.

Manizza, M, Buitenhuis ET, Le Quere C.  2010.  Sensitivity of global ocean biogeochemical dynamics to ecosystem structure in a future climate. Geophysical Research Letters. 37   10.1029/2010gl043360   AbstractWebsite

Terrestrial and oceanic ecosystem components of the Earth System models (ESMs) are key to predict the future behavior of the global carbon cycle. Ocean ecosystem models represent low complexity compared to terrestrial ecosystem models. In this study we use two ocean biogeochemical models based on the explicit representation of multiple planktonic functional types. We impose to the models the same future physical perturbation and compare the response of ecosystem dynamics, export production (EP) and ocean carbon uptake (OCU) to the same physical changes. Models comparison shows that: (1) EP changes directly translate into changes of OCU on decadal time scale, (2) the representation of ecosystem structure plays a pivotal role at linking OCU and EP, (3) OCU is highly sensitive to representation of ecosystem in the Equatorial Pacific and Southern Oceans. Citation: Manizza, M., E. T. Buitenhuis, and C. Le Quere (2010), Sensitivity of global ocean biogeochemical dynamics to ecosystem structure in a future climate, Geophys. Res. Lett., 37, L13607, doi: 10.1029/2010GL043360.

Manizza, M, Follows MJ, Dutkiewicz S, McClelland JW, Menemenlis D, Hill CN, Townsend-Small A, Peterson BJ.  2009.  Modeling transport and fate of riverine dissolved organic carbon in the Arctic Ocean. Global Biogeochemical Cycles. 23   10.1029/2008gb003396   AbstractWebsite

The spatial distribution and fate of riverine dissolved organic carbon ( DOC) in the Arctic may be significant for the regional carbon cycle but are difficult to fully characterize using the sparse observations alone. Numerical models of the circulation and biogeochemical cycles of the region can help to interpret and extrapolate the data and may ultimately be applied in global change sensitivity studies. Here we develop and explore a regional, three-dimensional model of the Arctic Ocean in which, for the first time, we explicitly represent the sources of riverine DOC with seasonal discharge based on climatological field estimates. Through a suite of numerical experiments, we explore the distribution of DOC-like tracers with realistic riverine sources and a simple linear decay to represent remineralization through microbial degradation. The model reproduces the slope of the DOC-salinity relationship observed in the eastern and western Arctic basins when the DOC tracer lifetime is about 10 years, consistent with published inferences from field data. The new empirical parameterization of riverine DOC and the regional circulation and biogeochemical model provide new tools for application in both regional and global change studies.

Manizza, M, Le Quere C, Watson AJ, Buitenhuis ET.  2008.  Ocean biogeochemical response to phytoplankton-light feedback in a global model. Journal of Geophysical Research-Oceans. 113   10.1029/2007jc004478   AbstractWebsite

Oceanic phytoplankton, absorbing solar radiation, can influence the bio-optical properties of seawater and hence upper ocean physics. We include this process in a global ocean general circulation model (OGCM) coupled to a dynamic green ocean model (DGOM) based on multiple plankton functional types (PFT). We not only study the impact of this process on ocean physics but we also explore the biogeochemical response due to this biophysical feedback. The phytoplankton-light feedback (PLF) impacts the dynamics of the upper tropical and subtropical oceans. The change in circulation enhances both the vertical supply in the tropics and the lateral supply of nutrients from the tropics to the subtropics boosting the subtropical productivity by up to 60 gC m(-2) a(-1). Physical changes, due to the PLF, impact on light and nutrient availability causing shifts in the ocean ecosystems. In the extratropics, increased stratification favors calcifiers (by up to similar to 8%) at the expense of mixed phytoplankton. In the Southern Ocean, silicifiers increase their biomass (by up to similar to 10%) because of the combined alleviation of iron and light limitation. The PLF has a small effect globally on air-sea fluxes of carbon dioxide (CO(2), 72 TmolC a(-1) outgassing) and oxygen (O(2), 46 TmolO(2) a(-1) ingassing) because changes in biogeochemical processes (primary production, biogenic calcification, and export production) highly vary regionally and can also oppose each other. From our study it emerges that the main impact of the PLF is an amplification of the seasonal cycle of physical and biogeochemical properties of the high-latitude oceans mostly driven by the amplification of the SST seasonal cycle.

