Presented at the 2018 Ocean Sciences MeetingIn this presentation we assess the impact of satellite sea surface salinity (SSS) observations on dynamical ENSO forecasts for the big 2015 El Niño event. From March to June 2015, the unprecedented availability of two overlapping satellite SSS instruments, Aquarius and SMAP, allows a unique opportunity to compare and contrast forecasts generated with the benefit of these two satellite SSS observation types. We will present four distinct experiments for the overlap period that include 1) freely evolving SSS (i.e. no satellite SSS), 2) climatological SSS (i.e. WOA13 SSS), 3) Aquarius, and 4) SMAP initialization. Coupled hindcasts are then generated from these initial conditions and these are validated against observations and evaluated with respect to the observed El Niño development. Differences from the impacts of the various SSS products can thus be evaluated for the buildup (until December 2015) and towards the demise (in March 2016) of the 2015 event. The coupled model that is used in this project is the new seasonal coupled forecast production model for NASA GMAO. This version couples the 1/2o resolution, 72 levels atmosphere with MOM Version 5 with 0.5o resolution and 40 vertical levels. For all the initialization experiments, all available along-track absolute dynamic topography and in situ observations are assimilated using the LETKF scheme of Penny et al., 2013. In order to minimize the transition from the old production coupled model to this current version, SST is relaxed to MERRA-2 values. Separate spin-up experiments are executed from January 2013 until March 2015 that relax to the various SSS products (WOA13, Aquarius climatology, and SMAP climatology) along with the control that allows SSS to vary freely (i.e. no SSS relaxation). Then for March- June 2015, observed interannual values of SSS from Aquarius and SMAP are utilized for SSS relaxation. From these initialization experiments, coupled experiments are run for each month, March to June 2015.