Solar-ENSO-Rainfall Connections
Abstract
There is now considerable evidence that solar activity influences terrestrial climate in the long term, but recent work shows significant connections also on shorter multidecadal scales, in particular between solar activity and Indian monsoon rainfall. Analysis of rainfall over the two test periods 1878-1913 and 1933-1964, each comprising three complete solar cycles which between them exhibit the highest contrast in solar activity since the beginning of accurate rainfall data collection in India, has shown significantly higher rainfall in the more solar-active period, see [1]. Several studies (see [2]) on the impact of ENSO on the Indian monsoon describe the temporal variability of the ENSO-monsoon interaction on the 2-7 y time scale. The main objective of this paper is to study the connections between the solar-ENSO-monsoon 'triad' using statistical time series and wavelet analysis using band-averaged wavelet power and cross power. The present investigation uses the statistical methods developed by the authors [3] for testing the significance of wavelet cross-spectra between solar activity and geophysical time series over a time scale of the order of 100 y. The methods are based on (i) comparison of statistics of band-averaged cross-spectra over two test periods respectively of low and high solar activity each comprising three complete solar cycles [BN05] and (ii) Monte Carlo simulations of spectrally-matched synthetic noise to provide the reference cross-spectrum for the statistical testing procedure. The tests indicate that, in the 8-16 y band, a decrease in an ENSO index, such as the Nino 3.4 tendency, is associated with an increase in sunspot numbers at confidence levels exceeding 99.5 %. Thus, the effect of solar activity on ENSO is strong, but not as that on Indian rainfall; the homogeneous Indian monsoon covaries with sunspot numbers at 99.99% confidence level by the same test. Interestingly, however, in the 2-7 y band the Nino 3.4 tendency (N34T) and rainfall indices show the opposite behaviour with variation in sunspot numbers, although it is weaker than in the 8-16 y band. Contribution to ENSO-rainfall cross-spectra in the 8-16 y band is by no means negligible compared to that in the 2-7 y band (22.1% vs. 51.2%, for N34T-HIM, for example.) The presence of such effects of opposite sign indicate the importance of interactions on different time scales in studying the influence of solar activity on monsoon rainfall. In order of decreasing confidence level, different index pairs may be ranked as follows: sunspots/rainfall ( 99.5%) , sunspot/Nino 3.4 tendency (95%), sunspot/Nino 3.4 (90%) and Nino 3.4 tendency/rainfall (70%). The evidence for multi-decadal connections between Indian rainfall and solar activity, directly and mediated through ENSO, is thus strong, as tested by two different procedures: time-domain analysis (which uses sunspot numbers only to select periods of greatest contrast in solar activity) and wavelet cross spectra (band- averaged and otherwise, which use quantitative solar activity data to analyse effects in different period bands). All these connections show regional variations across India, organized roughly along quasi-longitudinal belts tilting slightly eastward towards the north. (1) S. Bhattacharyya, R. Narasimha, 2005 (BN05), 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) S. Bhattacharyya, R. Narasimha, 2006, Evidence for multidecadal connections between Indian monsoon rainfall and solar activity, To be submitted shortly.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.A53E0252B
- Keywords:
-
- 3305 Climate change and variability (1616;
- 1635;
- 3309;
- 4215;
- 4513);
- 3309 Climatology (1616;
- 1620;
- 3305;
- 4215;
- 8408);
- 4522 ENSO (4922);
- 7536 Solar activity cycle (2162);
- 7974 Solar effects