Those types of deserts are caused by what is known as mountain shadow. The mountains cause the rain laden clouds to dump their moisture on the seaward side of them and thus they are dry before they make it to the other side with the exception of monsoon seasons and such. If you want deserts in that area, you just need to make sure the precipitation levels are very low.

I think I made a post up above explaining the geography and concept based on RobA's suggestions.