Try a search for
heat contentin the Users Group Archives at http://ferret.pmel.noaa.gov/static/Ferret_search/ferret_mail_archives.html. There are several threads that might help you get started. For instance, here is a question and answer from the archives on the topic.
Prakya Shree Ram wrote:
Dear Ferreters, I want to calculate the heat content from surface to 300 meters layer. How to do it in ferret? Any suggestions in this regard are highly appreciated. Regards, Sriram