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Ramesh K Reddy - One of the best experts on this subject based on the ideXlab platform.
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effects of Freshwater Input on trace element pollution in salt marsh soils of a typical coastal estuary china
Journal of Hydrology, 2015Co-Authors: Junhong Bai, Qingqing Zhao, Junjing Wang, Ramesh K ReddyAbstract:Summary Freshwater Input is an important pathway for the restoration of degraded coastal wetlands, however, little information is available on the negative effects of Freshwater Inputs on salt marsh soils in restored wetlands. Soil profile samples to a depth of 70 cm were collected in both degraded wetland (DW) and Freshwater restored wetland (RW) in the Yellow River Delta of China to analyze the trace element pollution effects of Freshwater Input on coastal wetland soils. Heavy metals (i.e. Cd, Cr, Cu, Ni, Pb and Zn) and arsenic (As) concentrations were determined using the inductively coupled plasma atomic absorption spectrometry to investigate their distributions, sources and ecotoxicity in marsh soils from both wetlands. Our results showed that these trace elements had moderate spatial variability in both DW and RW soils. The concentrations of As, Cr, Pb and Cd in all soil layers were generally higher in RW soils than those in DW soils (p
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effects of Freshwater Input on trace element pollution in salt marsh soils of a typical coastal estuary china
Journal of Hydrology, 2015Co-Authors: Qingqing Zhao, Junjing Wang, Qiongqiong Lu, Ramesh K ReddyAbstract:summary Freshwater Input is an important pathway for the restoration of degraded coastal wetlands, however, little information is available on the negative effects of Freshwater Inputs on salt marsh soils in restored wetlands. Soil profile samples to a depth of 70 cm were collected in both degraded wetland (DW) and Freshwater restored wetland (RW) in the Yellow River Delta of China to analyze the trace element pollution effects of Freshwater Input on coastal wetland soils. Heavy metals (i.e. Cd, Cr, Cu, Ni, Pb and Zn) and arsenic (As) concentrations were determined using the inductively coupled plasma atomic absorption spectrometry to investigate their distributions, sources and ecotoxicity in marsh soils from both wetlands. Our results showed that these trace elements had moderate spatial variability in both DW and RW soils. The concentrations of As, Cr, Pb and Cd in all soil layers were generally higher in RW soils than those in DW soils (p < 0.05), whereas the concentrations of Zn and Cu were higher in DW soils. Heavy metals had similar source in both wetlands, however, As and Zn in DW or As, Zn and Ni in RW might have another similar origin. The enrichment factor (EF) values for Cu, Ni and Pb in both wetlands indicated minimal enrichment levels, whereas both As and Cd were significantly enriched with EF values 3 or 6 times greater than 1.5, implying a significant natural or anthropogenic origin. As and Ni exceeded the effect range low (ERL) and threshold effect level (TEL) in both wetlands, even As exceeded the probable effect level (PEL) in RW soils. Cr, Cu and Cd were grouped into TELs-PELs, moreover, Cr concentrations in RW soils exceeded the ERL. However, both Pb and Zn concentrations were below the TELs in both wetlands. Generally, The toxic unit in more than 85% of DW or RW soil samples showed low toxicity with higher contribution of As and Ni. It is necessary to monitor and control trace elements in the Freshwater supplied to restored wetlands in coastal wetland restoration projects.
Stefan Rahmstorf - One of the best experts on this subject based on the ideXlab platform.
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rapid changes of glacial climate simulated in a coupled climate model
Nature, 2001Co-Authors: Andrey Ganopolski, Stefan RahmstorfAbstract:Abrupt changes in climate, termed Dansgaard-Oeschger and Heinrich events, have punctuated the last glacial period (approximately 100-10 kyr ago) but not the Holocene (the past 10 kyr). Here we use an intermediate-complexity climate model to investigate the stability of glacial climate, and we find that only one mode of Atlantic Ocean circulation is stable: a cold mode with deep water formation in the Atlantic Ocean south of Iceland. However, a 'warm' circulation mode similar to the present-day Atlantic Ocean is only marginally unstable, and temporary transitions to this warm mode can easily be triggered. This leads to abrupt warm events in the model which share many characteristics of the observed Dansgaard-Oeschger events. For a large Freshwater Input (such as a large release of icebergs), the model's deep water formation is temporarily switched off, causing no strong cooling in Greenland but warming in Antarctica, as is observed for Heinrich events. Our stability analysis provides an explanation why glacial climate is much more variable than Holocene climate.
