Cook, E.R., Seager, R., Cane, M.A. and Stahle, D.W. 2007. North American drought: Reconstructions, causes, and consequences. Earth-Science Reviews 81: 93-134.
In a review of multiple long tree-ring reconstructions of past drought (Palmer Drought Severity Index) conditions in North America, Cook et al. find evidence of "a number of unprecedented megadroughts over the past millennium that clearly exceed any found in the instrumental records," including an amazing "epoch of significantly elevated aridity that persisted for almost 400 years over the AD 900-1300 period." They also report that all major historical droughts of North America were caused by "the development of cool 'La Niņa-like' SSTs in the eastern tropical Pacific," and that both "model and data agree" that "if there is a heating over the entire tropics [our italics] then the Pacific will warm more in the west than in the east because the strong upwelling and surface divergence in the east moves some of the heat poleward," so "the east-west temperature gradient will strengthen, so the winds will also strengthen, so the temperature gradient will increase further ... leading to a more La Niņa-like state," which in turn leads to drought in North America. Also, they report that "La Niņa-like conditions were apparently the norm [our italics] during much of the Medieval period," during which time the truly unprecedented 400-year megadrought held sway. Consequently, because the AD 900-1300 time interval (representing the core of the Medieval Warm Period) was host to the most severe and long-lasting drought of the last eleven centuries, that period must also have experienced the warmest tropical temperatures of the last 1100 years.