Water density
Earth's ocean covers more than 70% of our planet's surface. There are five major ocean basins. The Pacific Ocean is the largest. It’s so large that it covers a third of the Earth's surface. The Atlantic Ocean is east of the Americas and west of Europe and Africa. The Indian Ocean is south of Asia and the Middle East and east of Africa. The Arctic Ocean is in the north polar region. The Southern Ocean surrounds Antarctica in the south polar region.
Seawater is salty. Ocean water is about 3.5% salt. That means that if the oceans dried up completely, enough salt would be left behind to build a 180-mile-tall, one- mile-thick wall around the equator. And more than 90 percent of that salt would be sodium chloride, or ordinary table salt. Salinity is the saltiness of the water. Scientists can measure salinity of seawater as well as its density and temperature using an instrument called a CTD. Overall, the chemistry of the seawater depends on the minerals and gases that become dissolved in it over time
The density of ocean water is rarely measured directly. If you wanted to measure the density of ocean water, you would have to collect a sample of sea water and bring it back to the laboratory to be measured. Density is usually calculated from in situ measurements of salinity, temperature and pressure. These in situ measurements are often made with a CTD instrument, where the instrument is placed in the ocean water from a ship or a platform. Once salinity, temperature and pressure have been measured, density can be derived from an equation called the equation of state of sea water.
Whereas the circulation of surface of the ocean are driven by winds, the circulation of the deep waters are driven by density differences. Circulation in the depths of the ocean is referred to as thermohaline circulation. Water tends to move horizontally 
throughout the deep ocean, moving along lines of equal density. Vertical circulation is thus limited because it is easier for water to move along lines of constant density (isopycnals) than across them.
Temperature has a greater effect on the density of water than salinity does. So a parcel of water with higher salinity can actual float on top of water with lower salinity if the parcel with higher salinity is quite a bit warmer than the lower salinity parcel.
You can see density increases with increasing depth. The pycnocline are layers of water where the water density changes rapidly with depth. This density-depth profile is typical of what you might expect to find at a latitude of 30-40 degrees south.
At the surface of the ocean, temperature measurements are usually taken by thermometers placed on buoys. Recently, the Argo program has taken on the task of monitoring the state of ocean surface waters across the globe. The Argo program is deploying floats that measure salinity and temperature throughout the surface layer of the ocean. 2500 floats will be deployed all over the ocean by 2003. Each float is programmed to sink 2000 meters down, drifting at that depth for about 11 days. The float then makes its way to the surface measuring temperature and salinity the whole time. Data is transmitted to a satellite once the float reaches the surface, so that scientists have access to the state of the ocean within hours of the data collection. Each float will last 6-7 years. At a greater depth in the water, measurements are often made with a CTD instrument, where the instrument is placed in the ocean water from a ship or a platform.
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