M. K. Gagan, L. K. Ayliffe(1), W. S. Hantoro(2), D. Hopley(3),
H. Scott-Gagan, C. D. Woodroffe(4), M. T. McCulloch
1) Laboratoire des Sciences du Climat et de l'Environnment, Gif-sur-Yvette
2) Research and Development Center for Geotechnology, Indonesian
Institute of Sciences.
3) Coastal and Marine Consultancies Pty. Ltd., Townsville, Qld.
4) School of Geosciences, University of Wollongong, NSW.
Changes in the magnitude
and frequency of El Niño-Southern Oscillation (ENSO) events
observed in instrumental records since the 1970s have generated
considerable debate about recent ENSO behaviour, and the possibility
that it may be responding to global warming. Ocean-atmosphere
dynamics and atmospheric circulation are known to be particularly
sensitive to large-scale changes in latitudinal temperature gradients.
Thus there is intense interest in documenting ENSO variability
since the mid-Holocene (~6,000 years ago), when climate boundary
conditions were comparable to today, yet the seasonal distribution
of insolation across Earth's surface was different because of
precession of the equinoxes.
A central issue limiting our understanding
of the palaeo-ENSO is that it is not known if the precipitation
anomalies in palaeorecords are directly related to changes in
El Niño temperature anomalies in the tropical Pacific.
Resolving the debate requires seasonal palaeoclimate data that
are capable of revealing the relative magnitude of the oceanic
and atmospheric signals diagnostic of the ENSO phenomenon. Coupled
measurements of Sr/Ca and 18O/16O in coral skeletons provide
a means for reconstructing 18O/16O in seawater, as well as temperature,
by removal of the temperature component of the coral 18O/16O
signal. In this study, we expand the coupled Sr/Ca-18O/16O technique
to generate records of the seasonal cycle of seawater 18O/16O
to examine the interaction of temperature, precipitation and
evaporation during individual palaeo-ENSO events.
We investigated the relationship between mean
climate and ENSO using Holocene Porites coral colonies preserved
in growth position on palaeo-reefs at Sumba (southern Indonesia)
and Orpheus Island (central Great Barrier Reef). Bi-monthly resolution
measurements of Sr/Ca and 18O/16O for these corals provide a
history of off-equator warming of the surface ocean, enhanced
evaporation in the austral spring, and reduced interannual variability
of monsoon rainfall from 4,800 to 6,200 calendar years before
present. Despite this different climate state, the ocean-atmosphere
feedbacks diagnostic of the ENSO remain predictably phase-locked
to the annual cycle. Yet, compared to ENSO behaviour of recent
decades, the response of Australian monsoon precipitation to
El Niño temperature anomalies is subdued in the mid-Holocene.
Records of skeletal 18O/16O for massive Porites
microatolls from Christmas Island (Kiritimati) in the central
equatorial Pacific provide high-resolution proxy records of ENSO
variability since 3,800 years ago. The comparative histories
indicate that ENSO anomalies were less intense between 3,800
and 2,800 years ago, and more pronounced 1,700 years ago.
Suppression of the ENSO during the mid-Holocene
and amplification ~2,000 years ago are consistent with model
predictions based on precessional changes in insolation seasonality.
However, the abrupt onset of ENSO variability in the Late Holocene
is unexpected, and appears to reflect stronger rainfall teleconnections.
We propose that this shift in Late Holocene ENSO behaviour reflects
enhanced interaction between the Southern Oscillation and the
Intertropical Convergence Zone during. The suppressed ENSO-monsoon
interaction observed during the mid-Holocene is best explained
by enhanced south-easterly wind-driven divergence in the tropical
Pacific, and northward displacement of the Intertropical Convergence
Zone. Tighter coupling between the Southern Oscillation and a
more southerly Intertropical Convergence Zone during the Late
Holocene could serve to amplify ENSO precipitation variability.
The results suggest that large-scale changes
in tropical atmospheric circulation can play an active role in
modulating ENSO-monsoon interactions and the precipitation response
to El Niño temperature anomalies. Transient greenhouse
warming simulations suggest that the distribution of global warming
will not be homogeneous in the 21st century, and that large-scale
changes in surface temperature gradients and atmospheric circulation
may result. Our findings predict that the impact of El Niño
temperature anomalies on precipitation will evolve along with
global climate change, even if El Niño temperature perturbations
remain relatively stable.