Vallina, SM, Simo R, Manizza M.  2007.  Weak response of oceanic dimethylsulfide to upper mixing shoaling induced by global warming. Proceedings of the National Academy of Sciences of the United States of America. 104:16004-16009.   10.1073/pnas.0700843104   AbstractWebsite

The solar radiation dose in the oceanic upper mixed layer (SRD) has recently been identified as the main climatic force driving global dimethylsulfide (DMS) dynamics and seasonality. Because DMS is suggested to exert a cooling effect on the earth radiative budget through its involvement in the formation and optical properties of tropospheric clouds over the ocean, a positive relationship between DMS and the SRD supports the occurrence of a negative feedback between the oceanic biosphere and climate, as postulated 20 years ago. Such a natural feedback might partly counteract anthropogenic global warming through a shoaling of the mixed layer depth (MILD) and a consequent increase of the SRD and DMS concentrations and emission. By applying two globally derived DMS diagnostic models to global fields of MILD and chlorophyll simulated with an Ocean General Circulation Model coupled to a biogeochemistry model for a 50% increase of atmospheric CO2 and an unperturbed control run, we have estimated the response of the DMS-producing pelagic ocean to global warming. our results show a net global increase in surface DMS concentrations, especially in summer. This increase, however, is so weak (globally 1.2%) that it can hardly be relevant as compared with the radiative forcing of the increase of greenhouse gases. This contrasts with the seasonal variability of DMS (1000-2000% summer-to-winter ratio). We suggest that the "plankton-DMS-clouds-earth albedo feedback" hypothesis is less strong a long-term thermostatic system than a seasonal mechanism that contributes to regulate the solar radiation doses reaching the earth's biosphere.

Le Quere, C, Harrison SP, Prentice IC, Buitenhuis ET, Aumont O, Bopp L, Claustre H, Da Cunha LC, Geider R, Giraud X, Klaas C, Kohfeld KE, Legendre L, Manizza M, Platt T, Rivkin RB, Sathyendranath S, Uitz J, Watson AJ, Wolf-Gladrow D.  2005.  Ecosystem dynamics based on plankton functional types for global ocean biogeochemistry models. Global Change Biology. 11:2016-2040.   10.1111/j.1365-2468.2005.01004.x   AbstractWebsite

Ecosystem processes are important determinants of the biogeochemistry of the ocean, and they can be profoundly affected by changes in climate. Ocean models currently express ecosystem processes through empirically derived parameterizations that tightly link key geochemical tracers to ocean physics. The explicit inclusion of ecosystem processes in models will permit ecological changes to be taken into account, and will allow us to address several important questions, including the causes of observed glacial-interglacial changes in atmospheric trace gases and aerosols, and how the oceanic uptake of CO(2) is likely to change in the future. There is an urgent need to assess our mechanistic understanding of the environmental factors that exert control over marine ecosystems, and to represent their natural complexity based on theoretical understanding. We present a prototype design for a Dynamic Green Ocean Model (DGOM) based on the identification of (a) key plankton functional types that need to be simulated explicitly to capture important biogeochemical processes in the ocean; (b) key processes controlling the growth and mortality of these functional types and hence their interactions; and (c) sources of information necessary to parameterize each of these processes within a modeling framework. We also develop a strategy for model evaluation, based on simulation of both past and present mean state and variability, and identify potential sources of validation data for each. Finally, we present a DGOM-based strategy for addressing key questions in ocean biogeochemistry. This paper thus presents ongoing work in ocean biogeochemical modeling, which, it is hoped will motivate international collaborations to improve our understanding of the role of the ocean in the climate system.

Manizza, M, Le Quere C, Watson AJ, Buitenhuis ET.  2005.  Bio-optical feedbacks among phytoplankton, upper ocean physics and sea-ice in a global model. Geophysical Research Letters. 32   10.1029/2004gl020778   AbstractWebsite

Phytoplankton biomass modifies the penetration of light and impacts the physical properties of the upper ocean. We quantify these impacts and the feedbacks on phytoplankton biomass for the global ocean using an Ocean General Circulation Model coupled to an ocean biogeochemistry model. Phytoplankton biomass amplifies the seasonal cycle of temperature, mixed layer depth and ice cover by roughly 10%. At mid and high latitudes, surface temperature warms by 0.1 - 1.5 degrees C in spring/ summer and cools by 0.1 - 0.3 degrees C in fall/ winter. In the tropics, phytoplankton biomass indirectly cools the ocean surface by 0.3 degrees C due to enhanced upwelling. The mixed layer stratifies by 4 - 30 m everywhere except at high latitudes. At high latitudes, the sea- ice cover is reduced by up to 6% in summer and increased by 2% in winter, leading to further feedbacks on vertical mixing and heat fluxes. Physical changes drive a positive feedback increasing phytoplankton biomass by 4 - 12% and further amplifies the initial physical perturbations.