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Solar Control of Asia's Southwest Monsoon
Tiwari, M. Ramesh, R., Somayajulu, B.L.K., Jull, A.J.T. and Burr, G.S. 2005. Solar control of southwest monsoon on centennial timescales. Current Science 89: 1583-1588.

What was done
The authors conducted a high-resolution (~50 years) oxygen isotope analysis of three species of planktonic foraminifera (Globigerinoides ruber, Gs. sacculifer and Globarotalia menardii) contained in a sediment core extracted from the eastern continental margin of the Arabian Sea (12.6N, 74.3E) that covered the past 13,000 years. Data for the final 1200 years of this period were compared with the reconstructed total solar irradiance (TSI) record developed by Bard et al. (2000), which is based on fluctuations of 14C and 10Be production rates obtained from tree rings and polar ice sheets.

What was learned
Tiwari et al. report that the Asian SouthWest Monsoon (SWM) "follows a dominant quasi periodicity of ~200 years, which is similar to that of the 200-year Suess solar cycle (Usokin et al., 2003)." This finding indicates, in their words, "that SWM intensity on a centennial scale is governed by variation in TSI," which "reinforces the earlier findings of Agnihotri et al. (2002) from elsewhere in the Arabian Sea."

What it means
The five researchers note that "variations in TSI (~0.2%) seem to be too small to perturb the SWM, unless assisted by some internal amplification mechanism with positive feedback." In this regard, they discuss two possible mechanisms. The first, in their words, "involves heating of the earth's stratosphere by increased absorption of solar ultraviolet (UV) radiation by ozone during periods of enhanced solar activity (Schneider, 2005)." According to this scenario, more UV reception leads to more ozone production in the stratosphere, which leads to more heat being transferred to the troposphere, which leads to enhanced evaporation from the oceans, which finally enhances monsoon winds and precipitation. The second mechanism, as they describe it, is that "during periods of higher solar activity, the flux of galactic cosmic rays to the earth is reduced, providing less cloud condensation nuclei, resulting in less cloudiness (Schneider, 2005; Friis-Christensen and Svensmark, 1997)," which then allows for "extra heating of the troposphere" that "increases the evaporation from the oceans." To read more about this latter mechanism, see Cosmic Rays in our Subject Index.

Agnihotri, R., Dutta, K., Bhushan, R. and Somayajulu, B.L.K. 2002. Evidence for solar forcing on the Indian monsoon during the last millennium. Earth and Planetary Science Letters 198: 521-527.

Bard, E., Raisbeck, G., Yiou, F. and Jouzel, J. 2000. Solar irradiance during the last 1200 years based on cosmogenic nuclides. Tellus B 52: 985-992.

Friis-Christensen, E. and Svensmark, H. 1997. What do we really know about the sun-climate connection? Advances in Space Research 20: 913-921.

Schneider, D. 2005. Living in sunny times. American Scientist 93: 22-24.

Usoskin, I.G. and Mursula, K. 2003. Long-term solar cycle evolution: Review of recent developments. Solar Physics 218: 319-343.

Reviewed 10 January 2007