CHAPTER 3 Physical Properties of Seawater 3.1. MOLECULAR PROPERTIES in the oceans, because water is such an effective OF WATER heat reservoir (see Section S15.6 located on the textbook Web site Many of the unique characteristics of the ; “S” denotes supplemental ocean can be ascribed to the nature of water material). itself. Consisting of two positively charged As seawater is heated, molecular activity hydrogen ions and a single negatively charged increases and thermal expansion occurs, oxygen ion, water is arranged as a polar mole- reducing the density. In freshwater, as tempera- cule having positive and negative sides. This ture increases from the freezing point up to about molecular polarity leads to water’s high dielec- 4C, the added heat energy forms molecular tric constant (ability to withstand or balance an chains whose alignment causes the water to electric field). Water is able to dissolve many shrink, increasing the density. As temperature substances because the polar water molecules increases above this point, the chains break align to shield each ion, resisting the recombina- down and thermal expansion takes over; this tion of the ions. The ocean’s salty character is explains why fresh water has a density maximum due to the abundance of dissolved ions. at about 4C rather than at its freezing point. In The polar nature of the water molecule seawater, these molecular effects are combined causes it to form polymer-like chains of up to with the influence of salt, which inhibits the eight molecules. Approximately 90% of the formation of the chains. For the normal range of water molecules are found in these chains. salinity in the ocean, the maximum density Energy is required to produce these chains, occurs at the freezing point, which is depressed which is related to water’s heat capacity. Water to well below 0C (Figure 3.1). has the highest heat capacity of all liquids Water has a very high heat of evaporation (or except ammonia. This high heat capacity is the heat of vaporization) and a very high heat of primary reason the ocean is so important in fusion. The heat of vaporization is the amount the world climate system. Unlike the land and of energy required to change water from a liquid atmosphere, the ocean stores large amounts of to a gas; the heat of fusion is the amount of heat energy it receives from the sun. This heat energy required to change water from a solid is carried by ocean currents, exporting or to a liquid. These quantities are relevant for importing heat to various regions. Approxi- our climate as water changes state from a liquid mately 90% of the anthropogenic heating in the ocean to water vapor in the atmosphere associated with global climate change is stored and to ice at polar latitudes. The heat energy Ó 2011. Lynne Talley, George Pickard, William Emery and James Swift. Descriptive Physical Oceanography 29 Published by Elsevier Ltd. All rights reserved. 30 3. PHYSICAL PROPERTIES OF SEAWATER Salinity FIGURE 3.1 Values of density st 0 10 20 30 40 (curved lines) and the loci of maximum 30 density and freezing point (at atmospheric pressure) for seawater as functions of temperature and salinity. The full density r 3 is 1000 þ st with units of kg/m . 20 0 5 10 15 20 25 30 10 90% of ocean Temperature (°C) Temperature Temp. of max. density Mean T&S Freezing point Most 0 abundant –2 involved in these state changes is a factor in pressure is the Pascal, where 1 Pa ¼ 1 N/m2. weather and in the global climate system. Atmospheric pressure is usually measured in Water’s chain-like molecular structure also bars where 1 bar ¼ 106 dynes/cm2 ¼ 105 Pa. produces its high surface tension. The chains Ocean pressure is usually reported in decibars resist shear, giving water a high viscosity for where 1 dbar ¼ 0.1 bar ¼ 105 dyne/cm2 ¼ its atomic weight. This high viscosity permits 104 Pa. formation of surface capillary waves, with wave- The force due to pressure arises when there is lengths on the order of centimeters; the restoring a difference in pressure between two points. The forces for these waves include surface tension as force is directed from high to low pressure. well as gravity. Despite their small size, capil- Hence we say the force is oriented “down the lary waves are important in determining the pressure gradient” since the gradient is directed frictional stress between wind and water. This from low to high pressure. In the ocean, the stress generates larger waves and propels the downward force of gravity is mostly balanced frictionally driven circulation of the ocean’s by an upward pressure gradient force; that is, surface layer. the water is not accelerating downward. Instead, it is kept from collapsing by the upward pressure gradient