Lim et al. (2005) examined the eolian quartz content (EQC) of a high-resolution sedimentary core taken from Cheju Island, Korea, creating a 6500-year proxy record of major Asian dust events produced by northwesterly winter monsoonal winds that carry dust from the inner part of China all the way to Korea and the East China Sea. The Asian dust time series was found to contain both millennial- and centennial-scale periodicities; and cross-spectral analysis between the EQC and a solar activity record showed significant coherent cycles at 700, 280, 210 and 137 years with nearly the same phase changes, leading the researchers to conclude that centennial-scale periodicities in the EQC could be ascribed primarily to fluctuations in solar activity.

Ji et al. (2005) used reflectance spectroscopy on a sediment core taken from Qinghai Lake in the northeastern part of the Qinghai-Tibet Plateau to construct a continuous high-resolution proxy record of the Asian monsoon over the past 18,000 years. As a result of this effort, monsoonal moisture since the late glacial period was shown to be subject to "continual and cyclic variations," including the well-known centennial-scale cold and dry spells of the Dark Ages Cold Period (DACP) and Little Ice Age, which lasted from 2100 to 1800 yr BP and 780 to 400 yr BP, respectively. Sandwiched between them was the warmer and wetter Medieval Warm Period, while preceding the DACP was the Roman Warm Period. Also, time series analysis of the sediment record revealed statistically significant periodicities (above the 95% level) of 123, 163, 200 and 293 years. The third of these periodicities corresponds well with the de Vries or Suess solar cycle, which suggests that cyclical changes in solar activity are important triggers for some of the cyclical changes in monsoon moisture at Qinghai Lake.

Citing studies that suggest the Indian summer monsoon may be sensitive to changes in solar forcing of as little as 0.25% (Overpeck et al., 1996; Neff et al., 2001; Fleitmann et al., 2003), Gupta et al. (2005) set out to test this hypothesis by comparing trends in the Indian summer monsoon with trends in solar activity across the Holocene. In this endeavor, temporal trends in the Indian summer monsoon were inferred from relative abundances of fossil shells of the planktic foraminifer Globigerina bulloides in sediments of the Oman margin, while temporal trends in solar variability were inferred from relative abundances of ¹⁴C, ¹⁰Be and haematite-stained grains.

Spectral analyses of the various data sets revealed statistically significant periodicities in the G. bulloides time series centered at 1550, 152, 137, 114, 101, 89, 83 and 79 years, all but the first of which periodicities closely matched periodicities of sunspot numbers centered at 150, 132, 117, 104, 87, 82 and 75 years. This close correspondence, in the words of Gupta et al., provides strong evidence for a "century-scale relation between solar and summer monsoon variability." In addition, they report that intervals of monsoon minima correspond to intervals of low sunspot numbers, increased production rates of the cosmogenic nuclides ¹⁴C and ¹⁰Be, and increased advection of drift ice in the North Atlantic, such that over the past 11,100 years "almost every multi-decadal to centennial scale decrease in summer monsoon strength is tied to a distinct interval of reduced solar output," and nearly every increase "coincides with elevated solar output," including a stronger monsoon (high solar activity) during the Medieval Warm Period and a weaker monsoon (low solar activity) during the Little Ice Age.

As for the presence of the 1550-year cycle in the Indian monsoon data, Gupta et al. consider it to be "remarkable," since this cycle has been identified in numerous climate records of both the Holocene and the last glacial epoch (including Dansgaard/Oeschger cycles in the North Atlantic), strengthening the case for a sun-monsoon-North Atlantic link. Given the remarkable findings of this study, it is no wonder that the researchers who conducted it say they are "convinced" there is a direct solar influence on the Indian summer monsoon in which small changes in solar output bring about pronounced changes in tropical climate.

In another impressive study, Dykoski et al. (2005) obtained high-resolution records of stable oxygen and carbon isotope ratios from a stalagmite recovered from Dongge Cave in southern China and utilized them to develop a proxy history of Asian monsoon variability over the last 16,000 years. In doing so, they discovered numerous centennial- and multi-decadal-scale oscillations in the record that were up to half the amplitude of interstadial events of the last glacial age, indicating that "significant climate variability characterizes the Holocene." As to what causes this variability, spectral analysis of δ¹⁴C data revealed significant peaks at solar periodicities of 208, 86 and 11 years, which they say is "clear evidence that some of the variability in the monsoon can be explained by solar variability."

