Which ocean lies between Africa and Australia?

The Indian Ocean, the only one named after a country, and the only major ocean effectively closed to the north - which changes everything about how it behaves. Landlocked at the top by Asia, it has no cold polar water entering from the north, so it is the warmest ocean in the world.

That closure produces the monsoon. The Asian landmass heats far faster than the sea in summer, drawing in moist air from the ocean and dumping it as rain over India and Southeast Asia, then reversing in winter as the land cools. Roughly a quarter of humanity depends on that reversal arriving on time, and a late or weak monsoon has historically meant famine.

It was also the first ocean with a systematic long-distance trade network, precisely because the winds reverse on schedule - sail one way with the monsoon, wait, sail back with it. Arab, Indian, Persian, Swahili and Chinese traders were running that circuit for well over a thousand years before Europeans arrived, and the Swahili language of the East African coast, full of Arabic and Persian loanwords, is a direct product of it.