El Niño and the Southern Oscillation (ENSO)
While Milankovitch's astronomical cycles are the largest source of millennial-scale climate variability, ENSO is the largest source of decadal climate variability. The possible interplay between the externally forced climate signal (Milankovitch cycles) and the internal source of climate variability (ENSO) is a fascinating new area of research. In 1982/83 one of the largest El Niño climate events in history created catastrophic climate anomalies around the world, yet is was several years later that the climate community began to understand the climate connections to the sea surface temperature and sea surface salinity anomalies in the remote central Pacific Ocean. In the past twenty years, ENSO research funding has expanded dramatically due to its direct impact on climate of the United States and elsewhere. The instrumental record of tropical Pacific sea surface temperature fades rapidly prior to 1970 and is nearly nonexistent prior to 1940. There are so few salinity measurements in the ENSO region that most researchers ignore this important measure of the location and size of the “fresh pool”, thought by some modelers to be important in the eastward propagation of El Niño. Although El Niño tends to occur every three to seven years, scientists are split between those who believe that models are already successfully predicting El Niño events and those who believe that it is impossible to predict this stochastic phenomenon. The latter argue that forecasters are simply .now-casting. the unfolding of an ENSO event after it has already started and has reached a “point of no return”.
Our research is focused on extending the record of ENSO using oxygen isotopes as an ENSO proxy measured in annually banded corals. We have established a network of ENSO monitoring sites along the equatorial Pacific spanning the region of maximum ENSO temperature, salinity, rainfall and thermocline anomalies. Our transect sites listed from east to west include Galapagos, Kiritimati, Kanton, Tarawa, Kapingamarangi, and Suluwesi. At each site we have cored century-old banded corals and generated monthly-resolution time-series of the oxygen isotope proxy for ENSO thereby extending the history of ENSO events in these remote under-sampled regions. We have mapped the changing frequency and intensity of ENSO back in time long before significant ocean measurements were recorded and compared our ENSO proxy to the much longer meteorological records available from major cities around the world. Remarkably, the correlation of our coral ENSO indices to continental climate anomalies are comparable to those computed for ocean instrumental records over the period of overlap. This gives us great confidence to utilize the coral ENSO indices back in time well beyond the instrumental records.
We continue to explore the fidelity of other ENSO proxies such as the proposed use of Sr/Ca, Mg/Ca, and U/Ca ratios as measures of sea surface temperature that have been adopted by many researchers in this field. Our empirical studies and experimental coral culture program lead us to abandon the Sr/Ca thermometer because it is controlled by many other variables. Other researchers have drawn the same conclusions. We remain cautious about using or interpreting Mg/Ca and U/Ca temperature proxies due to the shortcoming documented for Sr/Ca. In short, isotope proxies have may advantages over various metal to calcium ratios due to the fact that marine organisms actively transport Ca via enzymatic pathways and therefore many biologic rate processes interfere with the fidelity of metal/Ca proxies. Oxygen isotopes are not free of artifacts but it appears that it remains the most understood and reliable proxy of ENSO anomalies. The fact that oxygen isotopes are affected by both temperature and salinity means that this proxy is limited to measuring the presence or absence of ENSO events and only grossly may it be used to measure amplitude of the events.
Our present research is directed toward coring Holocene and deglacial reefs to investigate the influence of Milankovitch radiation cycles on ENSO frequency and intensity. We have some ENSO proxy results from mid-Holocene reefs in Tarawa that show a weaker ENSO signal compared to the past century but much more data are needed to fill in this picture. We completed our first Holocene coring program on Tarawa in early 2005 and cores just arrived at Lamont for analysis.
- [PDF] Charles, C.D., K.M. Cobb, M.D. Moore and R.G. Fairbanks, 2003. Monsoon/tropical ocean interaction in a network of coral records spanning the 20th century. Marine Geology, 201 (1-3), 207-222.
- [PDF] Evans, M., R.G. Fairbanks, and J.L Rubenstone, 1999. The thermal oceanographic signal of El Niño reconstructed from a Kiritimati Island coral. J. Geophysical Research, 104, 13409-13421.
- [PDF] Evans, M., R.G. Fairbanks, and J.L Rubenstone, 1998. A proxy of ENSO teleconnections. Nature, 394, 732-733.
- [PDF] Charles, C.D., D. Hunter, and R.G. Fairbanks, 1997. Interaction between the ENSO and the Asian Monsoon in a coral record of tropical climate. Science, 277, 925-928.
- [PDF] Fairbanks, R.G., M.N. Evans, J.L. Rubenstone, K. Broad, M.D. Moore, C.D. Charles, 1997. Evaluating climate indices and their geochemical proxies measured in corals. Coral Reefs, 16, 93-100.
- Fairbanks, R.G., M.N. Evans, J.L. Rubenstone, K. Broad, M.D. Moore, C.D. Charles, 1997. Evaluating climate indices and their geochemical proxies measured in corals. Proc. 8th Int Coral Reef Sym 1:107-116.
- [PDF] Cole, J.E., R.G. Fairbanks and G.T. Shen, 1993. Recent variability in the Southern Oscillation: isotopic results from a Tarawa Atoll coral. Science, 260, 1790-1793.
- Cole, J.E., G.T. Shen, R.G. Fairbanks and M. Moore, 1992. Coral monitors of ENSO dynamics across the equatorial Pacific. In: El Niño: Historical and Paleoclimatic Aspects of the Southern Oscillation, ed. by H. Diaz and V. Markgraf. Cambridge University Press, 349-375.
- Shen, G.T., L.L. Linn, T.M. Campbell, J.E. Cole and R.G. Fairbanks, 1992. A chemical indicator of westerly winds in corals from the central tropical Pacific. Jour. Geophy. Res., 97, no. C8, 12689-12697.
- Shen, G.T., J.C. Cole, D. Lea, L.J. Linn, T.A. McConnaughey, and R.G. Fairbanks, 1992. Surface variability at Galapagos from 1936-1982: calibration of geochemical tracers in corals. Paleoceanography, 7, no. 5, 563-588.
- Cole, J.E. and R. G. Fairbanks, 1990. The Southern Oscillation recorded in the stable isotopes of coral from Tarawa Atoll. Paleoceanography, 5, 669-683.
- [PDF] Fairbanks, R.G. and R.E. Dodge, 1979. Annual periodicity of the skeletal oxygen and carbon stable isotopic composition in the coral Montastrea Annularis. Geochim. et Cosmochim. Acta, 43 (7), 1-10.