How do I calculate a seasonal average from certain composite years and then subtract it from that of another set of years?

Q: How do I calculate a seasonal average from certain composite years and then subtract it from that of another set of years? Specifically, Iam interested in the (JAS) Y-Z cross-sectional average of temperature over 5S to 5N.

A: Start with the UMD Carton Global Ocean Analysis dataset and its temperature data variable here.

Step 1. Select the 5S to 5N latitudinal range and the JAS season from the years in the first set (1990, 1996, 1997, 2000) and find the average of this subset of data by entering the following lines below the text already in expert mode.
Y (5S) (5N) RANGE
T (Jul-Sep 1990) (Jul-Sep 1996) (Jul-Sep 1997) (Jul-Sep 2000) VALUES
[Y T]average

Step 2. Select the same subset of data for the years in the second set (1994, 1995, 1998, 1999) and find the average by entering the following lines below the text already in expert mode.
SOURCES .UMD .Carton .goa .beta7 .t
Y (5S) (5N) RANGE
T (Jul-Sep 1994) (Jul-Sep 1995) (Jul-Sep 1998) (Jul-Sep 1999) VALUES
[Y T]average

Step 3. To subtract the second average in the stack (1994, 1995, 1998, 1999) from the first average in the stack (1990, 1996, 1997, 2000) by adding the following Ingrid command below the text you just entered in expert mode.
sub Check

There other methods to making the data selections above. For more information, please refer to the Data Library Tutorial.