With the aim of improving climate models, research teams are studying periods of the past during which the climate was different from that of the present day. They are investigating especially the extent to which the ENSO phenomenon occurred at the time with the same frequency and intensity. IRD scientists from the Nouméa centre, in conjunction with the College of Marine Science (University of South Florida), have studied a colony of Porites (massive corals) which has been building up since the mid XVIIth century. This colony yields pure signals, by way of trace element analysis involving coupled Sr/Ca and U/Ca ratio determination, on the changes in the sea surface temperature (SST) that have occurred over 350 years (1).
The team focused its attention on a particularly cold period (1701-1761), which occurred in the middle of the Little Ice Age (1400 to 1850 A.D.). At this time, temperatures in temperate latitudes of the Northern Hemisphere were between 1 and 2 °C lower than average figures recorded at present. Drill-core coral samples taken from near the Amédée Lighthouse on the South-West side of New Caledonia show that the Little Ice Age also prevailed in the tropical South-West Pacific, with an average cooling of around 1°C. Was El Niño, which now manifests itself in this area of the Pacific every two to seven years by a fall in SSTs (by 0.5 to 1.5°C), reinforced by this generalized cooling? The reconstitution of a composite monthly SST record over the first 60 years of the XVIIIth century has led the researchers to some surprising observations. El Niño's behaviour then was similar to what it is now. In spite of a decrease in average temperatures, neither the strength nor the frequency of El Niño therefore appears to have been affected, even during the very coldest period.
The New Caledonia corals have, however, recorded a much more pronounced interdecadal oscillation (a period of 15 to 20 years) than is now observed (2) in 1720, 1730 and 1748 in particular decreases in annual average SSTs exceeded 1°C. This is an important result. The scientific community is now considering if there is interaction between El Niño (which recurs every 2 to 7 years) and lower-frequency climatic phenomena (recurrence every 10 to 60 years). The Amédée Lighthouse corals may not have shed new light on that, but they have proved they can yield useful information on these low-frequency cycles over time.
Massive corals are considered as the best memory of climatic variability of the tropical oceans. The coral skeleton incorporates a continual record, sometimes covering several centuries, of temperature, salinity and sunshine fluctuations. The fast formation rate and the existence of seasonal growth bands make them an ideal material for studying recent palaeoclimatology in relation with El Niño.
2 A 1°C decrease in average annual temperatures