This is the usual atmospheric surface pressure that we hear about in weather forecasts. (scientificamerican.com)
The method is a sea ice - ocean model ensemble run (without and with assimilation of sea - ice / ocean observations); the coupled ice - ocean model NAOSIM has been forced with atmospheric surface data from January 1948 to 7 June 2015. (arcus.org)
Trenberth et al (2002) Evolution of El Nino — Southern Oscillation and global atmospheric surface temperatures (See note 1) (skepticalscience.com)