Sign of Jonah in the Gospels

The Sign of Jonah appears in the Gospels according to Matthew and Luke and seems to come from some shared source other than Mark. (The big question mark of modern scientific study of the sources of Synoptic gospels is Q).


Matthew: Sign of Jonah
Matthew has a pericope that mentions wtihout further elaborating what the Sign of Jonah is:

The Pharisees and Sadducees came to Jesus and tested him by asking him to show them a sign from heaven.

He replied, “When evening comes, you say, ‘It will be fair weather, for the sky is red,’ and in the morning, ‘Today it will be stormy, for the sky is red and overcast.’ You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times. A wicked and adulterous generation looks for a sign, but none will be given it except the sign of Jonah.” Jesus then left them and went away.
Matthew 16:1-4 NIV

Luke: Jonah himself was a sign
The Gospel of Luke explains that Jonah was a sign to the Ninevites

As the crowds increased, Jesus said, “This is a wicked generation. It asks for a sign, but none will be given it except the sign of Jonah. For as Jonah was a sign to the Ninevites, so also will the Son of Man be to this generation. The Queen of the South will rise at the judgment with the people of this generation and condemn them, for she came from the ends of the earth to listen to Solomon’s wisdom; and now something greater than Solomon is here. The men of Nineveh will stand up at the judgment with this generation and condemn it, for they repented at the preaching of Jonah; and now something greater than Jonah is here.
Luke 11:29-32 NIV

Matthew: Jonah and the Big Fish
Matthew has another pericope that includes a discussion resembling that in Luke but precedes it with a powerful interpretation of the Sign of Jonah

He answered, “A wicked and adulterous generation asks for a sign! But none will be given it except the sign of the prophet Jonah. For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth.

The men of Nineveh will stand up at the judgment with this generation and condemn it; for they repented at the preaching of Jonah, and now something greater than Jonah is here. The Queen of the South will rise at the judgment with this generation and condemn it; for she came from the ends of the earth to listen to Solomon’s wisdom, and now something greater than Solomon is here.
Matthew 12:39-42 NIV