Figure 9 – Upper plate: The PCI (Polar Circulation Index) is a time series describing the dynamics (i.e., increase and decrease from mean values) of the well-mixed atmosphere represented by the dominant EOF of the major ions in the GISP2 ice core (Mayewski et al., 1994). The PCI provides a relative measure of the average size and intensity of polar atmospheric circulation. In general terms, PCI values increase (e.g., more continental dusts and marine contributions) during colder portions of the record (stadials) and decrease during warmer periods (interstadials and interglacials) (Mayewski et al., 1994). The PCI is contrasted with the sum of the bandpass components (>99% significance) estimated from this series. The sum represents ~90% of the variance in the original PCI series. Lower plates: Major bandpass components derived from the PCI series include those with periodicities close to elliptical precession, axial precession, precession of the equinoxes, lower order harmonics of the foregoing, and periodicities potentially related to ice sheet dynamics, internal ocean oscillations and solar variability. The 6100-year bandpass component aproximates the timing of H (Heinrich events) and the 1450-year bandpass component the timing of the rapid climate change events (Dansgaard/Oeschger) in the GISP2 record. Figure modified from Mayewski et al. (1997).