Strong correlation between potential vorticity (Q) and density field (R) at the surface in the assumption that the upper-ocean velocity does not vary too much in depth. They key explanation for this correlation is:

“the explanation relies on the similar forcings by a mean gradient of the surface buoyancy and interior PV and on the action of the eddy field that stirs these tracers in the same manner.”

The pertubation Q’ and R’ are advected by the same eddy field. The large-scale forcing gives the relation between the large-scale Q and surface R and thus the relation between Q’ and surface R’.

**Idea:** plots in the subtropical ocean and upper tropics, surface R’ and Q’ at various depths. See how deep the relation should hold.

“We have shown that large-scale unstable baroclinic flows possess the property of having potential vorticity mesoscale anomalies at depth correlated with surface buoyancy –or density– anomalies. Indeed, it is the structure of the large-scale forcing, in particular the similarity between the large-scale PV and surface density forcings, that leads to this correlation.”