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rapid changes of glacial climate simulated in a coupled climate model
Nature, 2001Co-Authors: Andrey Ganopolski, Stefan RahmstorfAbstract:Abrupt changes in climate, termed Dansgaard–Oeschger and Heinrich events, have punctuated the last glacial period (∼100–10 kyr ago) but not the Holocene (the past 10 kyr). Here we use an intermediate-complexity climate model to investigate the stability of glacial climate, and we find that only one mode of Atlantic Ocean circulation is stable: a cold mode with deep water formation in the Atlantic Ocean south of Iceland. However, a ‘warm’ circulation mode similar to the present-day Atlantic Ocean is only marginally unstable, and temporary transitions to this warm mode can easily be triggered. This leads to abrupt warm events in the model which share many characteristics of the observed Dansgaard–Oeschger events. For a large Freshwater Input (such as a large release of icebergs), the model's deep water formation is temporarily switched off, causing no strong cooling in Greenland but warming in Antarctica, as is observed for Heinrich events. Our stability analysis provides an explanation why glacial climate is much more variable than Holocene climate.
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bifurcations of the atlantic thermohaline circulation in response to changes in the hydrological cycle
Nature, 1995Co-Authors: Stefan RahmstorfAbstract:The sensitivity of the North Atlantic thermohaline circulation to the Input of fresh water is studied using a global ocean circulation model coupled to a simplified model atmosphere. Owing to the nonlinearity of the system, moderate changes in Freshwater Input can induce transitions between different equilibrium states, leading to substantial changes in regional climate. As even local changes in Freshwater flux are capable of triggering convective instability, quite small perturbations to the present hydrological cycle may lead to temperature changes of several degrees on timescales of only a few years.
Knut Aagaard - One of the best experts on this subject based on the ideXlab platform.
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the large scale Freshwater cycle of the arctic
Journal of Geophysical Research, 2006Co-Authors: Mark C Serreze, Andrew P Barrett, Andrew G Slater, Rebecca A Woodgate, Knut Aagaard, Richard B Lammers, Michael Steele, Richard E Moritz, Michael P Meredith, Craig M LeeAbstract:This paper synthesizes our understanding of the Arctic's large-scale Freshwater cycle. It combines terrestrial and oceanic observations with insights gained from the ERA-40 reanalysis and land surface and ice-ocean models. Annual mean Freshwater Input to the Arctic Ocean is dominated by river discharge (38%), inflow through Bering Strait (30%), and net precipitation (24%). Total Freshwater export from the Arctic Ocean to the North Atlantic is dominated by transports through the Canadian Arctic Archipelago (35%) and via Fram Strait as liquid (26%) and sea ice (25%). All terms are computed relative to a reference salinity of 34.8. Compared to earlier estimates, our budget features larger import of Freshwater through Bering Strait and larger liquid phase export through Fram Strait. While there is no reason to expect a steady state, error analysis indicates that the difference between annual mean oceanic inflows and outflows (∼8% of the total inflow) is indistinguishable from zero. Freshwater in the Arctic Ocean has a mean residence time of about a decade. This is understood in that annual Freshwater Input, while large (∼8500 km3), is an order of magnitude smaller than oceanic Freshwater storage of ∼84,000 km3. Freshwater in the atmosphere, as water vapor, has a residence time of about a week. Seasonality in Arctic Ocean Freshwater storage is nevertheless highly uncertain, reflecting both sparse hydrographic data and insufficient information on sea ice volume. Uncertainties mask seasonal storage changes forced by Freshwater fluxes. Of flux terms with sufficient data for analysis, Fram Strait ice outflow shows the largest interannual variability.
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revising the bering strait Freshwater flux into the arctic ocean
Geophysical Research Letters, 2005Co-Authors: Rebecca A Woodgate, Knut AagaardAbstract:[1] The Freshwater flux through the Bering Strait into the Arctic Ocean is important regionally and globally, e.g. for Chukchi Sea hydrography, Arctic Ocean stratification, the global Freshwater cycle, and the stability of the Atlantic overturning circulation. Aagaard and Carmack [1989] estimated the Bering Strait Freshwater flux as 1670 km3/yr (relative to 34.8 psu), assuming an annual mean transport (0.8 Sv) and salinity (32.5 psu). This is ∼1/3rd of the total Freshwater Input to the Arctic. Using long-term moored measurements and ship-based observations, we show that this is a substantial underestimate of the Freshwater flux. Specifically, the warm, fresh Alaskan Coastal Current in the eastern Bering Strait may add ∼400 km3/yr. Seasonal stratification and ice transport may add another ∼400 km3/yr. Combined, these corrections are larger than the interannual variability observed by near-bottom measurements and near-surface measurements will be necessary to quantify this flux and its interannual variability.
