I think the references should include the original papers: Madden R. and P. Julian, 1971: Detection of a 40-50 day oscillation in the zonal wind in the tropical Pacific, J. Atmos. Sci., 28, 702-708 https://doi.org/10.1175/1520-0469(1971)028<0702:DOADOI>2.0.CO;2 Madden R. and P. Julian, 1972: Description of global-scale circulation cells in the tropics with a 40-50 day period. J. Atmos. Sci., 29, 1109-1123 https://doi.org/10.1175/1520-0469(1972)029<1109:DOGSCC>2.0.CO;2 Dennis Shea, NCAR (retired)