beta: $\beta\ge 0$

Expansivity of salt. This is used in the gravity term on the right hand side of the Navier-Stokes equations in the form

$$-g\beta\left(s-s_0\right) ,$$

where $s(x,y,z,t)$ is the salinity and $s_0(z)$ is the initial salinity. If $\beta=0$ advection of salinity is not computed.