How do I plot the difference between data from two different times within a defined region?

Let's start with the IGOSS/NMC/MONTHLY dataset.

You can find it here, but if you would like to learn more about finding datasets in the Data Library, see Part I of the Data Library Tutorial.

For this example, let's subtract the SSTA of September 1990 from that of September 1991.

1. Select the Sea Surface Temperature Anomaly variable with the appropriate link under the Datasets and variable heading. Check Step
2. Click on "Expert Mode" in the function bar at the top of the page. Check Step
3. Below the text already in the text box, enter the following lines:

   a:
   T (Sep 1991) VALUE
   a:
   T (Sep 1990) VALUE
   :a
   

   You have just selected SSTA data from the two periods. You could have also done this with text that looks like this.
4. Subtract these two data values by entering the following text below the text that is already there and clicking "OK".

   sub
   

   Check Step

   You have now created data that represents the difference between the September 1990 and September 1991.

5. To view these data, click on the "Views" link below the text box.