Meridional Overturning and Dianeutral Transport in a z-Coordinate Ocean Model Including Eddy-Induced Advection

Abstract

The present study examines the marked changes in the patterns of meridional overturning and dianeutral motion that occur upon introduction of the Gent and McWilliams (GM) scheme for eddy-induced transport into a coarse-resolution global ocean model. Results from two versions of the ocean model are compared. The first version does not have the GM scheme and uses a standard background horizontal diffusivity. The second version includes the GM scheme and has zero horizontal diffusivity. Both versions include a weak vertical diffusivity and isoneutral tracer diffusion as implemented by Cox. First, representations of the meridional overturning circulation computed via integration along (i) level, (ii) potential density, and (iii) neutral density (?) surfaces are compared. Differences between the level surface representations are similar to those noted in previous studies. Differences between the density surface representations (not previously studied) are major at high densities (? or σ? > 27.0). The residual Deacon cell of the first version has completely vanished in the GM version. The separate direct Antarctic and deep circulation cells of the first version are fully merged in the GM version. In these respects, the solution of the GM version is broadly more consistent with that of the FRAM eddy-permitting simulation. However, introduction of the GM scheme does not aid long-standing model problems of excessive upwelling of deep water into the thermocline in the Pacific and excessive penetration of Antarctic Bottom Water into the North Atlantic. The marked differences between the versions noted above imply marked differences in the dianeutral transport of fluid. A direct calculation of dianeutral transport shows that this transport is much weaker in the Southern Ocean and in the western boundary currents in the GM run. A breakup of the dianeutral transport into components resulting from the individual model mixing processes shows that the sole factor responsible for these qualitative changes is the absence of dianeutral motion induced by horizontal diffusive fluxes in the GM version. The interior dianeutral transport in this version is characterized by widespread and gentle upward motion induced by vertical diffusive fluxes. The results are shown to be insensitive to details of the surface boundary restoration. Consequences for our understanding of (i) the role of the Deacon cell as a tracer transport mechanism and (ii) the nature of the eddy-induced transports as provided by the GM scheme are discussed. In particular, although the total effective transport in the ocean interior is nearly isoneutral in the GM case, the eddy-induced part of that transport is shown to have a large dianeutral component and so cannot be interpreted as an isoneutral ?bolus? transport.