Evidence for 18.6 year <i>M_N</i> signal in temperature and drought conditions in North America since AD 1800

Abstract

Evidence for the 18.6 year lunar nodal M N term in air temperature is found at 30 stations in eastern North America. Since AD 1800 the mean discrepancy in epoch between maxima in temperature and maxima in the Drought Area Index (DAI) for the western United States vis‐a‐vis, the maxima in the M N tide is 0.9 and 0.1 yr, respectively. Also, a cluster of 9 stations in western Canada yields the 18.6 yr term which is out of phase with stations in eastern North America and is indicative of a standing tidal wave in the atmosphere. This suggests that enhanced drought conditions in the western United States are neither ‘recurrent’ nor ‘rhythmic’ on a time scale of 20 yr, but rather periodic with a period of 18.6 yr. Finally, the Rocky Mountain system is seen as an influence for atmospheric tidal phenomena at epochs of maximum in the M N tide as well as for maxima in the temperature records and the DAI.