THE SUSCEPTIBILITY FACTOR IN THE ATMOSPHERIC RESPONSE TO PERIODIC FORCING

Evidence is presented of a periodic component in the inter-annual vari ability of precipitation and pressure data for India during June, the month of the onset of the Indian southwest monsoon. Two frequencies th at explain a statistically significant percent of the variance in thes e 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 th e frequencies the same but they are also in phase which strongly sugge sts 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 re lated to the long term change in northern hemispheric surface temperat ure. This susceptibility of the atmosphere to an external forcing resu lts in a nonlinear relationship between forcing and response. As a res ult, 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 o f the northern hemisphere temperature and the 18.6 year lunar nodal cy cle 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.