The geostrophic dynamics of meridional transport variability in the sub-polar North Atlantic

The North Atlantic meridional overturning 6 circulation (MOC) is believed to play an important role in regulating the Earth’s climate. Yet, there is still much uncertainty regarding the dynamics of the MOC and its variability. It is well established, however, that, through geostrophy, the zonally integrated meridional transport at a particular latitude and depth can be determined from the east-west bottom pressure difference across the basin. Therefore, rather than consider the MOC as a large-scale system, this paper focuses on the dynamics of this geostrophic relationship in two numerical ocean models at a single latitude (50◦14 N) in the sub-polar Atlantic. First, it is shown that the bottom pressure on the western boundary is sufficient to recover, with high fidelity, the interannual meridional transport variability at 50◦17 N over a one hundred year period in the climate model HadCM3. It is found that the variability of western boundary pressure is closely associated with density changes over the continental slope. These changes lead to a large zonal gradient in potential energy and an unfeasible, depth-mean velocity over the slope. The western boundary pressure, from which the meridional transport can be recovered, is generated as a compensation to this, and limits the depth-mean flow. This demonstrates that in numerical ocean models, at least, meridional transport variability is generated as a local response to density changes on the western slope. Whether this is a true representation of actual ocean variability is uncertain, but if it were, then meridional transport variability could largely be determined using only the density field on the western slope.