Holocene ocean-atmosphere interactions and seasonal expression of the El Niño-Southern Oscillation

Holocene ocean-atmosphere interactions and seasonal expression of the El Niño-Southern Oscillation

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 cedex, France.
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.