Circulation and intrusions northeast of Taiwan : chasing and predicting uncertainty in the cold dome

Author Posting. © The Oceanography Society, 2011. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 24 no. 4 (2011): 110–121, doi:10.5670/oceanog.2011.99.An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the "cold dome" frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome's dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broad-scale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7–8, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.We thank the National Science Council of Taiwan as well as the Office of Naval Research for generous support of this effort

Intravenous alteplase for stroke with unknown time of onset guided by advanced imaging: systematic review and meta-analysis of individual patient data

Background: Patients who have had a stroke with unknown time of onset have been previously excluded from thrombolysis. We aimed to establish whether intravenous alteplase is safe and effective in such patients when salvageable tissue has been identified with imaging biomarkers. Methods: We did a systematic review and meta-analysis of individual patient data for trials published before Sept 21, 2020. Randomised trials of intravenous alteplase versus standard of care or placebo in adults with stroke with unknown time of onset with perfusion-diffusion MRI, perfusion CT, or MRI with diffusion weighted imaging-fluid attenuated inversion recovery (DWI-FLAIR) mismatch were eligible. The primary outcome was favourable functional outcome (score of 0–1 on the modified Rankin Scale [mRS]) at 90 days indicating no disability using an unconditional mixed-effect logistic-regression model fitted to estimate the treatment effect. Secondary outcomes were mRS shift towards a better functional outcome and independent outcome (mRS 0–2) at 90 days. Safety outcomes included death, severe disability or death (mRS score 4–6), and symptomatic intracranial haemorrhage. This study is registered with PROSPERO, CRD42020166903. Findings: Of 249 identified abstracts, four trials met our eligibility criteria for inclusion: WAKE-UP, EXTEND, THAWS, and ECASS-4. The four trials provided individual patient data for 843 individuals, of whom 429 (51%) were assigned to alteplase and 414 (49%) to placebo or standard care. A favourable outcome occurred in 199 (47%) of 420 patients with alteplase and in 160 (39%) of 409 patients among controls (adjusted odds ratio [OR] 1·49 [95% CI 1·10–2·03]; p=0·011), with low heterogeneity across studies (I2=27%). Alteplase was associated with a significant shift towards better functional outcome (adjusted common OR 1·38 [95% CI 1·05–1·80]; p=0·019), and a higher odds of independent outcome (adjusted OR 1·50 [1·06–2·12]; p=0·022). In the alteplase group, 90 (21%) patients were severely disabled or died (mRS score 4–6), compared with 102 (25%) patients in the control group (adjusted OR 0·76 [0·52–1·11]; p=0·15). 27 (6%) patients died in the alteplase group and 14 (3%) patients died among controls (adjusted OR 2·06 [1·03–4·09]; p=0·040). The prevalence of symptomatic intracranial haemorrhage was higher in the alteplase group than among controls (11 [3%] vs two [<1%], adjusted OR 5·58 [1·22–25·50]; p=0·024). Interpretation: In patients who have had a stroke with unknown time of onset with a DWI-FLAIR or perfusion mismatch, intravenous alteplase resulted in better functional outcome at 90 days than placebo or standard care. A net benefit was observed for all functional outcomes despite an increased risk of symptomatic intracranial haemorrhage. Although there were more deaths with alteplase than placebo, there were fewer cases of severe disability or death. Funding: None

Interest of Gas Phase Processes for the Synthesis of Materials Activated under Light: Example in Photocatalysis and Photovoltaics

International audienceDue to its high activity under near UV illumination, one of the most studied materials in photocatalytic as well as in photovoltaic studies is titanium dioxide TiO$_2$ especially in its anatase crystalline form. In both cases, one of the phenomena limiting the efficiency is the recombination of electron-holes pair. In this context, the use of composites based on TiO$_2$ nanoparticles and carbon or metallic nano-objects is a relevant strategy towards more efficient electron transfer processes. This paper will present the one-step synthesis of such nanocomposites and some studies on their photocatalytic or photovoltaic applications. To achieve the synthesis of high quality nanocomposites presenting well-controlled physical properties, we use the laser pyrolysis method. This method is based on the interaction between a high power CO2 laser and a gaseous or liquid precursor. In all cases Titanium tetraisopropoxide (TTIP) was used as the TiO$_2$ precursor. Hydrogen tetrachloroaurate was dissolved in the TTIP solution to produce Au loaded TiO$_2$ nanoparticles (Figure 1, left). The efficiency of these nanoparticles was studied under air and N$_2$ for the photocatalytic decomposition of acetic acid. Graphene nanoparticles were dispersed in liquid TTIP to produce composite nanoparticles where TiO$_2$ is grown at the surface of the graphene layers (Figure 1, right). These composite nanoparticles were used to form the porous layer of a perovskite solar cell. In both cases, a significant effect is observed by comparison to the performances obtained from pure TiO$_2

Interest of Gas Phase Processes for the Synthesis of Materials Activated under Light: Example in Photocatalysis and Photovoltaics

International audienceDue to its high activity under near UV illumination, one of the most studied materials in photocatalytic as well as in photovoltaic studies is titanium dioxide TiO$_2$ especially in its anatase crystalline form. In both cases, one of the phenomena limiting the efficiency is the recombination of electron-holes pair. In this context, the use of composites based on TiO$_2$ nanoparticles and carbon or metallic nano-objects is a relevant strategy towards more efficient electron transfer processes. This paper will present the one-step synthesis of such nanocomposites and some studies on their photocatalytic or photovoltaic applications. To achieve the synthesis of high quality nanocomposites presenting well-controlled physical properties, we use the laser pyrolysis method. This method is based on the interaction between a high power CO2 laser and a gaseous or liquid precursor. In all cases Titanium tetraisopropoxide (TTIP) was used as the TiO$_2$ precursor. Hydrogen tetrachloroaurate was dissolved in the TTIP solution to produce Au loaded TiO$_2$ nanoparticles (Figure 1, left). The efficiency of these nanoparticles was studied under air and N$_2$ for the photocatalytic decomposition of acetic acid. Graphene nanoparticles were dispersed in liquid TTIP to produce composite nanoparticles where TiO$_2$ is grown at the surface of the graphene layers (Figure 1, right). These composite nanoparticles were used to form the porous layer of a perovskite solar cell. In both cases, a significant effect is observed by comparison to the performances obtained from pure TiO$_2

