Quote Originally Posted by Charerg View Post
Actually I'd say the southern Andes having such a drastic effect is the abnormal case here. Normally you'd expect cyclonic rainfall to not be so heavily affected. It's maybe a slight error in perception to view the regions that are affected by the westerlies as being constantly swept with moisture-laden westerly winds. In reality they are a belt of localised low pressure centers that circle the globe on a west-to-east vector (more-or-less), so the winds tend to be predominantly westerly when viewed globally, but not necessarily locally.

I suspect Patagonia has an unusual local wind pattern which for some reason favours constant westerly winds, though admittedly this is just a guess on my part.
South America is rather narrow at the southern end. That there is not much in the way of terrain features in the area may reduce some of the variation in wind patterns compared to wider continental areas with a variety of terrains over larger expanses? There is also very little land that far south globally. Air can pretty much just keep circling the globe without hitting much of anything until it reaches the Andes.