Arne Winguth

Atmospheric pCO2 sensitivity to the biological pump in the ocean

In models of the global carbon cycle, the pCO2of the atmosphere is more sensitive to the chemistry of the high-latitude surface ocean than the tropical ocean. Because sea-surface nutrient concentrations are generally high in the high latitudes,pCO2sensitivity to high-latitude forcing also determinespCO2sensitivity to the biological pump globally. We diagnose high-latitude sensitivity of a range of ocean models using atmosphericpCO2above an abiotic ocean; cold high-latitude waters pull abioticpCO2to low values. Box models are very high-latitude sensitive, while most global circulation models are considerably less so, including a two-dimensional overturning model, two primitive equation models, the Hamburg class of large scale geostrophic (LSG) general circulation models (GCMs), and the MICOM isopycnic GCM. High-latitude forcing becomes more important in a depth-coordinate GCM when lateral diffusion is oriented along isopycnal surfaces, rather than horizontally, followingRedi [1982]. In two different GCMs (a primitive equation model and LSG), addition of the Gent and McWillams[1990] isopycnal thickness diffusion scheme had only minor impact on high-latitude sensitivity. Using a simplified box model, we show that high-latitude sensitivity depends on a high-latitude monopoly on deep water formation. In an attempt to bridge the gap between box models and GCMs, we constructed a simple slab overturning model with an imposed stream function which can be discretized at arbitrary resolution from box model to GCM scale. High-latitude sensitivity is independent of model resolution but very sensitive to vertical diffusion. Diffusion acts to break the high-latitude monopoly, decreasing high-latitude sensitivity. In the isopycnal GCM MICOM, however, high-latitude sensitivity is relatively insensitive to diapycnal diffusion of tracers such as CO2. This would imply that flow pathways in MICOM take the place of vertical diffusion in the slab model. The two nominally most sophisticated ocean models in the comparison are the isopycnal model MICOM and the depth-coordinate GCM withRedi [1982] and Gent and McWilliams [1990] mixing. Unfortunately, these two models disagree in their abiotic CO2behavior; the depth-coordinate isopycnal mixing GCM is high-latitude sensitive, in accord with box models, while MICOM is less so. The rest of the GCMs, which have historically seen the most use in geochemical studies, are even less high-latitude sensitive than MICOM. This discrepancy needs to be resolved. In the meantime, the implication of the MICOM/traditional GCM result would be that box models overestimate high-latitude sensitivity of the real ocean. This would eliminate iron dust fertilization of the ocean as an explanation for the glacialpCO2 range of 180–200 μatm [Archer et al., 2000].

El Niño-Southern Oscillation related fluctuations of the marine carbon cycle

We investigate the response of a three-dimensional ocean circulation model (Hamburg LSG) coupled on-line with an oceanic carbon cycle model (HAMOCC-3) to El Nino-Southern Oscillation (ENSO) induced fluctuations of the wind field. During El Nino 1982/1983, when upwelling and biological productivity in the equatorial Pacific were strongly reduced and sea surface temperatures were increased, the oceanic CO2 partial pressure in this region decreased significantly. Consequently, in 1982/1983 the CO2 flux from the tropical ocean into the atmosphere was reduced. However, in 1983 the interannual deviations from the long-term trend in atmospheric CO2 showed in January low and in December high values with a total shift by more then 1.4 GtC. The model simulation supports the oceanic measurements and predicts a temporary uptake of 0.6 GtC during the ENSO year 1983. We conclude that the concurrent release of CO2 from the land biosphere must have been about 2 GtC.

Global decline in ocean ventilation, oxygenation, and productivity during the Paleocene-Eocene Thermal Maximum: Implications for the benthic extinction

The prominent global warming event at the Paleocene-Eocene boundary (55 Ma), referred to as the Paleocene-Eocene Thermal Maximum (PETM), was characterized by rapid temperature increase and changes in the global carbon cycle in

Simulating Permian–Triassic oceanic anoxia distribution: Implications for species extinction and recovery

