Question

why seas are more saline in tropical zones

Answer 1

This is partly because salinity levels in equatorial and tropical regions are roughly the same, though with slight variations due to more rainfall in the former. the map above clears your doubt.

If you notice, salinity is more towards the equator and the tropics and less towards the poles and the temperate regions.

According to the NOAA, salinity refers to the concentration of salt in seawater which is about 35 parts per thousand; meaning that about 3.5% of the weight of seawater comes from the dissolved salts. In every cubic mile of seawater, the weight of the salt would be about 120 million tons.