OC202 TOPIC 1: SEAWATER PROPERTIES

Density is defined as the mass of water
per unit volume and has units of grams per cubic centimeter (g /cm3), kilograms per liter (kg/ L) or kilograms
per cubic meter (kg/m3).The
density of freshwater at 4° C is 1.0000 g/cm3
or 1.000 kg/liter or 1000 kg/m3.
Why do you think the mass of water is defined at a specific
temperature?
The range of density in the oceans is
from about 1.020 to 1.070 g/cm3The changes in density are caused mainly by variations in
pressure, salinity and temperature:
·colder water more
dense
·saltier water more
dense
·higher pressure
causes density increase - pressure increases with depth due to the mass of
water above
Temperature
Effects on Density
Temperature changes effect seawater
density:as water cools its density
increases.As water cools, H2O molecules pack more closely together
(because the molecules are vibrating less at lower temperatures) and take up
less volume.The same number of water
molecules in smaller volume results in a higher density.
How much does the seawater density
increase upon cooling from 20° to 0°C?Seawater density increases from 1.0240 g/cm3at 20°C to 1.0273 g/cm3 at 0°C at a
constant salinity.Globally, there is an average
of about a 20°C temperature decrease from the surface to bottom of ocean. The
density increase with depth caused by the temperature decrease plays the
greatest role in determining the density of a sample of water.
Thus the increase in
density due to the decreasing temperature with depth dominates over the
salinity decrease and makes the deeper water more dense
than surface water.This means that in most regions the ocean is stable, that is, it
will take energy to mix the ocean vertically.This is not necessarily the situation in the polar
regions
Depth vs Temperature
Globally, the entire temperature range
for ocean water ranges from ~ -2°C to +40°C, which is much smaller than the
temperature range for air, which varies from -60°C to +60°C.
Surface water temperatures vary much
more than deep water temperatures.Most
of the ocean is warm at the surface and colder at increasing depths. The region
where temperature decrease is greatest with depth is called the thermocline.The rate of change of
temperature with depth is called the temperature gradient.The steepness of the
depth gradient in temperature depends on location.It is greatest in the warm tropical ocean
(warm at the surface and colder with increasing depth) and least in the cold
polar ocean (somewhat uniformly cold at the surface and at increasing depths).
Salinity
Effects
Salt in seawater
makes it denser than freshwater.How
much salt is in seawater? Typically seawater contains between 33 to 37 grams of
salt per liter of seawater, although
the extremes of salinity can range from 28 to 40 g/L.Oceanographers measure salinity in parts per
thousand (ppt), thus typical seawater is between 33
to 37 ppt.
To
make seawater, start with freshwater and add 35 grams
of salt to one liter (1 kg) of this freshwater (35 grams per 1000 grams).This resulting seawater is denser than
freshwater because of added mass of dissolved salt.
Typically,
the salinity decreases from the surface ocean to deep waters is very small,
from about 36 g/L (ppt) at the surface to 35 g/L (ppt) in the deep water, thus there is a very small density decrease with depth given a constant temperature.
The
salinity of seawater also affects its freezing point temperature.Freshwater
has a freezing point at 0°C.Seawaters
freezing point is at ~ -2°C.The
freezing point versus salinity explains why it is easier
to form ice on a lake (freshwater) than on a bay (seawater).
Density
and Water Movement
The density of
seawater determines its tendency to move vertically.If density of water at surface is higher than
below, the water will sink to a level of its own density.This situation the water column is unstable .
If density of water
at surface is lower than below, the water will not sink.In this situation the water column is stable .In this
situation it takes energy input (usually from the wind) to "push"
water downward
-e.g., like submerging a rubber duck in bathtub (you supply energy).
Sinking of surface water generally
occurs where there is cold air to cool water at surface.This situation found at high latitudes near
the poles.At these polar sites, surface
waters cool and become dense enough to sink thousands of meters.Sinking of surface waters is a very important
mechanism to replenish waters in the deep sea .
In contrast, for most of the ocean
(within ~50° of the equator) the surface waters are much warmer and less dense
than the cold waters found at depth.Under these conditions surface waters do not sink and thus there is no
direct contact with waters in the deep sea.
What controls surface salinity?Mainly the relative rates
of evaporation versus precipitation.When the rate of evaporation is greater than the
precipitation, then surface ocean salinity increases.When the rate of precipitation is greater
than the rate of evaporation, then the surface ocean salinity decreases.
·Surface heating and
precipitation promote water column stability by lowering the density of surface
seawater.
As pressure increases, so does water
density.The water molecules pack
together tighter as pressure increases -the pressure increase with depth, due
to the weight of the water above, and causes the greatest density changes in
seawater with depth (greater than the density changes due to temperature and
salinity changes).
Seawater Profiles vs
Depth
Thermoclineis a layer
within a body of water or air where the temperature changes rapidly with depth.
Because water is not perfectly transparent, almost all sunlight is absorbed in
the surface layer, which heats up. Wind and waves circulate the water in the
surface layer, distributing heat within it somewhat, and the temperature may be
quite uniform for the first few hundred feet. Below this mixed layer, however,
the temperature drops very rapidlyperhaps as much as 20 degrees Celsius with
an additional 150 m (500 ft) of depth. This area of rapid transition is the
thermocline.Below the thermocline, the
temperature continues to drop with depth, but far more gradually. In the
Earth's oceans, 90% of the water is below the thermocline. This deep ocean consists
of layers of equal density, being poorly mixed.
Pycnoclineis a layer where there is a rapid change in water density
with depth. In freshwater environments such as lakes this density change is
primarily caused by water temperature, while in seawater environments such as
oceans the density change may be caused by changes in water temperature and/or
salinity.
Haloclineis a
vertical salinity gradient. Because salinity (in concert with temperature)
affects the density of seawater, it can play a role in its vertical
stratification.Essentially, lower
salinity water (= lower density) "floats" on top of higher salinity
water (= higher density). The magnitude of the resulting density gradient plays
an important role in determining the impact of vertical mixing. A strong
salinity gradient will resist mixing, while a weak gradient can be mixed more
easily.Typically, ocean vertical
structure is dictated by temperature effects on density, but salinity and
haloclines play a dominant role in certain regions of the world ocean. The
sub-Arctic North Pacific is one such region.