The biggest mass extinction in the Phanerozoic, at the end of the Permian, has been associ- ated with oceanic changes, but the exact dynamics are still debated. Intensifi ed stratifi cation, widespread anoxia, chemocline excursions, and large-scale ocean overturn events have all been invoked as contributors to the extinction. In this study the effects of possible changes in environmental conditions, such as an increase in nutrient input or dust fl uxes into the ocean or an intensifi cation of the biological pump, on Permian-Triassic ocean chemistry are inves- tigated. Series of sensitivity experiments were performed with a fully coupled climate-carbon cycle model. None of the forcings alone generates extensive low-oxygen conditions in the deep sea. These are only simulated by an intense eutrophication in combination with an enhanced biological pump, but still confi ned to the central Panthalassic, Tethys, and the eastern Boreal Oceans. Our fi ndings support the conclusions from a recent geochemical study of a Japanese deep Panthalassa section, that around the Permian-Triassic boundary, the oxygen minimum zone expanded considerably, while the deep Panthalassa remained ventilated. The warming- induced increase in low-oxygen conditions within the water column aggravated adverse exist- ing conditions and likely contributed to the extinction peak. Upwelling of toxic water was probably regionally confi ned and hence not the main cause for the end-Permian marine and terrestrial mass extinction. Widespread deep-sea anoxia, generated by a strong increase in weathering and the related enhanced nutrient input into the oceans, is probably closely linked to the delayed recovery of species in the Early Triassic.

Assimilation of seasonal chlorophyll and nutrient data into an adjoint three-dimensional ocean carbon cycle model : Sensitivity analysis and ecosystem parameter optimization

[1] An adjoint method is applied to a three-dimensional global ocean biogeochemical cycle model to optimize the ecosystem parameters on the basis of SeaWiFS surface chlorophyll observation. We showed with identical twin experiments that the model simulated chlorophyll concentration is sensitive to perturbation of phytoplankton and zooplankton exudation, herbivore egestion as fecal pellets, zooplankton grazing, and the assimilation efficiency parameters. The assimilation of SeaWiFS chlorophyll data significantly improved the prediction of chlorophyll concentration, especially in the high-latitude regions. Experiments that considered regional variations of parameters yielded a high seasonal variance of ecosystem parameters in the high latitudes, but a low variance in the tropical regions. These experiments indicate that the adjoint model is, despite the many uncertainties, generally capable to optimize sensitive parameters and carbon fluxes in the euphotic zone. The best fit regional parameters predict a global net primary production of 36 Pg C yr -1 , which lies within the range suggested by Antoine et al. (1996). Additional constraints of nutrient data from the World Ocean Atlas showed further reduction in the model-data misfit and that assimilation with extensive data sets is necessary.

I/Ca evidence for upper ocean deoxygenation during the PETM

Anthropogenic global warming affects marine ecosystems in complex ways, and declining ocean oxygenation is a growing concern. Forecasting the geographical and bathymetric extent, rate, and intensity of future deoxygenation and its effects on oceanic biota, however, remains highly challenging because of the complex feedbacks in the Earth-ocean biota system. Information on past global warming events such as the Paleocene-Eocene Thermal Maximum (PETM, ~55.5 Ma), a potential analog for present and future global warming, may help in such forecasting. Documenting past ocean deoxygenation, however, is hampered by the lack of sensitive proxies for past oceanic oxygen levels throughout the water column. As yet no evidence has been presented for pervasive deoxygenation in the upper water column through expansion of oxygen minimum zones (OMZs). We apply a novel proxy for paleoredox conditions, the iodine to calcium ratio (I/Ca) in bulk coarse fraction sediment and planktonic foraminiferal tests from pelagic sites in different oceans, and compared our reconstruction with modeled oxygen levels. The reconstructed iodate gradients indicate that deoxygenation occurred in the upper water column in the Atlantic, Indian Oceans, and possibly the Pacific Ocean, as well during the PETM, due to vertical and potentially lateral expansion of OMZs.

A satellite view of internal waves induced by the Indian Ocean tsunami

At 08:45 local time (02:45 GMT) on 26 December 2004, 1 h and 45 min after the Sumatra Earthquakes of magnitude 9.3, a devastating tsunami struck the east coast of Sri Lanka. Nearly 2 h and 30 min after the wave hit the coast, a weather satellite passed over Sri Lankas coastal zone providing a rare glimpse of internal waves along the continental slope due to this tsunami. The satellite imagery indicates wave‐like features from the tsunami being reflected, diffracted, and scattered off the steep continental slope and submarine canyons adjacent to Sri Lanka. The energetic wave and its modification to internal waves possibly eroded sediment from the sea floor and transported it to the sea surface. Solitary features generated by internal waves can explain the observed pattern. Future modelling approaches considering these nonlinear interactions would be required for a better understanding of the tsunami behaviour in the coastal zone, where its destructive effects are most prominent.

