Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 189

Although the individual chemical species reflect various environmental
conditions as just presented, there are multiple sources for most of these
species. For instance, there is also a Ca 2 þ component in sea salt and Na þ may
be derived from continental sources. Consequently, it is necessary to discern
the source of the individual species, as well as the relative importance of each
chemical species in the overall glaciochemical record. This task is accom-
plished through an empirical orthogonal function (EOF) analysis of the
complete glaciochemical record (e.g. Mayewski et al. 1997 ). In fact, the
time series of individual EOF components has added information to the
overall environmental and climatic record available from the glaciochemical
record. The time series of EOF1 in the GISP2 core provides additional
evidence for the cyclical nature of Holocene climatic conditions in the NH
(Figure 6.3 ) , as this particular EOF explains the greatest amount of variability
in all of the individual glaciochemical time series.
The glaciochemical record from the GISP2 ice core developed by O'Brien
et al.( 1995 ) provides a continuous high-resolution record of cyclical changes
of cool climatic conditions during the Holocene (Figure 6.3 ) . As shown by the
time periods with high concentrations of individual chemical species and
high EOF1 values, there are five periods during the Holocene when overall
climatic conditions were characterized by cooler, drier conditions than other
time periods, and with overall greater circulation pattern development and the
resulting increase in wind speeds. The first period of such cooling following
the Younger Dryas, the final cold pulse of the last glacial, occurs between
8800 and 7800 years ago. Three more periods of cool Holocene climatic
conditions occur from 6100 to 5000, 2400 to 3100 and 600 years ago to
almost the present. The primary conclusion drawn from these results is that
NH climate, and probably global climate, has varied on approximately 2500-
year cycles during the Holocene. This periodicity matches the 2500-year
solar cycle as reflected in radiocarbon records from tree rings in the
Holocene. In general, O'Brien et al. suggest that these time periods are
characterized by an expanded polar vortex and possibly greater meridional
flow in the NH. Subsequently, milder NH conditions, and thus a contracted
polar vortex or weaker meridional flow, existed from 10 600 to 9300, 7900 to
6300, 2700 to 1500 and 960 to 610 years ago.
The use of EOF analysis on the GISP2 glaciochemical record (Mayewski
et al. 1997 ) provides excellent evidence for the regionalization of climate
during post-glacial times. EOF1, the most dominant component of the record,
explains 50% of the variance in all glaciochemical species during the
Holocene, but the amount of variance explained jumps to 70-80% during
interstadial and stadial events within the last glacial period, respectively.
Mayewski et al. suggest that EOF1 represents an expanded polar vortex
during glacial times when the large ice sheets are in existence. They believe