Ac. Hirst et al., THE MERIDIONAL OVERTURNING CELLS OF A WORLD OCEAN MODEL IN NEUTRAL DENSITY COORDINATES, Journal of physical oceanography, 26(5), 1996, pp. 775-791
A comparison is made of the meridional overturning circulation in a co
arse-resolution World Ocean model when the integration is performed al
ong (i) level, (ii) potential density, and (iii) neutral density surfa
ces. In the level-surface calculation, all the usual cells are evident
, including the Atlantic ''conveyor,'' the Deacon cell, and the direct
Antarctic cell. In the potential or neutral density calculations, all
cells remain present; however, the Deacon cell is greatly reduced in
strength (to just a few Sverdrups). An analysis of the thermodynamics
underlying the dianeutral motion is conducted. Most dianeutral motion
results from fluxes associated with the vertical diffusivity and the (
unphysical) horizontal diffusivity. Caballing is not important, despit
e the inclusion of isopycnal diffusivity. The mechanism of the residua
l Deacon cell involves densification near 40 degrees S resulting from
fluxes associated with the horizontal diffusivity. Horizontal diffusiv
ity results in substantial dianeutral motion in several other parts of
the ocean. Most significant is motion toward lesser density in the fa
r Southern Ocean, which integrates zonally and between 67 degrees S an
d 57 degrees S to give a transport of about 25 Sv across density surfa
ces. This transport dominates other dianeutral transports at high dens
ity in the ocean interior and indicates serious distortion of the solu
tion by the horizontal diffusivity. A second model run is conducted wh
ere the horizontal diffusivity is reduced to near the (experimentally
determined) limit for the numerical integrity of water properties on t
he large scale. Dianeutral transports associated with horizontal diffu
sivity generally decline modestly. In neutral density coordinates, the
Deacon cell now vanishes almost completely. The Deacon cell of the le
vel-surface integration results mainly from large-scale isopycnal moti
ons occurring on sloping density surfaces, which superpose to yield a
cell upon zonal integration at constant depth. Finally, it is apparent
that the neutral density coordinate provides a clearer picture of the
ocean circulation than do potential density coordinates, because of i
nherent ambiguity in choosing the reference pressure of potential dens
ity.