Post-glacial climatic change and variability - The Global Climate System: Patterns, Processes, and Teleconnections - page 209

the mid-latitude zones (Markgraf 1998 ), cooler temperatures over southern
Australia (Cook et al. 1995 ), and changes in spatial temperature and precipita-
tion patterns in Southern Africa (Stager and Mayewski 1997 ). Lindesay ( 1998 )
states that the northern displacement of the westerlies usually brings warmer,
wetter conditions to Southern Africa. Overall, sea ice extent expands northward,
and cyclonic activity increases. Goodwin et al.( 2004 ) demonstrate that
increased Na þ concentrations measured in ice cores show that transport of air
toward Antarctica was enhanced. There is a strong anti-correlation with the
AAO (see Section 5.4 ) and a high correlation with Rossby wave number 3
circulation in the SW Pacific and Indian Ocean region (see Chapter 4 ).
During warmer periods, the circumpolar vortex is closer to the pole, zonal
circulation dominates, and variations in the climate pattern are reduced.
Goodwin et al.( 2004 ) suggest that their Na þ data indicate a reduction in
circulation variability since 1600, more positive AAO values, and stronger
circumpolar zonal vortices in both the troposphere and stratosphere.
The details of what causes these changes are unknown. Jones and Mann ( 2004 )
and Mayewski et al.( 2004 ) describe the importance of changes in solar radiation
(associated with sunspots), and the extent and frequency of volcanic eruptions, as
major forcing factors. Oscillations in internal ocean circulation and deep water
dynamics (Cook et al. 1995 ) may also be important. Changes in regional circulation
dynamics, such as ENSO (Section 2.8 ),wouldalsoplayarole.IPCC( 2001 )states
that the ENSO and its teleconnections were very different in the mid-Holocene
compared to today. Between AD 700 and 1000, El Ni˜o events were at the highest
frequency in the Holocene period, gradually dropping off over the next fewcenturies
(Markgraf 1998 ). Answering the questions ''why?'' and ''how?'' past climate
change and variability occurred are important areas for future climate research.
6.5 Chapter summary
Assessment of past climate change and variability, and the forcing functions that
cause the changes, is a complex challenge. For the periods before AD 1850,
proxy analysis must be used. Fortunately, a range of very good proxies, such as
ice cores, coral cores, tree-rings, and speleothems, is available. Unfortunately,
there are difficulties with scale, length of record, calibration, and consistency.
Jones et al.( 1998 ) emphasize that often detailed proxy data have little in
common with each other. Regional impacts may interfere with global represen-
tativeness. Mayewski et al.( 2004 ) list several reasons for these differences,
including the complexities of the climate record, abrupt local or regional
changes, and spatial irregularities associated with multiple controls.
There are, however, enough data sets to begin to establish a broad global-scale
picture for the Holocene, the most recent 11 500 years. The NH data records are
more complete than those for the SH. A series of fluctuations between warm and
cool periods, which on a hemispheric average do not exceed 2 8C, occur in all