P

4.18 x 103 3.33 x 105 2.25 x 106 0.999 x 103 10-3 10-6 1.4 x 10-7

FIGURE 9.2. Contours of seawater density anomalies (a = p - pref in kgm-3) plotted against salinity (in psu = gkg-1 ) and temperature (°C) at the sea surface. Note that seawater in the open ocean has a in the range 20—29 kgm-3, T in the range 0—30°C and S in the range 33—36 psu. The panel on the right zooms in on the region of oceanographic relevance. An approximation to the equation of state in the vicinity of the point ao (To, So) is given by Eq. 9-5.

FIGURE 9.2. Contours of seawater density anomalies (a = p - pref in kgm-3) plotted against salinity (in psu = gkg-1 ) and temperature (°C) at the sea surface. Note that seawater in the open ocean has a in the range 20—29 kgm-3, T in the range 0—30°C and S in the range 33—36 psu. The panel on the right zooms in on the region of oceanographic relevance. An approximation to the equation of state in the vicinity of the point ao (To, So) is given by Eq. 9-5.

However for many purposes it suffices to assume that density is independent of pressure.

The dependence of the density of seawater on S and T at the surface of the ocean is shown in Fig. 9.2. What is actually plotted is the density anomaly a (see Section 7.3),

the difference between the actual density and a reference value pref = 1000kgm-3. From Fig. 9.2 we see that:

1. Salty water is more dense than fresh water; warm water is (almost always) less dense than cold water.

2. Fresh water (S = 0) has a maximum density at about 4°C: fresh water that is colder than this is less dense. This is why ice forms on the top of freshwater lakes. Cooling from the surface in winter forms ice rather than denser water.

3. In the (rather narrow) range of temperatures and salinities found in the open ocean (0-30° C and 33-36 psu, see Figs. 9.3 and 9.4), temperature typically influences density more than salinity. At the sea surface, a is typically 26 kg m-3 and varies monotonically with temperature.

The thermal expansion coefficient of water, aT, defined by:

Pref dr

(S and p kept constant) has a typical value of 1 x 10-4° C-1 and is larger at higher temperatures (note how the isopleths of a in Fig. 9.2

FIGURE 9.3. Average (a) annual-mean, (b) DJF, and (c) JJA sea-surface temperature (°C) from the Comprehensive Ocean-Atmosphere Data Set (COADS). Note that there are data void regions in the proximity of ice-covered areas, particularly around Antarctica in the southern hemisphere winter. The Antarctic ice edge is farthest north in September and occasionally crosses 60° S.

FIGURE 9.3. Average (a) annual-mean, (b) DJF, and (c) JJA sea-surface temperature (°C) from the Comprehensive Ocean-Atmosphere Data Set (COADS). Note that there are data void regions in the proximity of ice-covered areas, particularly around Antarctica in the southern hemisphere winter. The Antarctic ice edge is farthest north in September and occasionally crosses 60° S.

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