11,925 research outputs found
On the variability of the deep meridional transports in the tropical North Atlantic
No full textA 5-year-long time series of meridional transport below 1180 dbar—zonally integrated across a section spanning, the western basin of the tropical North Atlantic—is analyzed. It has been inferred from (i) zonally integrated meridional geostrophic transports derived from density and bottom pressure measurements at the end points of a 1000 km wide section bounded by the base of the western continental rise and the Mid-Atlantic Ridge and (ii) mooring-based direct current meter measurements over the steep Lesser Antilles continental rise. The southward time mean transport of North Atlantic Deep Water (NADW) transport is 15.9 Sv. The vertical shear of the geostrophic transport profiles in the western and eastern part of the section each show two layers of maximum southward transport within the NADW. The transport time series reveals changes of 7.7 Sv rms at periods of 1 month and longer, at times showing changes of up to 40 Sv within a month's time. The baroclinic (internal) contribution of the geostrophic flow (relative to 4950 dbar), yields fluctuations of 6.6 Sv rms. Adding transports over the steep continental rise reduces the overall transport variability to 5.2 Sv rms. As a result of this reduction in shorter-period variability, the lower-frequency variability becomes more pronounced, part of which is expected to be linked to the meridional overturning circulation (MOC). The transport variability is consistent with baroclinic Rossby waves (at periods between 3 and 9 months), dominating in the eastern and central part of the section, and with changes in deep western boundary current (DWBC) strength, DWBC re-circulation patterns and eddies that become important in the western part of the section. The reference-level (external) geostrophic transport variability displays long-wavelength (>2000 km) fluctuations of 7.5 Sv rms on periods less than 2 weeks that are consistent with barotropic Rossby waves.Numerical model simulations imply that the observed zonally integrated deep transport changes in the western basin have moderate skill in sensing changes in the MOC and in meridional heat transport, and that a now implemented extension of the array's integration scale into the eastern basin of the Atlantic would substantially improve the performance of the array as an MOC observing system.<br/
Coherent circulation changes in the deep North Atlantic from 16N and 26N transport arrays
No full textThe meridional overturning circulation (MOC) has been measured by boundary arrays in the Atlantic since 2000. Over the past decade of measurements, however, the reported tendencies in overturning circulation strength have differed between 16N and 26N. Here, we investigate these differences by diagnosing their origin in the observed hydrography, finding that both arrays show deep waters (below 1100 dbar) at the western boundary becoming fresher and less dense. The associated change in geopotential thickness is about 0.15 m2/s2 between 2004-2009 and 2010-2014, with the shift occurring between 2009- 2010 and earlier at 26N than 16N. In the absence of a similar density change on the east of the Atlantic, this mid-depth reduction in water density at the west would drive an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at 26N and 3.9 Sv at 16N. These transport anomalies result in an intensifying tendency in the MOC estimate at 16N, but at 26N, the method of correcting the geostrophic reference level results in an opposing (reducing) tendency of the MOC. The results indicate that both arrays are observing coherent, low frequency changes, but that there remain discrepancies in the methods of addressing the geostrophic reference level for boundary arrays measuring ocean circulation
Monitoring the integrated deep meridional flow in the tropical North Atlantic: long-term performance of a geostrophic array
No full textAs a component of the meridional overturning variability experiment in the tropical North Atlantic, a four-year-long time series of meridional transport of North Atlantic deep water has been obtained from moored end point measurements of density and bottom pressure. This study presents a quality assessment of the measurement elements. Rigorous pre- and post- deployment in situ calibration of the density sensors and subsequent data processing establish an accuracy of O(1.5 Sv) in internal transport in the 1200–5000 dbar range at subinertial time scales. A similar accuracy is reached in the bottom pressure-derived external transport fluctuations. However, for pressure, variability with periods longer than a deployment's duration (presently about one year) is not measurable. This effect is demonstrated using numerical simulations and a possible solution for detecting long-term external transport changes is presented. <br/
Seasonal variation of ocean bottom pressure derived from GRACE: Local validation and global patterns
No full textThe Gravity Recovery and Climate Experiment (GRACE) processing centers at the GeoForschungsZentrum Potsdam (GFZ) and the University of Texas Center for Space Research (UTCSR) provide time series of monthly gravity field solutions covering the period since mission launch in March 2002. Although the achieved accuracy still remains an order of magnitude below the mission's baseline goal, these time series have successfully been used to study terrestrial phenomena such as water storage variations. Over the oceans, the monthly gravity field solutions can be converted into estimates of the fluctuating ocean bottom pressure (OBP), which is the sum of atmospheric and oceanic mass variations. The GRACE products may be validated against in situ OBP observations which are available from a ground truth site in the tropical northwest Atlantic Ocean. Large differences are observed between the in situ and GRACE-derived OBP which are investigated by comparing the tidal and nontidal ocean models used at GFZ and UTCSR for dealiasing short-term (<2 months) mass variations from satellite measurements. Results show that the barotropic nontidal and tide models need improvement at periods shorter than 1 day and longer than 2 weeks. On a global scale the monthly OBP fields from GRACE generally overestimate the variability compared to ocean general circulation models, especially in tropical regions. This may be attributed to continuing deficiencies in GRACE data processing. Nevertheless, there is some initial evidence that GRACE possesses the potential to observe large-scale averages of bottom pressure fluctuations
