Stronger Solar-ENSO connections than ENSO-rainfall connections in the Indian Monsoon

Evidence presented in this paper shows that connections of ENSO with solar activity parameters are statistically stronger than those between ENSO and Indian summer monsoon rainfall. The global wavelet cross spectrum between solar irradiance and (a) monsoon rainfall and (b) the global ENSO index, show significant power around the dominant period of the 11 year solar cycle, passing the chi-squared test of significance proposed by Torrence and Compo (1998) at levels exceeding 95% and 97.5% respectively. In particular, the confidence level for the solar irradiance--ENSO index is as high as 97.5%. On the other hand the global wavelet cross power spectrum between the ENSO index and the all India summer monsoon rainfall (AISMR) shows a peak in the 3-4 year period range at significance levels of only 70%. The computed cross- spectrum shows that the average rainfall is higher and the average ENSO index lower during a test period (1933-1964) of greater solar activity; the z-test confidence levels for the solar irradiance-sunspot-AISMR- ENSO connections exceed 95%. Even over the slightly longer period of 1850-1998 over which the AISMR has been reconstructed, statistical analysis reveals significant influence by sunspots and solar irradiance, particularly over the two test-periods of 1878-1913 and 1933-1964, respectively representing three complete cycles of lowest and highest solar activity in the 148 y record. An increase in solar activity is accompanied by a decrease in ENSO and an increase in rainfall at the 8-16 y band. The link between ENSO and solar activity (95%) is weaker than the link between solar activity and rainfall (97.5%) but stronger than that between ENSO and rainfall (70%). The effect of Nino 3.4 positive tendencies on the monsoon rainfall is to decrease the rainfall. Both these effects occur in the test periods of lower solar activity. The global cross spectrum for ENSO and solar activity index exhibits peaks around the 11 y period at confidence levels of 90% or higher. This confirms that there is a stronger connection between ENSO and solar activity as compared to the ENSO- rainfall connection. Greater solar activity is associated with lower ENSO activity and vice-versa, supporting the well-known inverse relationship between ENSO and rainfall. Over the two test periods an increase in solar activity is associated with a decrease in the ENSO index and an increase in the monsoon rainfall in the 8-16 y period band, but has the opposite effect in the 2-7 y period band. The net effect of solar processes on rainfall thus appears to be the result of counteracting influences on short and long periods, the latter on the whole dominating over the former. The present results suggest that Indian rainfall is affected by solar processes in part indirectly through ENSO. (1) S. Bhattacharyya, R. Narasimha, 2005, Possible association between Indian monsoon rainfall and solar activity, GRL, 32, doi: 10.1029/2004GL021044, 2005. (2) Torrence C. and Webster P. J., 1999: Interdecadal Changes in the ENSO-Monsoon System, Journal of Climate, 12, 2679-2690. (3) Bhattacharyya, S., and R. Narasimha (2007), Regional differentiation in multidecadal connections between Indian monsoon rainfall and solar activity, J. Geophys. Res., 112, D24103, doi:10.1029/2006JD008353.