*2.2. HYCOM Reanalysis Products*

The background stratification was derived from a HYbrid Coordinate Ocean Model (HYCOM) of temperature and salinity with a temporal resolution of 3 h and a spatial resolution of 0.08◦ . The variables are interpolated onto 41 layers at depths between 0 and 5000 m. The model assimilates altimeter observations and in situ sea surface temperature, as well as vertical temperature and salinity profiles from Argo floats, expendable bathythermographs (XBTs), and moored buoys, by Navy Coupled Ocean Data Assimilation (NCODA) system [13,14]. The model data was used to estimate stratification before the ISW event.
