Questions about Stommel’s Two-Box Model (North Atlantic Current)

I have a question about a module on Stommel's Two-Box model.

I am uncertain about the following density equation: $ \Delta \rho = | R \Delta S – \Delta T| $

A larger difference between temperatures/salinity levels across the two regions of the North Atlantic should both increase the density difference. Why is this a subtraction? Is this because of the opposite relationship between salinity/temperature at each pole (i.e., high salinity increasing the density at the equatorial region with high temp decreasing density at the equatorial region, and vice versa at the poles)?