Why do the whales leave Alaska and Russia in the winter?

Whales leave the Arctic Ocean in the winter because there is no food and much of its surface freezes (whales need to breathe air and if the sea is covered in ice they will drown).

So whales migrate south to tropical waters. Hear there is no food, so they live off of their fat reserves. This is where they mate and where their calves are born. When winter is over, they head back north to find food and restock their fat reserves.