A second jersey? That's all I'm getting at. I don't really know if there is a definition of what people call base and mid layers in terms of what type of material or thickness or ...

Given your relatively mild winters, even the current one, these types of definitions aren't really necessary for your needs. In terms of upper body, you just need an extra layer which could come from anywhere in your closet but for the legs, if you're used to riding in cycling shorts, I wouldn't put jeans or other type of unstretchy pant over top as an extra layer (despite the fact that it might be sufficient to provide you the extra insulation you need) because you need to be able to comfortably move your legs.

If you're hands are normally in short gloves, do you have a thin glove you can put over top? If you normally wear short lycra socks, maybe a longer sock instead or better yet a wool sock?