The evaporation from the surface of the ocean is due to all meteorological effects - it depends on the near-surface temperature, humidity, wind speed and surface roughness (which is dependent on the waves). You can find the equations in the IFS Documentation, Part IV: Physical Processes, e.g. IFS Cycle 31r1 (close to version used in ERA-Interim) Doc, Eqn 7.6 in Chapter 7
ERA-Interim production to stop on 31st August 2019
As ERA5 is now available (What are the changes from ERA-Interim to ERA5?), we are preparing to stop the production of ERA-Interim on 31st August 2019. This means that the complete span of ERA-Interim data will be from 1st January 1979 to 31st August 2019.
Keeping in mind that ERA-Interim is published with an offset of about three months from the dataset's reference date, ERA-Interim August 2019 data will be made available towards the end of 2019.
For the time being and until further notice, ERA-Interim shall continue to be accessible through the ECMWF Web API. ERA5 is available from the Climate Data Store (CDS).