I have a question about the meaning of the verb "deepen" and by extension the adjective "deep" in this New York Times article:

Israel has full confidence in our intelligence-sharing relationship with the United States and looks forward to deepening that relationship in the years ahead under President Trump.

Most dictionaries define "deepen" as make something more "deep". So it comes down to the meaning of "deep". I check this dictionary, but I don't know which definition would work. Any suggestions from native speakers?