An ocean's salinity is affected by water and air temperature, the volume of water that flows into it, its age and the chemical composition of surrounding geological formations. Because the effect of these factors varies over the area of an ocean, different levels are found within the ocean.
The hotter the air surrounding an ocean, the more evaporation that takes place, particularly when accompanied by high winds. Only the water is lost in this process, so the salts become more concentrated.The Mediterranean Sea's high salinity level is attributed to the combination of warm air and low rainfall levels. Oceans that receive a lot of rain tend to have lower salinity levels near the surface where the water is more diluted. This effect is intensified by the lower density of fresh water, which stays at the surface while the more dense salt-rich water sinks.
Thawing ice burgs also add freshwater to oceans, as do the rivers that empty into it. The Baltic Sea has a very low salinity due to the hundreds of rivers that flow into it. In contrast, the water that enters the Dead Sea is full of various types of salts because of the mineral-rich river beds that feed into it. This sea has no outlet. The only water lost is through evaporation, making it the saltiest body of water on earth.