Western US is on track for a millenium event of mega drought regardless of year to year ENSO conditions, by mega drought meaning the maximum consistently hot and dry year round conditions year to year for a century or more. The Anasazi left their lands for river basins because of drought conditions much less severe than our current mega drought starting in 2010. Death Valley may end up having rival regions in Many regions of the west. Whether the jet stream was chaotic in medieval times as it is now I don't know but a lack of a strong consistent jet stream and its effects on ENSO and MJO are to blame as well as longer term PDO and Atlantic features as well.