状況１
You won't believe who is in our chimney at this very moment. There is . . . a SANTA Claus!
状況２
A lot of people say that there isn't a Santa Claus. They're are just ignorant.. Fact is ... there IS a Santa Claus.
状況３
A: Where? I don't see anything.
B: THERE is a Santa Claus!! Over there! On the roof!
状況３はちょっと微妙で、このthereは単なる存在の there is のthereでなく、「あそこに」という副詞。なので、A Santa Claus is there. の倒置と考えられる。ただこうなると冠詞の a はちょっと微妙で、
THERE is Santa Claus!
あるいは
THERE is the Santa Claus!
というのも（たぶん）あり得るはず。
