Kondrashov, D., Feliks, Y. and Ghil, M. 2005. Oscillatory modes of extended Nile River records (A.D. 622-1922). Geophysical Research Letters 32, L10702, doi:10.1029/2004GL022156, 2005.
The authors applied advanced spectral methods (Singular-Spectrum Analysis and Multi-Taper Method) to historical records of low- and high-water levels of the Nile River (~30°N, 31.2°E) in an effort to better characterize their interannual and interdecadal periodicities. The results revealed that "drastic changes" occurred around AD 1000 in the dominant lower frequencies of all the Nile River records they analyzed, suggesting that a "fairly abrupt" shift in climate occurred near the beginning of the Medieval Warm Period. In addition, Singular-Spectrum Analysis of the 7.2-year mode in the Nile River records revealed a large increase in amplitude between AD 950 and 1450; and Kondrashov et al. cite "several authors, who used different analysis methods," who "all found evidence of climate shifts at the beginning and/or at the end of the Medieval Warm Period."