Heat fluxes, evaporation, rain, river in flow, and freezing and melting of sea ice all influence the distribution of temperature and salinity at the ocean's surface. Changes in temperature and salinity can increase or decrease the density of water at the surface, which can lead to convection. If water from the surface sinks into the deeper ocean, it retains a distinctive relationship between temperature and salinity which helps oceanographers track the movement of deep water. In addition, temperature, salinity, and pressure are used to calculate density. The distribution of density inside the ocean is directly related to the distribution of horizontal pressure gradients and ocean currents. For all these reasons, we need to know the distribution of temperature, salinity, and density in the ocean.
Before discussing the distribution of temperature and salinity, let's first define what we mean by the terms, especially salinity.
6.1 Definition of Salinity
At the simplest level, salinity is the total amount of dissolved material in grams in one kilogram of sea water. Thus salinity is a dimensionless quantity. It has no units. The variability of dissolved salt is very small, and we must be very careful to define salinity in ways that are accurate and practical. To better understand the need for accuracy, look at Figure 6.1. Notice that the range of salinity for most of the ocean's water is from 34.60 to 34.80 parts per thousand, which is 200 parts per million. The variability in the deep North Pacific is even smaller, about 20 parts per million. If we want to classify water with different salinity, we need definitions and instruments accurate to about one part per million. Notice that the range of temperature is much larger, about 1°C, and temperature is easier to measure.
Writing a practical definition of salinity that has useful accuracy is difficult (see Lewis, 1980, for the details), and various definitions have been used.
A Simple Definition
A More Complete Definition
Salinity Based on Chlorinity
where chlorinity Cl is defined as "the mass of silver required to precipitate completely the halogens in 0.328 523 4kg of the sea-water sample."
As more and more accurate measurements were made, (6.1) turned out to be too inaccurate. In 1964 UNESCO and other international organizations appointed a Joint Panel on Oceanographic Tables and Standards to produce a more accurate definition. The Joint Panel recommended in 1966 (Wooster, Lee, and Dietrich, 1969) that salinity and chlorinity be related using:
This is the same as (6.1) for S = 35.
Salinity Based on Conductivity
where C (S, 15 , 0) is the conductivity of the sea-water sample at 15°C and atmospheric pressure, having a salinity S derived from (6.4), and C (35 , 15 , 0) is the conductivity of standard "Copenhagen" sea water. Millero (1996) points out that (6.3) is not a new definition of salinity, it merely gives chlorinity as a function of conductivity of seawater relative to standard seawater.
Practical Salinity Scale of 1978
The Practical Salinity Scale of 1978 is now the official definition:
where C (S, 15, 0) is the conductivity of the sea-water sample at a temperature of 15°C and standard atmospheric pressure, and C (KCl, t, 0) is the conductivity of the standard potassium chloride (KCl) solution at a temperature of 15°C and standard atmospheric pressure. The standard KCl solution contains a mass of 32.435 6 grams of KCl in a mass of 1.000 000kg of solution.
Lewis (1980) gives additional equations for calculating salinity at other temperatures:
An extension of (6.4) gives salinity at any pressure (see Millero 1996: 72).
The relationship between conductivity and salinity has an accuracy of around ± 0.003 in salinity. The very small error is caused by variations in constituents such as SiO2 which cause small changes in density but no change in conductivity.
Instruments for measuring salinity are calibrated using Normal Standard Seawater (P-series). The standard water is made from large samples of water from the north Atlantic, carefully diluted to S = 35, which is distributed in 275 ml sealed glass ampoules. Each is labeled for its conductivity ratio and salinity according to the Practical Salinity Scale 1978 and distributed worldwide by Ocean Scientific International in England since 1989. Each sample is carefully calibrated using the standard KCl solution.
|Department of Oceanography, Texas A&M University
Robert H. Stewart, firstname.lastname@example.org
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Updated on July 19, 2007