6.3 Density

Density describes the lot of mass every unit volume, such together grams every cubic centimeter (g/cm3). The thickness of new water is 1 g/cm3 in ~ 4o C (see section 5.1), yet the enhancement of salts and other liquified substances increases surface seawater density to between 1.02 and also 1.03 g/cm3. The thickness of seawater have the right to be boosted by to reduce its temperature, enhancing its salinity, or increasing the pressure. Pressure has the least impact on density as water is reasonably incompressible, therefore pressure results are not very significant except at extreme depths. However, if no for the slight compression of water as result of pressure, sea level would be about 50 m greater than it is today! That leaves temperature and salinity as the primary factors determining density, and also of these, temperature has the greatest influence (Figure 6.3.1).

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Figure 6.3.1 an international sea surface ar density. Cooler polar areas display greater densities than warmer tropical zones (By Plumbago (Own work) , via Wikimedia Commons).

Since temperature has actually the greatest result on density, density profiles room usually mirror pictures of temperature file (Figure 6.3.2). Thickness is lowest at the surface, whereby the water is the warmest. As depth increases, over there is a region of quickly increasing thickness with raising depth, i beg your pardon is referred to as the pycnocline. The pycnocline corresponds with the thermocline, together it is the sudden decrease in temperature the leads to the increase in density. Listed below the pycnocline, thickness may be fairly continuous (as is temperature), or the may continue to increase slightly in the direction of the bottom.

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Figure 6.3.2 Representative density profile because that the open s at mid-latitudes. The warm surface water reasons a to decrease in surface density (PW).

The profile over represents a secure state, or a high degree of stratification, whereby the warm, low thickness layer sit atop the colder, denser layer. If denser water occurred to kind at the surface, the water masses would be unstable, and also the denser water would certainly sink to the bottom, come be replaced by less dense water at the surface. This vertical activity of water masses based on density (as identified by temperature and salinity) is described as thermohaline circulation, i beg your pardon is the object of ar 9.8. By developing a stratified water column, the thermocline and also pycnocline together develop a barrier that prevents mixing in between the warmer, less dense surface water and also the colder, denser bottom water. In this way, nutrient-rich deep water might be prevent from comes to the surface to support major production.

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As through temperature, there are likewise latitudinal differences in density. In the tropics the surface ar water is warm and also low density, and also there is a express thermocline separating that from the colder, denser deep water. As declared above, this stratification avoids nutrient-rich water from reaching the surface and as a result tropical regions frequently have short productivity. In the high latitudes the water is uniformly cold at every depths, so there is little density stratification. The lack of a pycnocline (or a thermocline) allows cold, nutrient-rich deep water to an ext easily mix with the surface water, top to higher primary production in polar regions.


a region in the water obelisk where there is a large change in density over a little change in depth (6.3)


a an ar in the water pillar where there is a dramatic change in temperature over a small change in depth (6.2)