Ronald J Stouffer - One of the best experts on this subject based on the ideXlab platform.
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coupled ocean atmosphere model response to Freshwater Input comparison to younger dryas event
Paleoceanography, 1997Co-Authors: Syukuro Manabe, Ronald J StoufferAbstract:This study explores the responses of a coupled ocean-atmosphere model to the discharge of Freshwater into the North Atlantic Ocean. In the first numerical experiment in which Freshwater is discharged into high North Atlantic latitudes over a period of 500 years, the thermohaline circulation (THC) in the Atlantic Ocean weakens, reducing surface air temperature over the northern North Atlantic Ocean and Greenland and, to a lesser degree, over the Arctic Ocean, the Scandinavian peninsula, and the Circumpolar Ocean and the Antarctic continent of the southern hemisphere. Upon the termination of the water discharge at the 500th year, the THC begins to intensify, regaining its original intensity in a few hundred years. With the exception of the Pacific sector of the Circumpolar Ocean of the southern hemisphere, where the surface air temperature recovery is delayed, the climate of the northern North Atlantic and surrounding regions rapidly resumes its original distribution. The evolution of the ocean-atmosphere system described above resembles the Younger Dryas event as inferred from the comprehensive analysis of ice cores and deep-sea and lake sediments. In the second experiment, in which the same amount of Freshwater is discharged into the subtropical North Atlantic again over a period of 500 years, the THC and climate evolve in a manner qualitatively similar to the first experiment. However, the magnitude of the THC response is 4–5 times smaller. It appears that Freshwater is much less effective in weakening the THC if it were discharged outside high North Atlantic latitudes.
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simulation of abrupt climate change induced by Freshwater Input to the north atlantic ocean
Nature, 1995Co-Authors: Syukuro Manabe, Ronald J StoufferAbstract:TEMPERATURE records from Greenland ice cores1,2 suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and postglacial periods; one example is the Younger Dryas cold event. Broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean–atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the thermohaline circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean–atmosphere system recorded in ice and deep-sea cores4. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and its neighbourhood, but is relatively small in the rest of the world.
Henk A Dijkstra - One of the best experts on this subject based on the ideXlab platform.
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an indicator of the multiple equilibria regime of the atlantic meridional overturning circulation
Journal of Physical Oceanography, 2010Co-Authors: Selma E Huisman, Matthijs Den Toom, Henk A Dijkstra, Sybren DrijfhoutAbstract:Recent model results have suggested that there may be a scalar indicator S monitoring whether the Atlantic meridional overturning circulation (MOC) is in a multiple equilibrium regime. The quantity S is based on the net Freshwater transport by the MOC into the Atlantic basin. It changes sign as soon as the steady Atlantic MOC enters the multiple equilibrium regime because of an increased Freshwater Input in the northern North Atlantic. This paper addresses the issue of why the sign of S is such a good indicator for the multiple equilibrium regime. Changes in the Atlantic Freshwater budget over a complete bifurcation diagram and in finite amplitude perturbation experiments are analyzed in a global ocean circulation model. The authors show that the net anomalous Freshwater transport into or out of the Atlantic, resulting from the interactions of the velocity perturbations and salinity background field, is coupled to the background (steady state) state Freshwater budget and hence to S. The sign of S precisely shows whether this net anomalous Freshwater transport is stabilizing or destabilizing the MOC. Therefore, it can indicate whether the MOC is in a single or multiple equilibrium regime.
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stability of the atlantic overturning circulation competition between bering strait Freshwater flux and agulhas heat and salt sources
Journal of Physical Oceanography, 2001Co-Authors: Wilbert Weijer, Wilhelmus P M De Ruijter, Henk A DijkstraAbstract:Abstract The role played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean is examined. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, Drake Passage, and Agulhas Leakage) are represented by sources and sinks. The profiles and amplitudes of these sources are based mainly on the heat and salt fluxes in a high-resolution ocean model (OCCAM). When applying realistic sources and sinks, a circulation is favored that is characterized by major downwelling in the Northern Hemisphere (northern sinking pole to pole circulation, NPP), and resembles the present-day Atlantic overturning circulation. The Southern Ocean sources appear to stabilize this circulation, whereas Bering Strait Freshwater Input tends to destabilize it. Already a small buoyancy Input at southerly latitudes is enough to prohibit the existence of a southern sinking circulation (SPP), leaving the NPP circul...
