Increasing Evidence for the Medieval Drought-Deluge-Drought Sequence in the Western U.S

The term Medieval Climatic Anomaly was first used in 1991 to describe the aberrant (by modern standards) hydroclimatic conditions that characterized portions of the western United States and southern South America between roughly AD 850 and 1350. During this period much of California and Nevada experienced two profound and persistent (century-scale) droughts that drew lakes, rivers, and marshes to levels well below the natural lowstands of the past 160 years. Relict tree- and shrub-stumps, rooted in place on lands exposed by these drought-induced drawdowns, provide the most conspicuous, and most readily dated, evidence of the dramatically lowered water levels; geomorphic features, sedimentary sequences, and scatters of artifacts bolster the evidence. The two epic droughts were separated by a multi-decade period of wetness that forced hydrographically closed lakes to levels higher than those of the past 160 years; indeed, during this brief period Mono Lake attained its third-highest level of the Holocene's latter half, and Owens Lake appears to have reached its Holocene highstand. These wide swings in wetness--from drought, to deluge, and back to drought (and then into the Little Ice Age)--undoubtedly reflect large-scale shifts in the wintertime circumference and/or configuration of the northern hemisphere's circumpolar vortex, with the vortex edge (i.e. the storm track) well to the north of California during the periods of drought, and over California during periods of wetness. Such changes in atmospheric circulation (at least insofar as vortex circumference is concerned) may well be driven by hemispheric warmings (leading to vortex contraction and thus wintertime drought over California and Nevada), and coolings (forcing vortex expansion and bringing wetness to the region).