William H Campbell
Articles written in Journal of Earth System Science
Volume 102 Issue 1 March 1993 pp 157-173
Evidence is presented of a periodic component in the inter-annual variability of precipitation and pressure data for India during June, the month of the onset of the Indian southwest monsoon. Two frequencies that explain a statistically significant percent of the variance in these data sets are the same as the two that explain most of the variance of the average monthly lunar tidal potential for June. Not only are the frequencies the same but they are also in phase which strongly suggests that lunar tides in the atmosphere do, in fact, produce an element of climatic variability. The amplitude of the atmospheric response to this periodic forcing was not constant in time but was found to be related to the long term change in northern hemispheric surface temperature. This susceptibility of the atmosphere to an external forcing results in a nonlinear relationship between forcing and response. As a result, nonlinear regression had to be used in order to adequately define the magnitude of the response at a given frequency. The ramifications of this nonlinear response are discussed. The nonlinear interaction of the northern hemisphere temperature and the 18.6 year lunar nodal cycle results in a modulation of the frequency which appears in a linear spectral analysis near 22 years. Thus, the 22-year cycle often found in meteorological data sets may instead be the result of the modulated nodal cycle.