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Linda T. Smith

Abstract

A primitive equation numerical ocean model is used to investigate the response of an basic flow, initially horizontally and vertically uniform, to the presence of an isolated seamount at the center of a 4000-m-deep periodic channel. The model is configured with 5-km horizontal resolution and 15 constant-density layer in the vertical. The Coriolis parameter, fluid stratification, coefficient of lateral friction, and length scale or the topography are held constant while the men downstream flow speed and height of the obstacle vary up to an extreme case of a 20 cm s−1 current impinging on a 3500 seamount. The results include the familiar anticyclonic vortex straddling the topograhy while a cyconic vortex propagates downgstream. Both advective and vorticity interaction effects are noted in the experiment with the extent of perturbations to the density field being influenced by variations in the mean flow speed and obstacle height. Possible modifications to the model are discussed in relation to observed conditions at Fieberling Guyot, a Pacific Ocean seamount whose geometric parameters are approximated by the model topography.

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Linda T. Smith, Eric P. Chassignet, and Rainer Bleck

Abstract

A series of numerical experiments for the North Atlantic have been carried out with a primitive equation density-coordinate model, incorporating a relaxation zone south of 65°N designed to simulate the production of dense water in regions outside the model domain. The principal experiments vary both the horizontal grid resolution of the model (0.9° and 1/3°) and the width of the buffer zone (4° and 4/3°); an additional model experiment addresses the issue of the forcing used within the restoring region. The North Atlantic water mass transformations simulated in the various experiments and the subsequent pathways of converted water from the northern boundary region into the interior basin are documented both by time-evolution and time-mean analysis of the model results, focusing on the mass content of the isopycnic layers, the spread of water masses tagged with a passive tracer, the meridional overturning streamfunction and associated transport of heat, and mass transport budgets within specified density classes. Primarily because of its inherent ability to retain the characteristics of dense water masses, the density-coordinate model is found to be relatively insensitive to the configuration of the northern boundary region when compared to similarly configured models in depth coordinates. However, the density-coordinate experiments with finer grid resolution produce higher values of northward heat transport, particularly north of 35°N, and the strongest meridional overturning and heat transport are seen in an experiment combining wide buffer zones and high horizontal resolution. Comparison to observations indicates that the use of σ 2 rather than σ θ as the coordinate representation of density, together with refinements in vertical resolution, will likely improve the model’s ability to accurately represent the water mass distributions and three-dimensional circulation of the Atlantic.

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Rainer Bleck, Claes Rooth, Dingming Hu, and Linda T. Smith

Abstract

An isopycnic-coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented. Special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation. The model performance is illustrated by a 30-year spinup run with coarse horizontal resolution (2° mesh) in a domain with North Atlantic topography extending from 10° to 60°N latitude. The vertical structure encompasses 10 isopycnic layers in steps of 0.2 σ units, capped by a thermodynamically active mixed layer. From an initially isohaline state with isopycnals prescribed by zonally averaged climatology, the model is forced by seasonally varying wind stress, radiative and freshwater fluxes, and by a thermal relaxation process at the surface. After a mechanical spinup time of about 15 years, a quasi-stationary pattern of mean circulation and annual variability ensues, characterized by pronounced subtropical mode-water formation and a gradual growth in the salinity contrast between the subtropics and the subpolar region. The effect of the freshwater flux forcing on the ventilation of the thermocline is a key point of discussion. Finally, a low-viscosity experiment suggests that the thermohaline processes represented in the model are quite insensitive to dynamic noise development at the grid resolution limit.

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Eric P. Chassignet, Linda T. Smith, Rainer Bleck, and Frank O. Bryan

Abstract

A series of medium-resolution (∼1°) numerical simulations for the equatorial and North Atlantic basin have been performed with two primitive equation models, one employing depth and the other density as the vertical coordinate. The models have been configured for this exercise in as similar a fashion as their basic formulations allow, and with fundamentally identical initialization, boundary conditions, and forcing functions for each of the experiments. The purpose of comparing the models’ results is twofold: 1) to understand the degree to which model-generated circulation fields depend on the particular model architecture by examining the rate of divergence of the solutions of an isopycnic and a depth coordinate model given the same initial conditions and 2) to uncover and remedy possible defects in either model design. The comparison is focused on the importance in each simulation of the choice of mixing parameterization, which has a crucial impact on the meridional overturning circulation, on the associated northward heat transport, and on the evolution of water masses. Although the model-generated horizontal fields viewed at specific times during the integrations do not appear to be strongly dependent on the design of each model and are in good agreement with one another, the integrated properties of the depth coordinate model and the isopycnic coordinate model diverge significantly over time, with the depth coordinate model being unable to retain its most dense water masses after long integration periods.

