39.9.8 Overturning streamfunction in the $(\phi ,\rho ^{(p)})$ plane

Another choice is to set $s = \rho^{(p)}$, where $\rho^{(p)}$ is the density referenced to a particular pressure level. The only difference with the $s = \rho^{(0)}$ discussion is that the reference pressure for evaluating the thermal and saline expansion coefficients is altered.