非表示:
キーワード:
Sea surface temperature; Arabian Sea; Summer monsoon; Winter monsoon; Holocene; Global climate variability
要旨:
The seasonal monsoon cycle with winds from the southwest (SW) in summer and from the northeast (NE) in winter strongly impacts on modern regional sea surface temperature (SST) patterns in the Arabian Sea (northern Indian Ocean). To reconstruct the temporal and spatial variation in the dynamically coupled winter and summer monsoon strength over the last 25 ka, we analyzed alkenone-derived SST variations in one sediment core from the northwestern Arabian Sea, that is influenced by the summer monsoon (SST affected by upwelling processes), and in one core from the northeastern Arabian Sea, where SST is mainly governed by the winter monsoon (no upwelling). Comparison of the SST records reveals an antagonistic relationship of summer and winter monsoon strength throughout the late deglaciation and the Holocene. Upwelling along the Arabian Peninsula associated with peak SW monsoonal wind strength was strongest during the early Holocene climate optimum between 11 and 8 ka, and coincided with the northernmost position of the Intertropical Convergence Zone (ITCZ) marked by maximum precipitation over northern Oman. The SW monsoon weakened over the middle to late Holocene, while the NE monsoon gained strength. This different evolution was caused by the southward displacement of the ITCZ throughout the Holocene. Superimposed over the long-term trend are variations in northeast monsoon wind strength at time scales of centuries that were synchronous with late Holocene climate variations recorded on the Asian continent and in the high-latitude Northern Hemisphere. Their likely driving forces are insolation changes associated with sunspot cycles. Enhanced by feedback mechanisms (e.g. land-sea thermal contrast) they enforced centennial scale fluctuations in wind strength and temperature in the northern Arabian Sea monsoon system.