On the Send-Synchronizability Problem for Mailbox Communication
No full textInternational audienceA system of communicating automata is send-synchronizable if its set of send sequences (i.e., the projection on send actions of its executions) is the same when communications are asynchronous and when they are rendez-vous synchronizations. Send-synchronizability was claimed to be decidable for the mailbox semantics (Basu and Bultan, 2011) and for the peer-to-peer semantics (Basu and Bultan, 2016). Finkel and Lozes showed in 2017 that the proofs of these results are flawed, and they proved that send-synchronizability is in fact undecidable for peer-to-peer systems. The sendsynchronizability problem for mailbox systems was left open. A partial solution was recently proposed in (Di Giusto, Laversa and Peters, 2024). In this paper, we revisit the send-synchronizability problem for mailbox systems. Firstly, we show that send-synchronizability is undecidable for mailbox systems, thus closing the question left open in (Finkel and Lozes, 2023) and (Di Giusto, Laversa and Peters, 2024). Secondly, we show that send-synchronizability is decidable for the class of 1-schedulable mailbox systems. A system is 1-schedulable if every execution can be re-scheduled into an equivalent execution where each send is either immediately followed by its matching receive, or is never matched. Despite the apparent similarity between send-synchronizability and 1-schedulability, the proof that send-synchronizability is decidable for 1-schedulable mailbox systems is quite involved. We believe that the techniques that we develop in this proof could be used to address other problems on mailbox systems, such as the realizability problem
Author Guidelines
No full textReproduction Rights Universitas Scientiarum is licensed under Creative Commons: Attribution - Noncommercial - No derivative works, 2.5 of Colombia. Consequently, it can be electronically reproduced, distributed and publically communicated, provided the authors and the Pontificia Universidad Javeriana are acknowledged. Commercial use of this work is not permitted. Modifications to the original work, and distribution of modified copies are prohibited. Upon acceptance of the article, the author will send the journal’s Office a printed and signed license of economic rights transfer, which authorizes its reproduction in Universitas Scientiarum (the license can be downloaded from the journal website; http://revistas.javeriana.edu.co/scientarium/web; http://revistas.javeriana.edu.co/scientarium/ojs). Such permission will infer the author’s acceptance of the article’s dissemination not only on paper but also in electronic media, that is, in websites and in national and international bibliographic databases. The magazine accepts pre-print articles, previously uploaded to e-print servers (OAI - such as arxiv.org)
Author Guidelines
No full textReproduction Rights Universitas Scientiarum is licensed under Creative Commons: Attribution - Noncommercial - No derivative works, 2.5 of Colombia. Consequently, it can be electronically reproduced, distributed and publically communicated, provided the authors and the Pontificia Universidad Javeriana are acknowledged. Commercial use of this work is not permitted. Modifications to the original work, and distribution of modified copies are prohibited. Upon acceptance of the article, the author will send the journal’s Office a printed and signed license of economic rights transfer, which authorizes its reproduction in Universitas Scientiarum (the license can be downloaded from the journal website; http://revistas.javeriana.edu.co/scientarium/web; http://revistas.javeriana.edu.co/scientarium/ojs). Such permission will infer the author’s acceptance of the article’s dissemination not only on paper but also in electronic media, that is, in websites and in national and international bibliographic databases. The magazine accepts pre-print articles, previously uploaded to e-print servers (OAI - such as arxiv.org)
Trade Update Notes; Report of the Task Force on The Aggregate Measure of Support: Potential Use by GATT for Agriculture
No full textInternational Relations/Trade,
European Economic Integration and the Consequences for U.S. Agriculture
No full textThe pace of political-economic change in former East Bloc nations of Europe defies accurate prediction. Some events such as more price-directed markets are predictable enough but integration of former East Bloc countries into the European Community remains a matter of speculation. Analysis indicates that the economics of agriculture favors acceptance by the European Community of members of the European Free Trade Association before former members of the. East Bloc. Analysis also indicates the considerable agricultural production potential of Central and East Europe will be unleased first by market-directed economies and later by integration with the EC -- if the latter occurs. US consumers gain more than producers lose so the economic welfare of Americans is raised modestly.International Relations/Trade,
A Critique of Computable General Equilibrium Models for Trade Policy Analysis
No full textThe paper will deal in turn with three sets of modelling issues: the question of 'data'; the 'micro' problem of specifying market behaviour, and the. 'macro' issue of 'closing' the models in aggregate. I will conclude with some suggestions for future research. The basic theme of the paper is this: CGE modelling is essentially a conservative or 'neoclassical' scientific endeavour, and exhibits the strengths and weaknesses of neoclassicism. And as for the recent injection of apparently nonneoclassical imperfect competition or industrial organization (IO) concepts into CGE, though, as an 10 specialist myself I certainly welcome this in principle, I have doubts about the usefulness of the practice.International Relations/Trade,
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