Solar Forcing and the Western US Bi-Decadal Drought Rhythm:  An Analysis Back to AD 800


Author:  Edward R. Cook
Affiliation: Lamont-Doherty Earth Observatory


The existence of a bi-decadal (~20 years) drought rhythm in the western USA has been documented in instrumental drought records covering the past 100+ years, and its longer-term existence has been demonstrated as well in long tree-ring reconstructions of past drought extending back to AD 1700. This mode of variability is strongly related to forcing by the 22-yr Hale solar magnetic cycle and the 18.6-yr lunar nodal tidal cycle (Cook et al., 1997), an interaction that modulates the overall bi-decadal drought rhythm at centennial timescales. A new drought area reconstruction for the western USA now extends back to AD 800.  This development has provided an opportunity for a much longer evaluation of solar and lunar tidal forcing to be made then was previously possible.  In so doing, the results strongly show that solar and lunar tidal forcing have jointly influenced bi-decadal drought formation in the western USA for the past 1200 years and probably much farther back in time.  In addition, the long tree-ring-based drought area reconstruction provides evidence for multi-centennial drought variability that might indicate longer-term solar forcing as well.  This new information is described and compared to indicators of long-term solar variability in 14C and 10Be records.  Overall, the results of this study provide compelling long-term statistical evidence for solar forcing of drought in the western USA.  The challenge now is to understand the physics behind such forcing so that it can be properly modeled.



Cook, E.R., Meko, D.M. and Stockton, C.W. 1997. A new assessment of possible solar and lunar forcing of the bi-decadal drought rhythm in the western United States. Journal of Climate 10:1343-1356.