It is coming from the Merged Hadley-NOAA/OI Sea Surface Temperature & Sea-Ice Concentration (Hurrell et al, 2008)see also: http://climatedataguide.ucar.edu/guidance/merged-hadley-noaaoi-sea-surface-temperature-sea-ice-concentration-hurrell-et-al-2008Then it is processed with the tools icesst, This tools computes a climatology and modifies monthly mean values to produce mid-month values such that linear time interpolation (e.g. as done in a model) will reproduce the monthly means. Algorithms devised by Karl Taylor of PCMDI are used to accomplish these tasks. Taylor, K.E., D. Williamson, and F. Zwiers (2000): The sea surface temperature and sea-ice concentration boundary conditions for AMIP II simulations. PCMDI Report No. 60 and UCRL-MI-125597, Lawrence Livermore National Laboratory, Livermore, CA, 25 pp.
pdf file available at http://www-pcmdi.llnl.gov/pcmdi/pubs/ab60.html