Density of the ocean water is determined mainly by temperature, salinity and pressure. However, other regional factors also determine the water density. These further govern the vertical movement of ocean water resulting in its stratification according to density.

• Temperature: Depends upon the heat exchange between sea surface and atmosphere. At higher temperature water expands reducing its density, while lower temperature increases density of water. Thus, the water at poles is denser than in tropics.
• Salinity: High salinity seawater is denser and sinks below lower salinity water, leading to stratification.
• Pressure: Increasing density values demonstrate the compressibility of seawater under the tremendous pressures present in the deep ocean.
• Other Factors: Rainwater, surface run-off brought by rivers flowing into oceans, and melt water from ice and snow are minor factors that lower down density. While, evaporation, cooling of surface water and process of ice formation tend to increase density of the ocean water. Partially or completely enclosed seas don’t intermix much with surrounding water. In the tropics they have higher evaporation resulting in higher density while near poles they have lesser evaporation resulting in comparatively low density.

Latitudinal Variation

Between 20 degrees and 30 degree N and S, water has high salinity due to high rate of evaporation, thereby leading to higher density.

1. Temperature of surface ocean water decreases between 45-degree N-S and poles. Lower surface temperature corresponds to high water density.
2. Salinity is lower than average 35 % in the equatorial waters due to high daily rainfall and and high relative humidity.

Seasonal Variation

• As salinity, temperature and density of water are interrelated; any change in one factor influences other.
• For instance, in Red Sea, salinity is as high as 41 ppt, while in the Arctic, it fluctuates from 0-35 ppt, seasonally. In hot and dry regions, where evaporation is high, the salinity sometimes reaches to 70 ppt. Ocean salinity declines due to rains as well as in summers when ice melts.

Relationship with Ocean Currents

• Waters of low salinity and density flow on the surface of waters of high salinity, while waters of high salinity flow at the deep oceans towards waters of low salinity. For example, the less saline waters of the Atlantic flow on the surface into the enclosed Mediterranean Sea, this is compensated for by an outflow of the denser bottom water from the Mediterranean.
• In the northern Atlantic Ocean, creation of gyres due to Gulf Stream, Canary Current and North Equatorial Current, increases the salinity.
• Also, in areas where convergence of ocean currents occurs, water of relatively higher density moves downwards, while, in areas of divergences, ascending motion of bottom water of higher density and lower temperature towards the surface.