force. Therefore pressure 3.2. PRESSURE increases with increasing depth. This balance of downward gravity force and upward pres- Pressure is the normal force per unit area sure gradient force, with no motion, is called exerted by water (or air in the atmosphere) on hydrostatic balance (Section 7.6.1). both sides of the unit area. The units of force The pressure at a given depth depends on the are (mass  length/time2). The units of pressure mass of water lying above that depth. A pressure are (force/length2) or (mass/[length  time2]). change of 1 dbar occurs over a depth change of Pressure units in centimeters-gram-second (cgs) slightly less than 1 m (Figure 3.2 and Table 3.1). are dynes/cm2 and in meter-kilogram-second Pressure in the ocean thus varies from near (mks) they are Newtons/m2. A special unit for zero (surface) to 10,000 dbar (deepest). Pressure PRESSURE 31 is usually measured in conjunction with other 6000 seawater properties such as temperature, salinity, and current speeds. The properties are often presented as a function of pressure rather than depth. 4000 Horizontal pressure gradients drive the hori- zontal flows in the ocean. For large-scale currents (of horizontal scale greater than a kilo- meter), the horizontal flows are much stronger Depth (m) than their associated vertical flows and are 2000 usually geostrophic (Chapter 7). The horizontal pressure differences that drive the ocean currents are on the order of one decibar over hundreds or thousands of kilometers. This is much smaller than the vertical pressure 0 0 2000 4000 6000 gradient, but the latter is balanced by the down- Pressure (dbar) ward force of gravity and does not drive a flow. Horizontal variations in mass distribution FIGURE 3.2 The relation between depth and pressure, using a station in the northwest Pacific at 41 53’N, 146 create the horizontal variation in pressure in 18’W. the ocean. The pressure is greater where the water column above a given depth is heavier either because it is higher density or because it is thicker or both. TABLE 3.1 Comparison of Pressure (dbar) and Depth Pressure is usually measured with an elec- (m) at Standard Oceanographic Depths tronic instrument called a transducer. The accu- Using the UNESCO (1983) Algorithms racy and precision of pressure measurements is high enough that other properties such as Pressure (dbar) Depth (m) Difference (%) temperature, salinity, current speeds, and so 000forth can be displayed as a function of pressure. 100 99 1 However, the accuracy, about 3 dbar at depth, is not sufficient to measure the horizontal pressure 200 198 1 gradients. Therefore other methods, such as the 300 297 1 geostrophic method, or direct velocity measure- 500 495 1 ments, must be used to determine the actual flow. Prior to the 1960s and 1970s, pressure 1000 990 1 was measured using a pair of mercury ther- 1500 1453 1.1 mometers, one of which was in a vacuum 2000 1975 1.3 (“protected” by a glass case) and not affected by pressure while the other was exposed to the 3000 2956 1.5 water (“unprotected”) and affected by pressure, 4000 3932 1.7 as described in the following section. More 5000 4904 1.9 information about these instruments and methods is provided in Section S6.3 of the 6000 5872 2.1 supplementary materials on the textbook Web Percent difference ¼ (pressure À depth)/pressure  100%. site. 32 3. PHYSICAL PROPERTIES OF SEAWATER 3.3. THERMAL PROPERTIES OF A change of 1C is the same as a change of 1 K. A SEAWATER: TEMPERATURE, temperature of 0C is equal to 273.16 K. The HEAT, AND POTENTIAL range of temperature in the ocean is from the TEMPERATURE freezing point, which is around À1.7C (depen- ding on salinity), to a maximum of around 30C One of the most important physical charac- in the tropical oceans. This range is considerably teristics of seawater is its temperature. Temper- smaller than the range of air temperatures. As ature was one of the first ocean parameters to be for all other physical properties, the tempera- measured and remains the most widely ture scale has been refined by international observed. In most of the ocean, temperature is agreement. The temperature scale used most the primary determinant of density; salinity is often is the International Practical Temperature of primary importance mainly in high latitude Scale of 1968 (IPTS-68). It has been superseded regions of excess rainfall or sea ice processes by the 1990 International Temperature Scale (Section 5.4). In the mid-latitude upper ocean (ITS-90). Temperatures should be reported in (between the surface and 500 m), temperature ITS-90, but all of the computer algorithms is the primary parameter determining sound related to the equation of state that date from speed.