Extreme and persistent drought in California and Patagonia during mediaeval time

Studies from sites around the world have provided evidence
for anomalous climate conditions persisting for several hundred years
before about AD 1300. Early workers emphasized the temperature increase
that marked this period in the British Isles, coining the terms
'Mediaeval Warm Epoch' and 'Little Climatic Optimum', but many sites
seem to have experienced equally important hydrological changes. Here I
present a study of relict tree stumps rooted in present-day lakes,
marshes and streams, which suggests that California's Sierra Nevada
experienced extremely severe drought conditions for more than two
centuries before ad ~ 1112 and for more than 140 years before ad ~ 1350.
During these periods, runoff from the Sierra was significantly lower
than during any of the persistent droughts that have occurred in the
region over the past 140 years. I also present similar evidence from
Patagonia of drought conditions coinciding with at least the first of
these dry periods in California. I suggest that the droughts may have
been caused by reorientation of the mid-latitude storm tracks, owing to
a general contraction of the circumpolar vortices and/or a change in the
position of the vortex waves. If this reorientation was caused by
mediaeval warming, future natural or anthropogenically induced warming
may cause a recurrence of the extreme drought conditions.