Building upon this work, as well as that of Yuan et al. (2004), Wang et al. (2005) developed a shorter (9000-year) but higher-resolution (4.5-year) absolute-dated δ¹⁸O monsoon record for the same location, which they compared with atmospheric ¹⁴C data and climate records from lands surrounding the North Atlantic Ocean. This work indicated their monsoon record broadly followed summer insolation but was punctuated by eight significantly weaker monsoon periods lasting from one to five centuries, most of which coincided with North Atlantic ice-rafting events. In addition, they found that "cross-correlation of the decadal- to centennial-scale monsoon record with the atmospheric ¹⁴C record shows that some, but not all, of the monsoon variability at these frequencies results from changes in solar output," similar to "the relation observed in the record from a southern Oman stalagmite (Fleitmann et al., 2003)."

In a News item (Kerr, 2005) that accompanied the report of Wang et al., one of the report's authors (Hai Cheng of the University of Minnesota) was quoted as saying their study suggests that "the intensity of the summer [East Asian] monsoon is affected by solar activity." Dominik Fleitman, who worked with the Oman stalagmite, also said that "the correlation is very strong," stating that it is probably the best monsoon record he had seen, calling it "even better than ours." Last of all, Gerald North of Texas A & M University, who Kerr described as a "longtime doubter," admitted that he found the monsoon's solar connection "very hard to refute," although he stated that "the big mystery is that the solar signal should be too small to trigger anything."

Clearly, it is time to put more effort into solving North's "big mystery," for as is readily evident from the wealth of materials archived under the many sub-headings of the Solar Effects section of our Subject Index, there are a host of climatic phenomena that owe their existence to some type of solar-climate connection, not the least of which may be a significant portion of the global warming of the past century.

References
Dykoski, C.A., Edwards, R.L., Cheng, H., Yuan, D., Cai, Y., Zhang, M., Lin, Y., Qing, J., An, Z. and Revenaugh, J. 2005. A high-resolution, absolute-dated Holocene and deglacial Asian monsoon record from Dongge Cave, China. Earth and Planetary Science Letters 233: 71-86.

Fleitmann, D., Burns, S.J., Mudelsee, M., Neff, U., Kramers, J., Mangini, A. and Matter, A. 2003. Holocene forcing of the Indian monsoon recorded in a stalagmite from southern Oman. Science 300: 1737-1739.

Gupta, A.K., Das, M. and Anderson, D.M. 2005. Solar influence on the Indian summer monsoon during the Holocene. Geophysical Research Letters 32: 10.1029/2005GL022685.

Ji, J., Shen, J., Balsam, W., Chen, J., Liu, L. and Liu, X. 2005. Asian monsoon oscillations in the northeastern Qinghai-Tibet Plateau since the late glacial as interpreted from visible reflectance of Qinghai Lake sediments. Earth and Planetary Science Letters 233: 61-70.

Kerr, R.A. 2005. Changes in the sun may sway the tropical monsoon. Science 308: 787.

Lim, J., Matsumoto, E. and Kitagawa, H. 2005. Eolian quartz flux variations in Cheju Island, Korea, during the last 6500 yr and a possible Sun-monsoon linkage. Quaternary Research 64: 12-20.

Neff, U., Burns, S.J., Mangini, A., Mudelsee, M., Fleitmann, D. and Matter, A. 2001. Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago. Nature 411: 290-293.

Overpeck, J.T., Anderson, D.M., Trumbore, S. and Prell, W.L. 1996. The southwest monsoon over the last 18,000 years. Climate Dynamics 12: 213-225.

Wang, Y., Cheng, H., Edwards, R.L., He, Y., Kong, X., An, Z., Wu, J., Kelly, M.J., Dykoski, C.A. and Li, X. 2005. The Holocene Asian monsoon: Links to solar changes and North Atlantic climate. Science 308: 854-857.

Yuan, D., Cheng, H., Edwards, R.L., Dykoski, C.A., Kelly, M.J., Zhang, M., Qing, J., Lin, Y., Wang, Y., Wu, J., Dorale, J.A., An, Z. and Cai , Y. 2004. Timing, duration, and transitions of the last interglacial Asian monsoon. Science 304: 575-578.