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ArticleLagrangian ocean analysis : fundamentals and practices(Elsevier, 2017-11-24) van Sebille, Erik ; Griffies, Stephen M. ; Abernathey, Ryan ; Adams, Thomas P. ; Berloff, Pavel S. ; Biastoch, Arne ; Blanke, Bruno ; Chassignet, Eric P. ; Cheng, Yu ; Cotter, Colin J. ; Deleersnijder, Eric ; Döös, Kristofer ; Drake, Henri F. ; Drijfhout, Sybren ; Gary, Stefan F. ; Heemink, Arnold W. ; Kjellsson, Joakim ; Koszalka, Inga M. ; Lange, Michael ; Lique, Camille ; MacGilchrist, Graeme ; Marsh, Robert ; Mayorga-Adame, Claudia G. ; McAdam, Ronan ; Nencioli, Francesco ; Paris, Claire B. ; Piggott, Matthew D. ; Polton, Jeff ; Rühs, Siren ; Shah, Syed H.A.M. ; Thomas, Matthew D. ; Wang, Jinbo ; Wolfram, Phillip J. ; Zanna, Laure ; Zika, Jan D.Lagrangian analysis is a powerful way to analyse the output of ocean circulation models and other ocean velocity data such as from altimetry. In the Lagrangian approach, large sets of virtual particles are integrated within the three-dimensional, time-evolving velocity fields. Over several decades, a variety of tools and methods for this purpose have emerged. Here, we review the state of the art in the field of Lagrangian analysis of ocean velocity data, starting from a fundamental kinematic framework and with a focus on large-scale open ocean applications. Beyond the use of explicit velocity fields, we consider the influence of unresolved physics and dynamics on particle trajectories. We comprehensively list and discuss the tools currently available for tracking virtual particles. We then showcase some of the innovative applications of trajectory data, and conclude with some open questions and an outlook. The overall goal of this review paper is to reconcile some of the different techniques and methods in Lagrangian ocean analysis, while recognising the rich diversity of codes that have and continue to emerge, and the challenges of the coming age of petascale computing.
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ArticleSinking of dense North Atlantic waters in a global ocean model : location and controls(John Wiley & Sons, 2018-04-23) Katsman, Caroline A. ; Drijfhout, Sybren ; Dijkstra, Henk A. ; Spall, Michael A.We investigate the characteristics of the sinking of dense waters in the North Atlantic Ocean that constitute the downwelling limb of the Atlantic Meridional Overturning Circulation (AMOC) as simulated by two global ocean models: an eddy‐permitting model at 1/4° resolution and its coarser 1° counterpart. In line with simple geostrophic considerations, it is shown that the sinking predominantly occurs in a narrow region close to the continental boundary in both model simulations. That is, the regions where convection is deepest do not coincide with regions where most dense waters sink. The amount of near‐boundary sinking that occurs varies regionally. For the 1/4° resolution model, these variations are in quantitative agreement with a relation based on geostrophy and a thermodynamic balance between buoyancy loss and alongshore advection of density, which links the amount of sinking to changes in density along the edge of the North Atlantic Ocean. In the 1° model, the amount and location of sinking appears not to be governed by this simple relation, possibly due to the large impact of overflows and nonnegligible cross‐shore density advection. If this poor representation of the processes governing the sinking of dense waters in the North Atlantic Ocean is a generic feature of such low‐resolution models, the response of the AMOC to changes in climate simulated by this type of models needs to be evaluated with care.
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ArticleHistorical and idealized climate model experiments : an intercomparison of Earth system models of intermediate complexity(Copernicus Publications on behalf of the European Geosciences Union, 2013-05-16) Eby, Michael ; Weaver, Andrew J. ; Alexander, K. ; Zickfeld, K. ; Abe-Ouchi, A. ; Cimatoribus, A. A. ; Crespin, E. ; Drijfhout, Sybren ; Edwards, N. R. ; Eliseev, A. V. ; Feulner, G. ; Fichefet, T. ; Forest, Chris E. ; Goosse, H. ; Holden, P. B. ; Joos, Fortunat ; Kawamiya, M. ; Kicklighter, David W. ; Kienert, H. ; Matsumoto, K. ; Mokhov, I. I. ; Monier, Erwan ; Olsen, Steffen M. ; Pedersen, J. O. P. ; Perrette, M. ; Philippon-Berthier, G. ; Ridgwell, Andy ; Schlosser, A. ; Schneider von Deimling, T. ; Shaffer, G. ; Smith, R. S. ; Spahni, R. ; Sokolov, Andrei P. ; Steinacher, M. ; Tachiiri, K. ; Tokos, K. ; Yoshimori, M. ; Zeng, Ning ; Zhao, F.Both historical and idealized climate model experiments are performed with a variety of Earth system models of intermediate complexity (EMICs) as part of a community contribution to the Intergovernmental Panel on Climate Change Fifth Assessment Report. Historical simulations start at 850 CE and continue through to 2005. The standard simulations include changes in forcing from solar luminosity, Earth's orbital configuration, CO2, additional greenhouse gases, land use, and sulphate and volcanic aerosols. In spite of very different modelled pre-industrial global surface air temperatures, overall 20th century trends in surface air temperature and carbon uptake are reasonably well simulated when compared to observed trends. Land carbon fluxes show much more variation between models than ocean carbon fluxes, and recent land fluxes appear to be slightly underestimated. It is possible that recent modelled climate trends or climate–carbon feedbacks are overestimated resulting in too much land carbon loss or that carbon uptake due to CO2 and/or nitrogen fertilization is underestimated. Several one thousand year long, idealized, 2 × and 4 × CO2 experiments are used to quantify standard model characteristics, including transient and equilibrium climate sensitivities, and climate–carbon feedbacks. The values from EMICs generally fall within the range given by general circulation models. Seven additional historical simulations, each including a single specified forcing, are used to assess the contributions of different climate forcings to the overall climate and carbon cycle response. The response of surface air temperature is the linear sum of the individual forcings, while the carbon cycle response shows a non-linear interaction between land-use change and CO2 forcings for some models. Finally, the preindustrial portions of the last millennium simulations are used to assess historical model carbon-climate feedbacks. Given the specified forcing, there is a tendency for the EMICs to underestimate the drop in surface air temperature and CO2 between the Medieval Climate Anomaly and the Little Ice Age estimated from palaeoclimate reconstructions. This in turn could be a result of unforced variability within the climate system, uncertainty in the reconstructions of temperature and CO2, errors in the reconstructions of forcing used to drive the models, or the incomplete representation of certain processes within the models. Given the forcing datasets used in this study, the models calculate significant land-use emissions over the pre-industrial period. This implies that land-use emissions might need to be taken into account, when making estimates of climate–carbon feedbacks from palaeoclimate reconstructions.