MadSci Network: Earth Sciences

Re: What has the greatest effect on density of seawater: degrees or salinity?

Date: Wed Feb 4 09:54:25 1998
Posted By: Karen Culver-Rymsza, Grad student oceanography
Area of science: Earth Sciences
ID: 885860977.Es

Dr. Edwards,

The short answer is salinity has the greatest effect on density. As 
you can see below, a change in salinity of 0.5ppt will change 
sigma-t by about 0.4 units at any temperature or salinity while the effect 
of a change of 1oC varies in salty and fresh water from decreasing 
salinity by approximately 0.3 sigma-t units to increasing density 
slightly. However, the effect of both temperature and salinity on density 
varies with changes in temperature and salinity. For example, the change in 
sigma-t for water at 0 ppt salinity and 20oC will change by -0.3 sigma-t 
units for every 10C increase in temperature. The same water (0ppt,20oC) 
will change by 0.4 sigma-t units for every 0.5ppt increase in salinity. 
But, as mentioned the response to changes in salinity and temperature 
varies so that a salty warm water parcel (30oC, 40ppt) is less sensitive to 
changes in salinity than a cold, fresh water parcel, but more sensitive to 
changes in temperature. The following table shoud help. It is drawn from 
"DESCRIPTIVE PHYSICAL OCEANOGRAPHY,An introduction" by Pickard and Emery.

Figure 3.1: Variation in sigma-t with variations in temperature and 
salinity as functions of temperature and salinity

                    CHANGE IN SIGMA-T AS A FUNCTION OF:
             1oC increase in temp         0.5ppt increase in salinity

Salinity      0       20       40             0       20      40

Temp (oC)
   30        -0.30   -0.33   -0.34          0.39     0.38     0.38
   20        -0.21   -0.24   -0.27          0.40     0.38     0.38
   10        -0.09   -0.14   -0.18          0.41     0.39     0.39
    0        +0.07   -0.01   -0.17          0.43     0.40     0.40

     I hope this answers your question and I also hope the table looks like 
a table. I checked a few intro Ocenography texts and they did not contain 
adequate information on this subject, so I urge you to consult an 
introductory physical oceanography text for more detailed information.

Good Luck!

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