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Eric P. Chassignet, Linda T. Smith, George R. Halliwell, and Rainer Bleck

Abstract

The viability of a generalized (Hybrid) Coordinate Ocean Model (HYCOM), together with the importance of thermobaricity and the choice of reference pressure, is demonstrated by analyzing simulations carried out using the World Ocean Circulation Experiment (WOCE) Community Modeling Experiment (CME) Atlantic basin configuration. The standard hybrid vertical coordinate configuration is designed to remain isopycnic throughout as much of the water column as possible while smoothly making a transition to level (pressure) coordinates in regions with weak vertical density gradients, such as the surface mixed layer, and to terrain-following coordinates in shallow-water regions. Single-coordinate (pressure or density) experiments illustrate the flexibility of the model but also bring forward some of the limitations associated with such a choice. Hybrid experiments with potential density referenced to the surface (σ θ) and to 20 MPa (∼2000 m) (σ 2) illustrate the increased influence of pressure errors with increasing distance from the reference pressure. The σ θ hybrid experiment does not properly reproduce the northward flow of Antarctic Bottom Water (AABW), and large errors in near-surface pressure gradients in the σ 2 experiment produce a wind-driven gyre circulation that is too strong, when compared with observations, and a North Atlantic Current that follows an unrealistic path. These near-surface and near-bottom pressure errors are removed when thermobaric effects are included, resulting in a more accurate representation of the upper-ocean gyre circulation, the northward AABW flow near the bottom, and the meridional overturning circulation and heat flux.

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M. Ades, R. Adler, Rob Allan, R. P. Allan, J. Anderson, Anthony Argüez, C. Arosio, J. A. Augustine, C. Azorin-Molina, J. Barichivich, J. Barnes, H. E. Beck, Andreas Becker, Nicolas Bellouin, Angela Benedetti, David I. Berry, Stephen Blenkinsop, Olivier. Bock, Michael G. Bosilovich, Olivier. Boucher, S. A. Buehler, Laura. Carrea, Hanne H. Christiansen, F. Chouza, John R. Christy, E.-S. Chung, Melanie Coldewey-Egbers, Gil P. Compo, Owen R. Cooper, Curt Covey, A. Crotwell, Sean M. Davis, Elvira de Eyto, Richard A. M de Jeu, B.V. VanderSat, Curtis L. DeGasperi, Doug Degenstein, Larry Di Girolamo, Martin T. Dokulil, Markus G. Donat, Wouter A. Dorigo, Imke Durre, Geoff S. Dutton, G. Duveiller, James W. Elkins, Vitali E. Fioletov, Johannes Flemming, Michael J. Foster, Richard A. Frey, Stacey M. Frith, Lucien Froidevaux, J. Garforth, S. K. Gupta, Leopold Haimberger, Brad D. Hall, Ian Harris, Andrew K Heidinger, D. L. Hemming, Shu-peng (Ben) Ho, Daan Hubert, Dale F. Hurst, I. Hüser, Antje Inness, K. Isaksen, Viju John, Philip D. Jones, J. W. Kaiser, S. Kelly, S. Khaykin, R. Kidd, Hyungiun Kim, Z. Kipling, B. M. Kraemer, D. P. Kratz, R. S. La Fuente, Xin Lan, Kathleen O. Lantz, T. Leblanc, Bailing Li, Norman G Loeb, Craig S. Long, Diego Loyola, Wlodzimierz Marszelewski, B. Martens, Linda May, Michael Mayer, M. F. McCabe, Tim R. McVicar, Carl A. Mears, W. Paul Menzel, Christopher J. Merchant, Ben R. Miller, Diego G. Miralles, Stephen A. Montzka, Colin Morice, Jens Mühle, R. Myneni, Julien P. Nicolas, Jeannette Noetzli, Tim J. Osborn, T. Park, A. Pasik, Andrew M. Paterson, Mauri S. Pelto, S. Perkins-Kirkpatrick, G. Pétron, C. Phillips, Bernard Pinty, S. Po-Chedley, L. Polvani, W. Preimesberger, M. Pulkkanen, W. J. Randel, Samuel Rémy, L. Ricciardulli, A. D. Richardson, L. Rieger, David A. Robinson, Matthew Rodell, Karen H. Rosenlof, Chris Roth, A. Rozanov, James A. Rusak, O. Rusanovskaya, T. Rutishäuser, Ahira Sánchez-Lugo, P. Sawaengphokhai, T. Scanlon, Verena Schenzinger, S. Geoffey Schladow, R. W Schlegel, Eawag Schmid, Martin, H. B. Selkirk, S. Sharma, Lei Shi, S. V. Shimaraeva, E. A. Silow, Adrian J. Simmons, C. A. Smith, Sharon L Smith, B. J. Soden, Viktoria Sofieva, T. H. Sparks, Paul W. Stackhouse Jr., Wolfgang Steinbrecht, Dimitri A. Streletskiy, G. Taha, Hagen Telg, S. J. Thackeray, M. A. Timofeyev, Kleareti Tourpali, Mari R. Tye, Ronald J. van der A, Robin, VanderSat B.V. van der Schalie, Gerard van der SchrierW. Paul, Guido R. van der Werf, Piet Verburg, Jean-Paul Vernier, Holger Vömel, Russell S. Vose, Ray Wang, Shohei G. Watanabe, Mark Weber, Gesa A. Weyhenmeyer, David Wiese, Anne C. Wilber, Jeanette D. Wild, Takmeng Wong, R. Iestyn Woolway, Xungang Yin, Lin Zhao, Guanguo Zhao, Xinjia Zhou, Jerry R. Ziemke, and Markus Ziese
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