As inferred by the name "thermohaline" with
respect to density-driven currents in the ocean, the main properties of ocean
water we measure are temperature and salinity. Functionally salinity means
conductivity, because the salt content of ocean water is related directly to
how electrically conductive it is, and conductivity is easy to measure. One of
the most basic tools for oceanographers is the CTD, short for Conductivity-Temperature-Depth.
In research the CTD is used to infer density. In applied
science it is used more often to determine the speed of sound in water, for use
in acoustical applications. Electrical conductivity is measured as a proxy for
salinity. Temperature is measured directly, and depth is inferred from water
pressure. In this way scientists (both applied, as I am, and researchers) can
obtain a vertical set of measurements through the water (or "water
column", being a vertical section from sea surface to sea floor).
Because water is a polar molecule, salt dissolves readily in
it. About 85% of salt in the ocean is sodium chloride, which we know as table
salt. Since salt molecules are bonded ionically--the sodium (Na+) atom
surrenders an electron to the chlorine (Cl-)--water easily separates them and
carries them in solution. (This is why hydrocarbons like gasoline and oil do
not dissolve in water: hydrocarbons are not polar, so they are not sensitive to
water's hydrogen bonding and stratify according to density.) The sources of
ocean salt are thought to be eroded continental rock and volcanic outgassing
(including discharges from hydrothermal vents, which are heated by magma below
the sea floor).
Over billions of years, with the constant cycling of water
around the surface of the planet, the ionic content of the oceans has only
increased, because salt ions do not evaporate. For this reason, at any given
time, salt content of the ocean is considered constant. The total salt content
of the ocean is estimated to be 1.4*1020 g, or 1.4 sextillion grams.
The mean salinity of the ocean is 35 parts per thousand (ppt), or physically,
35 g salt/1000 g water. An alternate measure is the practical salinity unit, or
PSU, which is simply ppt - 1000. (1035 ppt = 35 PSU.)
Ocean salinity is not constant around the world. It is lower
where there is an influx of fresh water. For example, rain, a regular
occurrence in the northeast Pacific, produces a comparatively fresh layer on
the surface of the ocean, as does the discharge from large rivers like the Nile
or Amazon. And where evaporation dominates, like in the eastern Mediterranean
or Red Sea (or in extreme cases like the landlocked Dead Sea), salinity is
higher.
The ocean's mean density is 1029 grams per cubic centimeter
(g/cm3), rather more than pure water (1000 g/cm3). The
density changes with temperature. Seawater is at its densest around 4°C
(39°F).
It expands as it gets colder from there, with ice being about 90% as dense as
water (why 9/10 of icebergs are below the surface). Water also expands as it
gets warmer, which is why the ocean stratifies as it does with the warmest
water on top, and the coldest on the bottom.
A basic means of representing the state of water column at a location is the Temperature-Salinity Diagram, or TS diagram for short. A vertical section of water is profiled with a CTD and the results are plotted with temperature on the vertical axis, salinity on the horizontal. The diagram can be used in a number of ways, but the two most common are to plot a given water mass as a region, showing the temperature and salinity properties common to that type of water. In the first figure, North Atlantic Central Surface Water = NACSW, Mediterranean Intermediate Water = MIW, Antarctic Intermediate Water = AAIW, North Atlantic Deep Water = NADW, and Antarctic Bottom Water = AABW. Another method is to profile the CTD cast as a point plot, showing the changing properties of the water column with depth (which can be roughly inferred from temperature). In the second figure, the surface water smoothly blends into the intermediate water. The thermocline is more of a halocline--temperature is consistent while salinity and density increase dramatically--followed by colder, saltier bottom water. Plots from around the world will show vastly different water properties at different depths.
Tomorrow: dissolved oxygen.
Be well!
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