In the calculation of the SST anomalies in this dataset, the following climatology was subtracted from the SST dataset:
http://iridl.ldeo.columbia.edu/SOURCES/.IGOSS/.nmc/.climatology/.
Documentation for this climatology can be found here:
Documentation for the dataset in general, including relevant references, can be found here:
I should point out that an updated version (version 2) of this Reynolds and Smith SST dataset is now available, and can be found here:
http://iridl.ldeo.columbia.edu/SOURCES/.IGOSS/.nmc/.Reyn_SmithOIv2/
If you go to the monthly data or the climatology, a link to the relevant documentation can be found in the blue menu bar on the left side of the page. The Reynolds and Smith version 2 climatology is for the years 1971-2000.
I should also note that there will soon no longer be monthly updates to the Reynolds and Smith version 1 dataset -- the version 2 dataset will continue to be updated, however.
-- Michael Bell