The Impact of Lateral Boundary Conditions and Horizontal Resolution on North Atlantic Water Mass Transformations and Pathways in an Isopycnic Coordinate Ocean Model

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 σ2 rather than σ? 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.