on top of vs on top of that

on top of

prep
  • In addition to something else. 

  • Atop 

  • Fully informed about, and in control of, something; up to speed with. 

  • Very close to, in any orientation or in time. 

on top of that

prep
  • Also, additionally, moreover. 

How often have the words on top of and on top of that occurred in a corpus of books? (source: Google Ngram Viewer )