Salinity of Seawater and Freezing
Point Depression –
A simple model using ArcGIS
GIS and the Marine Environment
University of Alaska Anchorage Spring 2004
John Field
Study Area
John Field
GIS 333 - Seawater and Salinity
Arctic Ocean Conditions
• Looking at surface water in the polar
Arctic Ocean, the Bering Sea and Gulf of
Alaska
• Arctic Ocean has freshwater influx from
large river systems
• Low salinity: 30-32ppt* vs 35ppt average
for world’s oceans
* ppt - parts per thousand
John Field
GIS 333 - Seawater and Salinity
Satellite image of Sea Ice
http://www.arcticice.org/seaice.htm
Barrow Sea Ice Cam
John Field
GIS 333 - Seawater and Salinity
Sea ice concentration for May, average 1978-1992
Schweitzer, Peter N., 1995, Monthly average polar sea-ice concentration: USGS Digital Data Series DDS-27, USGS Reston, Virginia.
John Field
GIS 333 - Seawater and Salinity
Terminology
• Degree Day – A measure of the departure of the mean
daily temperature form a given standard: one degree
day for each degree (C or F) of departure above or
below the standard for one day.
• Freezing Degree Day (FDD) – Days average temp is
below freezing, adjusted to -1.8°C for sea water
• Freezing Point Depression (FPD) – The amount of
temperature in degrees (C or F) below the normal
freezing point of a solution (for ex. water and salt).
www.wikipedia.org
John Field
GIS 333 - Seawater and Salinity
A method to calculate Freezing
Point Depression (FPD) based on
Salinity?
Chemistry approach base on Molarity, conversions of ppt to molar concentration.
There must be some existing models out there…
Tip #1: Don’t know where to look?
http://www.usna.edu/Oceanography/courses/
google it. ;-)
SO426_maksym/text/chapter3_iceformation.htm
John Field
GIS 333 - Seawater and Salinity
Tf –T0 = mC
Because of the presence of salts in seawater, the equilibrium freezing point is
depressed below that of fresh water. It turns out that for temperatures above -8.2 °C,
the freezing point of sea ice, Tf, can be approximated as a linear function of C
concentration.
Tf –T0 = mC
where m, a constant, is the slope of the solidus-liquidus line, and T0 is 0 °C.
In other words, the temperature at the ice/water interface is fixed by the interfacial
solute concentration. This means that for seawater at 35 ppt, typical of the Antarctic,
the freezing point is about -1.9 °C. In the Arctic it is -1.8 °C.
John Field
GIS 333 - Seawater and Salinity
Tf – T0 = mC
• Domain for the model
-8.2°C – 8.2°C (17.2-46.7F)
• C is the salinity in ppt
• m is the calculated slope
Calculate m aka slope with built in Excel
function SLOPE
Calculated field in Excel FPD_sal: The
freezing point depression due to salinity
John Field
GIS 333 - Seawater and Salinity
Checking slope
First result for slope -0.046672593
Recalculate slope using only two known
points from the graph
(0,0) (24.69, -1.33)
Which results in m = -0.05386
John Field
GIS 333 - Seawater and Salinity
Map of
seawater
salinity
John Field
GIS 333 - Seawater and Salinity
Freezing
Point
Depression
and Salinity
John Field
GIS 333 - Seawater and Salinity