Logical reason why survivors in a post-nuclear world would not migrate south?

Question

I’m building a post apocalyptic world right. In my world, after the Nuclear War there is a brief nuclear winter.

Realistically, some have told me, the survivors would migrate south as the Southern Hemisphere would be less affected then the North. However, in my world this cannot happen under any circumstances.

See, the survivors in the North are supposed to be the ones who repopulate the continent of North America after WW3. If they leave off to Central America, there will be no one left.

What is a logical reason for survivors not to migrate south during a nuclear winter?

Answer 1

Make the south worse. Here are just a few reasons.

1. Ashfall: A southern hemisphere super-volcano has erupted, and the global stresses from this drive the nuclear war. You might have your north American survivors fending off invasions from the south.
2. Pandemic: A global disease outbreak pushes the nuclear war. Before the ball went up, North America nuked Central America to contain the outbreak. Sounds like a good reason for someone to hate the USA's guts.
3. Global Warming: The south has become an uninhabitable wasteland of 130 degree weather, and maybe in a couple generations it will be better. Catastrophic climate change is one of the most likely things to trigger nuclear war.
4. The Enemy: Either the south IS the enemy (a South American/expansionist alliance and a nuclear-armed unified Africa) or the war started with the enemy gaining control of the south by alliance or conquest; not a bad reason to start a nuclear war...

Answer 2

They wouldn't have to. Models for a nuclear winter after a large nuclear exchange predict a 36 degree Fahrenheit drop in the average temperature in some parts of North America for a few years. Significant parts of the USA, such as the south, would have average drops of less than 18 degrees Fahrenheit.

While this is a lot- a 36 degree F drop in Chicago is equivalent to average -15 or -20 degree lows in January- it's not apocalyptic. A lot of people, especially in Russia, Canada, and Alaska, routinely live in much colder environments without having any nuclear wars. An -18 degree F temperature change in Austin or Dallas Texas still keeps the average temperature above freezing year-round.

To be clear: this is certainly apocalyptic from the perspective of the climate, agriculture, and biosphere. But if you just need a few hundred or thousand people to survive on canned food and peanut butter for a few years without freezing, that's eminently doable.

Answer 3

Rumors that American's moving south become slaves in the former third world. ...and badly treated slaves because everyone resents us for blowing up the world.