Temperature of Ocean Water

There is a boundary between surface waters of the ocean and deeper layers that are not mixed. The boundary usually begins around 100-400 meters and extends several hundred of meters downward from there. This boundary region, where there is a rapid decrease of temperature, is called the thermocline. 90 % of the total volume of ocean is found below the thermocline in the deep ocean. Here, temperatures approach 0 degrees Celsius. So even though surface waters can be a comfortable 20 degrees Celsius (good for swimming in!), the majority of our ocean water has a temperature between 0-3 degrees Celsius (32-37.5 degrees Fahrenheit).

The density of ocean water continuously increases with decreasing temperature until the water freezes. Ocean water, with an average salinity of 35 psu, freezes at -1.94 degrees Celsius (28.5 degrees Fahrenheit). At very high latitudes, ocean water can reach these low temperatures and freeze. Dissolved salts in the water tend to be rejected by the forming ice so that sea ice is only about 1 % salt. An interesting tidbit for those of you interested in survival tactics - because of the lessened amount of salt, melted sea ice would be fit to drink even if sea water is not! Sea ice formation at high latitudes ultimately drives circulation of the deep waters of the ocean.

At the surface of the ocean, temperature measurements are often 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. 3,000 floats will be deployed all over the ocean by 2003. Each float is programmed to sink 2,000 meters down, drifting at that depth for about 10 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 4-5 years. At a greater depth in the water, 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.