Indian Ocean Monsoon Dynamics recorded in a Speleothem from Socotra, Yemen

On the arid Indian Ocean Socotra Island (12º30'36"
53º55'12"), the Intertropical Convergence Zone (ITCZ)
induces a bimodal distribution of the precipitations. Rain falls
only as the northward migrating ITCZ passes in May-June and
as it returns southward from September to December. The
watershed effect of the 1540m-high SW/NE oriented
Haggeher Mountains forces precipitations to concentrate on
the windward side of the range [1]. Multi-proxy analyses
("18O, "13C & greyscale) are carried out at high resolution
(500?m = time-scale ~3 years) on two stalagmites collected
in the eastern part of the island: STM1 from the Hoq Cave
and STM5 from Casecas cave, 6 km away. Based on TIMS
U/Th-dating they resp. cover the last 6000 years and the last
1000 years. Spectral analysis of the obtained records reveals
an important ~205-years component in STM1. Comparing the
*18O & *13C records with a reconstruction of solar activity
[2] indicates that for most of the last 6000 years periods of
lower precipitation and less vegetation are associated with
periods of high solar activity. Periods of low solar activity
induce more precipitations and higher vegetation abundance.
In Oman, solar activity is positively related with the intensity
of the rainy seasons because of changes in the latitudinal
position of the ITCZ and the convective activity [3]. The
Socotra results independently confirm this hypothesis and
refine it by identifying the 205-years "De Vries / Suess"
sunspot cycle as the dominant forcing cycle for the Indian
Monsoon Dynamics on a centennial scale.
[1] Scholte, P., De Geest, P., (2010) Journal of Arid
Environments 74, 1507; [2] Steinhilber, et al., (2009)
Geophysical Research Letters 36, 1; [3] Fleitmann et al.
(2007) Quaternary Science Reviews 26, 170.