I am confused about the system of studying for a PhD in the US. I have just finished an undergraduate masters physics degree (4yrs), want to do research, but want to do it in the earth sciences (specifically, ocean and atmospheric science). If I were to apply in the US would I have to take more courses before doing a research project, or would I be able to go straight into research? I like the idea of being able to sample a range of topics before choosing a final PhD topic, is this what happens? Finally, what universities in the US have good research departments in ocean and atmospheric science?