This is why seawater at Antarctica is very salty. The salt that is rejected forms brine beneath the ice and becomes more and more salty until it becomes so dense that it sinks, displaces less dense seawater that moves to the surface. As ice forms in the sea, the salt cannot form part of the ice crystal so the ice is almost pure water.The salinity of seawater is about 3.5% and it freezes at about -1.9☌.The addition of salt to water makes a solution that is denser than fresh water – it freezes at a lower temperature.The hydrogen of the water molecule is attracted to chlorine ions and the oxygen to the sodium ions. This allows the sodium and chlorine ions to be pulled apart by the water molecules. Salt dissolves in water because the attraction between the water molecules and the sodium ions or chlorine ions is stronger than the attraction between the sodium ions and chloride ions in the lattice.For every sodium ion you will find one chlorine ion (1:1 ratio). Salt is made up of many sodium and chlorine ions stacked together in a lattice.The lack of a pycnocline (or a thermocline) allows cold, nutrient-rich deep water to more easily mix with the surface water, leading to higher primary production in polar regions. In the high latitudes the water is uniformly cold at all depths, so there is little density stratification. As stated above, this stratification prevents nutrient-rich water from reaching the surface and as a result tropical regions often have low productivity. In the tropics the surface water is warm and low density, and there is a pronounced thermocline separating it from the colder, denser deep water. In this way, nutrient-rich deep water may be prevented from coming to the surface to support primary production.Īs with temperature, there are also latitudinal differences in density. By creating a stratified water column, the thermocline and pycnocline together create a barrier that prevents mixing between the warmer, less dense surface water and the colder, denser bottom water. This vertical movement of water masses based on density (as determined by temperature and salinity) is referred to as thermohaline circulation, which is the topic of section 9.8. If denser water happened to form at the surface, the water masses would be unstable, and the denser water would sink to the bottom, to be replaced by less dense water at the surface. The profile above represents a stable state, or a high degree of stratification, where the warm, low density layer sits atop the colder, denser layer.

The warm surface water causes a decrease in surface density (PW). Figure 6.3.2 Representative density profile for the open ocean at mid-latitudes. Below the pycnocline, density may be fairly constant (as is temperature), or it may continue to increase slightly towards the bottom. The pycnocline coincides with the thermocline, as it is the sudden decrease in temperature that leads to the increase in density. The exact value is actually slightly lower. The density of water is about 1 gram per milliliter (g/ml), 1 gram per cubic centimeter (g/cm 3 ), 1000 kg/m 3, or 62 pounds per cubic foot (lb/ft 3 ). For all practical purposes, this is the same as the weight of water per unit of volume. As depth increases, there is a region of rapidly increasing density with increasing depth, which is called the pycnocline. The density of water is the mass of water per unit of volume. Density is lowest at the surface, where the water is the warmest.

Since temperature has the greatest effect on density, density profiles are usually mirror images of temperature profiles (Figure 6.3.2).

Colder polar regions display higher densities than warmer tropical zones (By Plumbago (Own work), via Wikimedia Commons). However, if not for the slight compression of water due to pressure, sea level would be approximately 50 m higher than it is today! That leaves temperature and salinity as the primary factors determining density, and of these, temperature has the greatest impact (Figure 6.3.1).įigure 6.3.1 Global sea surface density. The snow density is derived by collocated MRR and Parsivel from the ICE-POP 2017/2018. Pressure has the least impact on density as water is fairly incompressible, so pressure effects are not very significant except at extreme depths.

The density of seawater can be increased by reducing its temperature, increasing its salinity, or increasing the pressure. The density of fresh water is 1 g/cm 3 at 4 o C (see section 5.1), but the addition of salts and other dissolved substances increases surface seawater density to between 1.02 and 1.03 g/cm 3. Density refers to the amount of mass per unit volume, such as grams per cubic centimeter (g/cm 3).

Water density in gcm3