Stability of the Atlantic overturning circulation: competition between Bering Strait freshwater flux and Agulhas heat and salt sources

In this study we examine the role that is played by interocean fluxes of buoyancy in stabilizing the present-day overturning circulation of the Atlantic Ocean. A 2D model of the Atlantic overturning circulation is used, in which the interocean fluxes of heat and salt (via the Bering Strait, the Drake Passage and via 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 (NPP circulation), 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),lea ving the NPP circulation as a unique and stable solution. A large, factor three increase in Bering Strait freshwater import would be necessary to bring the SPP circulation back into existence. Especially the Indian-Atlantic transfer of heat and salt, brought about by Agulhas Leakage,c ontributes considerably to the strength and, in particular, the stability of the northern sinking circulation. According to this model,sh utting off Agulhas Leakage, and consequently the so-called warm water route for North Atlantic Deep Water (NADW) compensation, leads to a reduction of the overturning strength by 10% at most. These results imply that the way in which the NADW renewal takes place has implications for both the strength and stability of the Atlantic overturning circulation,giv ing the discussion about the warm vs. cold water route for NADW compensation dynamical significance. Moreover, when the stabilizing effect of Agulhas Leakage on the overturning disappears, the destabilizing influence of the Bering Strait freshwater input becomes more effective. The system is then close to a regime where the northern and southern overturning circulations coexist as stable solutions. Perturbations in Bering Strait inflow may then easily lead to switches between the two circulation states. These results suggest that the absence of Agulhas Leakage during the last ice-age may have contributed to weakening the glacial overturning circulation in the Atlantic. It may have made the thermohaline circulation vulnerable to variability, caused either by regime switches, or by the excitation of oscillatory modes. The sudden restart of the Atlantic overturning circulation at the beginning of the Holocene may well have been stimulated by the coincident reopening of the Agulhas gap. The presence of Agulhas Leakage may contribute to the relative stability of Holocene climate. Present-day climate may thus be more stable than previously thought

Cardiovascular risk management in old age

The aim of this thesis was to study cardiovascular risk management in old age, in order to facilitate the development of age specific guidelines. In part one the current status of cardiovascular prevention in old age is described, including a study into general practitioners__ attitudes and perceived barriers in this respect. The second part explores the incremental value of routine-ECGs for cardiovascular risk management in older persons from the general population, beyond existing information from medical records. The third part focuses on primary prevention, exploring the performance of classic risk factors, and some new biomarkers, in predicting cardiovascular mortality in very old people from the general population. It was concluded that a homocysteine level alone accurately identifies those at high risk of cardiovascular mortality, whereas classic risk factors included in the Framingham risk score do not. Next, in various age strata from age 55 years onwards, the association between blood pressure and mortality was studied. Finally, a systematic review into the diagnostic accuracy of natriuretic peptides for the diagnosis of chronic heart failure in older persons from the general population was performed, followed by a study in a cohort of nonagenarians into the prognostic value of NT-proBNP. A general discussion is provided, including directions for future research.UBL - phd migration 201

Mach's Principle and Model for a Broken Symmetric Theory of Gravity

We investigate spontaneous symmetry breaking in a conformally invariant gravitational model. In particular, we use a conformally invariant scalar tensor theory as the vacuum sector of a gravitational model to examine the idea that gravitational coupling may be the result of a spontaneous symmetry breaking. In this model matter is taken to be coupled with a metric which is different but conformally related to the metric appearing explicitly in the vacuum sector. We show that after the spontaneous symmetry breaking the resulting theory is consistent with Mach's principle in the sense that inertial masses of particles have variable configurations in a cosmological context. Moreover, our analysis allows to construct a mechanism in which the resulting large vacuum energy density relaxes during evolution of the universe.Comment: 9 pages, no figure

Response of the Atlantic overturning circulation to South Atlantic sources of buoyancy

The heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage is found to influence the Atlantic overturning circulation in a low-resolution Ocean General Circulation Model. The model used is the Hamburg Large-Scale Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas Leakage is parameterized by sources of heat and salt in the upper South Atlantic Ocean, that extend well into the intermediate layers. It is shown that the models overturning circulation is sensitive to the applied sources of heat and salt. The response of the overturning strength to changes in the source amplitudes is mainly linear, interrupted once by a stepwise change. The South Atlantic buoyancy sources influence the Atlantic overturning strength by modifying the basin-scale meridional density and pressure gradients. The nonlinear, stepwise response is caused by abrupt changes in the convective activity in the northern North Atlantic. Two additional experiments illustrate the adjustment of the overturning circulation upon sudden introduction of heat and salt sources in the South Atlantic. The North Atlantic overturning circulation responds within a few years after the sources are switched on. This is the time it takes for barotropic and baroclinic Kelvin waves to reach the northern North Atlantic. The advection of the anomalies takes 3 decades to reach the northern North Atlantic. The model results give support to the hypothesis that the re-opening of the Agulhas Gap at the end of the last ice-age, as indicated by palaeoclimatological data, may have stimulated the coincident strengthening of the Atlantic overturning circulation

Acquisition Hardware for Imaging

In electron microscopy images can either be recorded in parallel (Transmission Electron Microscopy) or acquired as the variation in a signal as a probe is scanned over the specimen (Scanning Electron Microscopy). To extract the most information from an image requires that the best possible systems are used for acquiring image data. Ultimately, the limit to information capture is achieved when every electron from the scattering event of interest is recorded. The ideal system can be realised both for parallel recording with scientific grade CCD cameras, and for scanning microscopy with single electron counting electronics. The data rates from these different systems impose different constraints on the computer systems needed to acquire and display the incoming images

Impact of interbasin exchange on the Atlantic overturning circulation

The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a data set based on WOCE hydrographic data. It is shown that the salt and heat transports brought about by the South Atlantic subtropical gyre play an essential role in the Atlantic heat and salt budgets. It is found that on average the exported North Atlantic Deep Water (NADW) is fresher than the return flows (basically composed ofth ermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater from the Atlantic. The sensitivity ofth e OC to interbasin fluxes of heat and salt is studied in a 2D model, representing the Atlantic between 60°N and 30°S. The model is forced by mixed boundary conditions at the surface, and by realistic fluxes of heat and salt at its 30°S boundary. The model circulation turns out to be very sensitive to net buoyancy fluxes through the surface. Both net surface cooling and net surface saltening are sources ofp otential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the strati fication, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition oft he NADW return transports, whether by a change in the ratio thermocline/intermediate water, or by a change in their thermohaline characteristics, might influence the Atlantic OC considerably. It is also shown that the circulation is much more sensitive to changes in the shape ofth e lateral buoyancy flux than to changes in the shape ofth e surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes ofhe at and salt are important for the strength and operation of the Atlantic thermohaline circulation, and should be correctly represented in models that are used for climate sensitivity studies

Hidden bedside rationing in the Netherlands:a cross-sectional survey among physicians in internal medicine

Background: Healthcare rationing can be defined as withholding beneficial care for cost reasons. One form in particular, hidden bedside rationing, is problematic because it may result in conflicting loyalties for physicians, unfair inequality among patients and illegitimate distribution of resources. Our aim is to establish whether bedside rationing occurs in the Netherlands, whether it qualifies as hidden and what physician characteristics are associated with its practice. Methods: Cross-sectional online questionnaire on knowledge of -, experience with -, and opinion on rationing among physicians in internal medicine within the Dutch healthcare system. Multivariable ordinal logistic regression was used to explore relations between hidden bedside rationing and physician characteristics. Results: The survey was distributed among 1139 physicians across 11 hospitals with a response rate of 18% (n = 203). Most participants (n = 129; 64%) had experience prescribing a cheaper course of treatment while a more effective but more expensive alternative was available, suggesting bedside rationing. Subsequently, 32 (24%) participants never disclosed this decision to their patient, qualifying it as hidden. The majority of participants (n = 153; 75%) rarely discussed treatment cost. Employment at an academic hospital was independently associated with more bedside rationing (OR = 17 95%CI 6.1–48). Furthermore, residents were more likely to disclose rationing to their patients than internists (OR = 3.2, 95%CI 2.1–4.7), while salaried physicians were less likely to do so than physicians in private practice (OR = 0.5, 95%CI 0.4–0.8). Conclusion: Hidden bedside rationing occurs in the Netherlands: patient choice is on occasion limited with costs as rationale and this is not always disclosed. To what extent distribution of healthcare should include bedside rationing in the Netherlands, or any other country, remains up for debate.</p

