Subtropical waters are typically more saline than equatorial waters because of

Subtropical waters are typically more saline than equatorial waters because of the bigger temperature and much lower precipitation.
The precipitation manages to lower the salinity on the surface of the water even create ''pools'' with fresh water in the middle of the saline water. In the equatorial region there's rain almost every day, so the salinity is lower, while in the subtropical regions there's seasonal rain and it is in much lower amount, the temperature is higher which makes the water evaporate more and the salt to be more concentrated, thus the higher salinity.