Expanded oxygen minimum zones during the late Paleocene‐early Eocene: Hints from multiproxy comparison and ocean modeling

Anthropogenic warming could well drive depletion of oceanic oxygen in the future. Important insight into the relationship between deoxygenation and warming can be gleaned from the geological record, but evidence is limited because few ocean oxygenation records are available for past greenhouse climate conditions. We use I/Ca in benthic foraminifera to reconstruct late Paleocene through early Eocene bottom and pore water redox conditions in the South Atlantic and Southern Indian Oceans and compare our results with those derived from Mn speciation and the Ce anomaly in fish teeth. We conclude that waters with lower oxygen concentrations were widespread at intermediate depths (1.5–2 km), whereas bottom waters were more oxygenated at the deepest site, in the Southeast Atlantic Ocean (>3 km). Epifaunal benthic foraminiferal I/Ca values were higher in the late Paleocene, especially at low-oxygen sites, than at well-oxygenated modern sites, indicating higher seawater total iodine concentrations in the late Paleocene than today. The proxy-based bottom water oxygenation pattern agrees with the site-to-site O2 gradient as simulated in a comprehensive climate model (Community Climate System Model Version 3), but the simulated absolute dissolved O2 values are low (< ~35 µmol/kg), while higher O2 values (~60–100 µmol/kg) were obtained in an Earth system model (Grid ENabled Integrated Earth system model). Multiproxy data together with improvements in boundary conditions and model parameterization are necessary if the details of past oceanographic oxygenation are to be resolved.

The Urban Heat Island of the North-Central Texas Region and Its Relation to the 2011 Severe Texas Drought

AbstractHourly surface temperature differences between Dallas–Fort Worth, Texas, metropolitan and rural sites have been used to calculate the urban heat island from 2001 to 2011. The heat island peaked after sunset and was particularly strong during the drought and heat wave in July 2011, reaching a single-day instantaneous maximum value of 5.4°C and a monthly mean maximum of 3.4°C, as compared with the 2001–11 July average of 2.4°C. This severe drought caused faster warming of rural locations relative to the metropolitan area in the morning as a result of lower soil moisture content, which led to an average negative heat island in July 2011 of −2.3°C at 1100 central standard time. The ground-based assessment of canopy air temperature at screening level has been supported by a remotely sensed surface estimate from the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the Terra satellite, highlighting a dual-peak maximum heat island in the major city centers of Dallas and Fort Worth. Both grou...

Vegetation-climate feedbacks in transient simulations over the last interglacial (128 000 - 113 000 yr BP)

Abstract The presently developed MPI/UW 3D — Earth system model for long-term integrations is applied to simulate the climate of the last interglacial. The model consists of an atmospheric and oceanic general circulation model, a dynamical terrestrial vegetation model and a marine carbon cycle model. The model was forced with time-varying insolation from 129000 to 113000 years before present (yr BP), revealing substantial feedbacks from the land biosphere on climate. These turned out to be important both for the simulated temperature in high northern latitudes and for the precipitation in the northern hemisphere monsoon belt. During the Eemian warm period, the simulated boreal forest extends in many places till the Arctic Ocean, and during the following cold period the transition between tundra and taiga migrated further south. Furthermore, associated albedo changes strongly amplify the simulated temperature changes. The intensified summer insolation during the Eemian leads to a higher precipitation over continents in the northern hemisphere. The strongest response is seen in the tropics and the African-Asian monsoon belt due to increased land-sea temperature contrasts. Vegetation is established in the Western Sahara desert. Compared to simulations with land vegetation prescribed at presentday pattern, the amount of precipitation in the Sahara is more than twice as large. The simulations show strong impacts on the time-transient climate response by triggering nonlinear delays and accelerations seen in various atmospheric and oceanic temperature time series. The simulated storage of carbon in the terrestrial biosphere is relatively large. The carbon storage in land vegetation is increased by more than 10% during the Eemian compared to the following cold period. The associated changes in storage in soil and litter account for more than 100 GtC.