Density

Measurements Impacts Beyond the Gulf References

What is density?

Density is weight divided by the amount of space it occupies. Density can be expressed as kilograms per cubic meter (mass divided by volume). For instance, if a cubic meter (~264 gallons) of seawater weighs 1025 kg (~1 ton), it has a density of 1025 kg/m^3. Another way you will see density expressed is as sigma-t, which is simply the density minus 1000. Seawater with a density of 1025 kg/m^3 has a sigma-t of 25.

Gulf of Maine water density

Water density depends on temperature, salinity, and pressure. As salinity increases, density increases; and as temperature increases, density decreases. This generally means that colder water will sink below warmer water, and saltier water will sink below fresher water. Environmental forces that change water temperature and salinity -- e.g., air temperature, freshwater input from rivers -- can produce layers of different densities in the water column.

Waters in the Gulf are usually well mixed by wind and tides and thus only seasonally layered. Higher temperatures in the summer and fall warm the surface waters and contribute to a layered water column. Through winter and spring, lack of sunlight and low air temperatures cool surface waters. The sinking of these denser waters and the increased wind from storms breaks down the layers, mixing Gulf waters.

At left, summer data from the Gulf of Maine. Low density waters, marked by high temperature and low salinity, form the upper layers of the water column.
At right, spring data from the Gulf of Maine. Other than relatively warm waters in Jordan Basin, temperatures are well mixed everywhere. High density, high salinity waters are found at depth everywhere (especially Jordan Basin).
Summer Temperature
Summer Salinity 		Spring Salinity
Spring Salinity

Click the white dots to discover their sigma-t values during these seasons

What does GoMOOS measure?

If we had to figure out the density of seawater by weight and volume, it would take a LOT of work. Instead, we can calculate the density based on the temperature and salinity of the water. Every hour, many GoMOOS buoys measure salinity and temperature at several depths, from which scientists calculate density.

Impacts of density

When ocean waters are layered, sunlit surface waters are separated from deeper nutrient-rich waters. This prevents phytoplankton from getting what they need to thrive: sunlight AND nutrients. Seawater mixing can bring nutrients in the sunlit zone. This often results in a phytoplankton bloom, a "population explosion" of phytoplankton.

Beyond the Gulf: How does density affect global circulation?

Thermohaline

Ocean waters are not consistently layered by density. The surface layer of the ocean is warmed by the sun and moved by wind-driven currents. If warm air and wind cause evaporation, surface water salinity will increase, density will increase, and uppers water will sink. Likewise, when frigid air cools the ocean surface, density will increase and water can sink; in this case, deeper waters will rise to take their place. This is called "thermohaline" circulation: movement of water due to temperature and salinity-related effects on density.

References