Homocysteine levels and treatment effect in the prospective study of pravastatin in the elderly at risk

Objectives: To assess the effect of preventive pravastatin treatment on coronary heart disease (CHD) morbidity and mortality in older persons at risk for cardiovascular disease (CVD), stratified according to plasma levels of homocysteine.&lt;p&gt;&lt;/p&gt; Design: A post hoc subanalysis in the PROspective Study of Pravastatin in the Elderly at Risk (PROSPER), started in 1997, which is a double-blind, randomized, placebo-controlled trial with a mean follow-up of 3.2 years.&lt;p&gt;&lt;/p&gt; Setting: Primary care setting in two of the three PROSPER study sites (Netherlands and Scotland).&lt;p&gt;&lt;/p&gt; Participants: Individuals (n = 3,522, aged 70–82, 1,765 male) with a history of or risk factors for CVD were ranked in three groups depending on baseline homocysteine level, sex, and study site.&lt;p&gt;&lt;/p&gt; Intervention: Pravastatin (40 mg) versus placebo.&lt;p&gt;&lt;/p&gt; Measurements: Fatal and nonfatal CHD and mortality.&lt;p&gt;&lt;/p&gt; Results: In the placebo group, participants with a high homocysteine level (n = 588) had a 1.8 higher risk (95% confidence interval (CI) = 1.2–2.5, P = .001) of fatal and nonfatal CHD than those with a low homocysteine level (n = 597). The absolute risk reduction in fatal and nonfatal CHD with pravastatin treatment was 1.6% (95% CI = −1.6 to 4.7%) in the low homocysteine group and 6.7% (95% CI = 2.7–10.7%) in the high homocysteine group (difference 5.2%, 95% CI = 0.11–10.3, P = .046). Therefore, the number needed to treat (NNT) with pravastatin for 3.2 years for benefit related to fatal and nonfatal CHD events was 14.8 (95% CI = 9.3–36.6) for high homocysteine and 64.5 (95% CI = 21.4–∞) for low homocysteine.&lt;p&gt;&lt;/p&gt; Conclusion: In older persons at risk of CVD, those with high homocysteine are at highest risk for fatal and nonfatal CHD. With pravastatin treatment, this group has the highest absolute risk reduction and the lowest NNT to prevent fatal and nonfatal CHD.&lt;p&gt;&lt;/p&gt

Stikstofwerking van organische meststoffen op bouwland : resultaten van veldonderzoek in Wageningen in 2010/2011 : tussentijdse rapportage

De stikstofwerking van organische meststoffen afkomstig uit mestverwerking is in 2010 vergeleken met de stikstofwerking van onbewerkte mesten. Daartoe is het effect van mineralenconcentraat (MC), varkensdrijfmest (VDM), rundveedrijfmest (RDM), dikke fractie van gescheiden varkensdrijfmest (DF), dikke fractie van het gescheiden digestaat van suikerbietenloof (SBDF), rundveestalmest (SM) en KAS-kunstmest (KM) op de opbrengst van snijmaïs bepaald. De gevonden stikstofwerkingen bedroegen voor MC 77% (forfaitair 100%), voor VDM 65% (forfaitair 70%), voor RDM 60% (forfaitair 60%), voor DF 64% (forfaitair 55%), voor SM 33% (forfaitair 55%) en voor SBDF 26% (forfaitair 40%). Vooral gevonden werkingen van MC, SM en SBDF blijven achter bij de wettelijke forfaits. De proef is er ook op gericht om het niet-werkzame deel van de stikstofgift nader te verklaren. Daartoe zijn waarnemingen na de oogst van de mais voortgezet. Alle organische mesten, inclusief MC, lieten per kg toegediende werkzame N minder onbenutte minerale bodem-N achter dan kunstmest-N. Deze bodem-N bleek een goede voorspeller van de hoeveelheid nitraat in het bovenste grondwater