Conceptual model for millennial climate variability: a possible combined solar-thermohaline circulation origin for the ~1,500-year cycle
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Dansgaard-Oeschger and Heinrich events are the most pronounced climatic changes over the last 120,000 years. Although many of their properties were derived from climate reconstructions, the associated physical mechanisms are not yet fully understood. These events are paced by a ~1,500-year periodicity whose origin remains unclear. In a conceptual model approach, we show that this millennial variability can originate from rectification of an external (solar) forcing, and suggest that the thermohaline circulation, through a threshold response, could be the rectifier. We argue that internal threshold response of the thermohaline circulation (THC) to solar forcing is more likely to produce the observed DO cycles than amplification of weak direct ~1,500-year forcing of unknown origin, by THC. One consequence of our concept is that the millennial variability is viewed as a derived mode without physical processes on its characteristic time scale. Rather, the mode results from the linear representation in the Fourier space of nonlinearly transformed fundamental modes.
KeywordsRectification Solar forcing Thermohaline circulation Millennial variability Fundamental Derived climate modes
This study was supported by Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (MARCOPOLI), by Alexander von Humboldt Foundation and University of Bucharest, Faculty of Physics, Romania through contract CEEX-112/2005. Thanks go to two anonymous referees for constructive and helpful comments and Drs. Norel Rimbu and Thomas Laepple for fruitful discussions.
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