Tropical ocean waters are more saline than colder waters because tropical waters

get salt from currents

get more sediments

have more fish

evaporate more rapidly

evaporate more rapidly

Explanation:

the faster the water evaporates the more salt it leaves behind.

The reason for more salinity of Tropical Ocean when compared to cold water is rapid evaporation.  

The rate of evaporation is very high in case of Tropical Ocean than the cold water. Tropical seas receive more solar radiation consistently. The water present in tropical seas is warm, around 27°C (80.6ºF).

Due to this reason, the rate of evaporation is very high in which the salt at the end of evaporation remains in the ocean and leads to high salinity of the tropical ocean.

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