"Because of the ambiguity regarding the identification of Tarshish, we are not able to say with confidence where Jonah was heading when he set sail from Joppa. All we can be sure of is that he was going west, and that he thought he would be leaving his God behind."
The Anchor Bible dictionary, ed. David Noel Freedman (6 vols., New York, NY: Doubleday, 1992), s.v. "Tarshish," by David W. Baker (vol. 6, p. 333 (331-333)). Cf. C. H. Gordon in the Interpreter's dictionary of the Bible (5 vols., Nashville, TN: Abingdon Press, 1962, 1976): "A far-off, and sometimes idealized, port that cannot be identified with any one location" (vol. 4, p. 517).