Doesn't the Koppen climate system distinguishes between full-on Desert climates (both hot and cold) and Semi-Arid ones, (also divided in hot and cold) to represent the transition zones from Desert areas to non-Desert ones. I think the issue that arises with how PC does climate is that it can't quite distinguish between the Gobi Desert and cold Eurasian steppes that are both cold and arid, but one is much more inhospitable and drier than the other. Or, like in this case, between the actual Sahara Desert and the semi-arid zones that exist between the Mediterranean Coast and the Sahara.