Laser pyrolysis synthesis of nanoparticles for energy applications

International audienceThe laser pyrolysis method has proved efficient for the synthesis of various oxide and non-oxide nanoparticles such as SiC, Si, TiO$_2$... Recently we have developed the method for the one-step production of Si@C core shell nanoparticles and composites of TiO$_2$-carbon nanostructures. Much attention has been paid to Si as an anode material in Li-Ion batteries in order to improve storage capacity (the theoretical limit being 3579 mAh/g in the Li$_{15}$Si$_4$ alloy vs 372 mAh/g for graphitic carbon). However, due large volume changes during cycling, silicon suffers several drawbacks, including rapid pulverization and SEI ripening, limiting its use. Nanostructuration and protection of silicon by a carbon coating are proven methods to improve the behavior of the silicon based anodes. We have developed a two stage laser pyrolysis reactor to achieve the synthesis of silicon-carbon core-shell nanoparticles in a continuous way, without intermediate manipulations between the synthesis of the core and the shell. At lab scale, the reactor is capable of stable run times up to several hours with production rates of ~8-10 g/hour. The size of the core was tuned from 20 to 80 nm diameter with associated carbon content of up to 20 wt%. EIS spectroscopy clearly shows the influence of the carbon coating on the growth of SEI during the first cycle, with a significantly lower resistance of the SEI when Si@C nanoparticles are used by comparison with Si. The carbon coating also enables improved utilization of silicon content in the first cycles as shown by galvanostatic cycling. By adding germane in the reaction zone, we were also able to synthesize nanoparticles of SixGe1-x with x in the range 20-80. These particles also present a core@shell organization with a silicon shell at the surface of the alloy particle. These NPs present improved columbic efficiency and stability when tested in coin cells

Synthesis of advanced nanomaterials for Energy Applications by Laser Pyrolysis

International audienceLaser pyrolysis is an effective method to synthesize a variety of oxide and non-oxide nanoparticles such as SiC, Si, TiO2, etc. This paper will present one example each of oxide and non-oxide materials being studied in our labs, describing both their synthesis methods and applications. First, we will discuss silicon and silicon alloy core-shell nanoparticles and their performance as anode materials in Li-ion batteries. Next we will cover TiO2-carbon nanocomposites as active materials in perovskite solar cells. Due to increasing demand in energy storage, much attention has been paid to Si as an anode material in Li-Ion batteries because of its theoretical capacity (3579 mAh/g in the Li15Si4 alloy vs 372 mAh/g for graphitic carbon). However, silicon suffers several drawbacks, including rapid pulverization and continuous ripening of the solid-electrolyte interphase, limiting its use. Nanostructuration and protection of silicon by a carbon coating are proven methods to improve the behavior of the silicon-based anodes. We will present the development of a two stage laser pyrolysis reactor, wherein the continuous synthesis of silicon-carbon core-shell nanoparticles was achieved. That is, there are no intermediate manipulations between the synthesis of the core (from silane precursor) and the shell (from ethylene). The protective influence of the carbon coating is clearly seen by impedance spectroscopy. Another strategy to enhance the performance and lifetime of of Si-based anode materials is the use of Si alloys. By adding germane in the reaction zone, we were able to synthesize nanoparticles of SixGe1-x with x in the range 20-80. These particles also present a core@shell organization with a silicon shell at the surface of the alloy particle. Li-ion coin cells incorporating these materials demonstrate improved coulombic efficiency and stability

Weak oceanic heat transport as a cause of the instability of glacial climates

The stability of the thermohaline circulation of modern and glacial climates is compared with the help of a two dimensional ocean-atmosphere-sea ice coupled model. It turns out to be more unstable as less freshwater forcing is required to induce a polar halocline catastrophy in glacial climates. The large insulation of the ocean by the extensive sea ice cover changes the temperature boundary condition and the deepwater formation regions moves much further South. The nature of the instability is of oceanic origin, identical to that found in ocean models under mixed boundary conditions. With similar strengths of the oceanic circulation and rates of deep water formation for warm and cold climates, the loss of stability of the cold climate is due to the weak thermal stratification caused by the cooling of surface waters, the deep water temperatures being regulated by the temperature of freezing. Weaker stratification with similar overturning leads to a weakening of the meridional oceanic heat transport which is the major negative feedback stabilizing the oceanic circulation. Within the unstable regime periodic millennial oscillations occur spontaneously. The climate oscillates between a strong convective thermally driven oceanic state and a weak one driven by large salinity gradients. Both states are unstable. The atmosphere of low thermal inertia is carried along by the oceanic overturning while the variation of sea ice is out of phase with the oceanic heat content. During the abrupt warming events that punctuate the course of a millennial oscillation, sea ice variations are shown respectively to damp (amplify) the amplitude of the oceanic (atmospheric) response. This sensitivity of the oceanic circulation to a reduced concentration of greenhouse gases and to freshwater forcing adds support to the hypothesis that the millennial oscillations of the last glacial period, the so called Dansgaard-Oeschger events, may be internal instabilities of the climate system