The temperature of the sea is dependent on the amount

A characteristic of seawater is that it is salty. Salinity is usually measured in parts per thousand (ppt), and the open ocean has about 35 grams (1.2 oz) solids per litre, a salinity of 35 ppt. The Mediterranean Sea is slightly higher at 37 ppt, and the Dead Sea has as much as 300 grams (11 oz) dissolved solids per litre (300 ppt). Although sodium chloride is the main salt present, constituting about 85 percent of the solids in solution, there are also 5 grams (0.18 oz) per litre of the chlorides of other metals such as potassium and magnesium and 3 grams (0.11 oz) of sulphates, carbonates, bromides and other salts. A kilogram (2.2 lb) of salt can thus be found in 28 litres or one cubic foot of typical ocean water. Despite variations in the levels of salinity in different seas, the relative composition of the dissolved salts is stable throughout the world's oceans.[15]

The circumstances that cause the salinity of a body of water to vary include evaporation from its surface (increased by high temperatures, wind and wave motion), precipitation on its surface, the freezing or melting of sea ice, the melting of glaciers, the influx of fresh river water, and the mixing of bodies of water of different salinities. The Baltic Sea, for example, is in a cool climatic region with low evaporation, and has many rivers flowing into it and intermittent replenishment from the open ocean. The occasional influx of water from the North Sea creates a cold, dense under layer that hardly mixes with the surface layers. The uppermost layer may have a salinity of 10 to 15 ppt, with even lower levels in the estuaries.[16] The Red Sea experiences high atmospheric temperatures causing high evaporation but little precipitation; few rivers flow into it, and the Bab-el-Mandeb, joining it to the Gulf of Aden, is narrow. The Red Sea's salinity averages 40 ppt.[17]

The temperature of the sea is dependent on the amount of solar radiation falling on the surface. In the tropics, with the sun nearly overhead, the temperature of the surface layers can rise to over 30 °C (86 °F) while near the poles the temperature in equilibrium with the sea ice is about -2 °C (28 °F). Cold water is denser than warm water and tends to sink. There is a continuous circulation of water in the oceans. Warm surface currents cool as they move away from the tropics, and the water becomes denser and sinks. The cold water moves back towards the equator as a deep sea current, driven by changes in the temperature and density of the water, before eventually welling up again towards the surface. Deep seawater has a temperature between -2 °C (28 °F) and 5 °C (41 °F) in all parts of the globe.