What is the soil temperature variable over the oceans in IFS and ERA5?

Hello,

I am using soil temperature data (stl, or sot depending on the age of the data) from the IFS and ERA5, and can’t work out what the data over the oceans for this variable is.

Given the ocean is not land (obviously) I would have thought it would be nans, but it’s not - it looks like its related to the sea surface temperature. I know it’s not skin temperature because it varies with ‘soil level’ but I can’t find anywhere in the documentation which describes it. Is it ocean temperature at ocean levels matching the soil level depths? Or something else?

Note that I do need this data so I don’t want to replace it with nans - I just want to understand exactly what it is.

Thanks in advance!

Cat