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The early Holocene SWW core maximum coincides with a widespread warming
at southern hemisphere mid-latitudes as evidenced by sea-surface temperature
(SST) records from the Atlantic Southern Ocean (53
S; Fig.
1
g) (Bianchi and
Gersonde
2004
) as well as offshore southern (Caniupan et al.
2011
), central (Lamy
et al.
2002
) and northern Chile (Kaiser et al.
2008
). At the same time, SSTs in the
eastern tropical Paci
°
c were relatively cold (Koutavas and Sachs
2008
) reducing
the low to mid-latitude SST gradient in the South Paci
c (Fig.
1
f) and consequently
the SWW at their northern margin as during present-day summer. Warming in the
eastern tropical Paci
c and cooling further south during the late Holocene possibly
enhanced the latitudinal SST gradients resulting in stronger winds at the northern
margin of the SWW (Lamy et al.
2010
) and reduced winds across the southern tip
of South America as presently occurring during winter.
Some of our proxy data are inconsistent with our numerical model results which
suggest that the annual and seasonal mean SWW is subjected to an overall
strengthening and poleward shifting trend during the course of the mid-to-late
Holocene under the in
uence of orbital forcing, except for the austral spring season,
where the SWW exhibit an opposite trend of shifting towards the equator (Fig.
1
j)
(Varma et al.
2012a
). The major change in the proxy data occurs during the early
Holocene (not yet covered by the transient model runs) rather than the mid- and late
Holocene. However, the modeled poleward shifting and strengthening of the
westerlies during most of the year from the mid to the late Holocene is inconsistent
with the trend to more humid conditions and stronger northern margin westerlies in
central Chile shown by the data (Fig.
1
a
c). An exception is the modeled austral
spring enhancement of the northern westerlies, which is however not the major
rainfall season in central Chile assumed to be recorded by the proxies (Fig.
1
j).
Further work is therefore needed to reconcile these model-data inconsistencies
including an extension of the transient experiments into the early Holocene where
processes such as the ocean-driven bipolar see-saw become important (Fig.
1
h).
More consistent data-model results have been obtained for centennial-scale
SWW changes in central Chile over the past 3 ka (Fig.
2
) (Varma et al.
2011
,
Varma et al.
2012b
). The proxy and model results suggest that centennial-scale
periods of lower (higher) solar activity caused equatorward (southward) shifts of the
annual mean SWW. Under a
-
mechanism, where applied changes in
total solar irradiance mostly affect the climate system through shortwave absorption
by the surface, the strength and position of the SWW are strongly related to
meridional surface temperature gradients. By contrast, a
“
bottom-up
”
“
top-down
”
mechanism
in
uences the troposphere via stratospheric ozone responses to variations in
ultraviolet radiation and dynamical coupling between the atmospheric layers. The
SWW response in simulations with varying stratospheric ozone (EGMAM2) is
more pronounced and robust compared to the one with
xed ozone (EGMAM1)
(Fig.
2
) suggesting an important contribution from the middle atmosphere through a
“
”
top-down
mechanism.
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