Due to high surface evaporation, when the water evaporates more, salinity remains. Regarding the second question. Probably due to the lower level of rain. Salinity is lower in subtropical regions

Dr Ghasem Shekari thank you for your contribution to the discussion

In tropical zone, temperature is generally higher than the other parts and so the rate of evaporation is also higher, therefore, salinity is higher. Salinity is formally defined as the total concentration of dissolved ions in water, and it is typically expressed in units of parts-per-thousand or g/L. Freshwater has less than 1 g/L total salinity, and seawater in tropical areas can be 35 g/L salinity or more. The equatorial region of the ocean has lower salinity than the tropical region. This is because although evaporation is high at the equator, precipitation is also high. High rainfall reduces salinity in equatorial regions; hence overall the salinity is less than in tropical regions. Thus, salinity decreases with depth in the equatorial and tropical regions. The rate of evaporation is relatively higher at high latitudes. The increase in evaporation rate results in increasing the salinity. In subtropical latitudes, high surface evaporation creates high salinity near the sea surface. In sub polar latitudes, high precipitation creates low salinity near the sea surface. As these waters flow into the ocean interior, they create layers of high and low salinity. This is because equatorial regions also get a high volume of rain on a regular basis, which dilutes the surface water along the equator. So the higher salinities are found at subtropical, warm latitudes with high evaporation and less precipitation. Ocean water is more salty in the subtropics due to higher evaporation of water into the atmosphere, with salt left behind in the ocean. In the tropical regions, the temp is high, so the evaporation rates are high enough to increase salinity. Along the equator, there is a lot of rain, which causes runoffs, which causes a decrease in salinity because of the added water.There is more evaporation in the subtropics which increases salinity and more precipitation in equatorial which decreases salinity.