Research led by the University of Edinburgh seeks to understand how the El-Niño Southern Oscillation (ENSO) phenomenon may change over the coming decades. The research aims to shed light on why some ENSO events are much stronger than others, why some decades show much stronger ENSO activity, and how ENSO will be affected by future climate change.
A fundamental challenge in understanding systems such as ENSO is that the directly observed record of climate is limited to the 20th century, a period insufficiently long to characterize the extent of natural variability. Consequently, researchers are indirectly reconstructing tropical Pacific climate over the past millennium using the physical and chemical properties of natural archives, such as annually-banded corals and trees. In particular, the current project is generating many centuries of new data from living and ‘fossil’ corals from the Galápagos Islands in the Eastern Pacific, an area highly sensitive to ENSO variability.
In order to understand the origins of changes in ENSO behaviour over the last millennium, climate model simulations are compared to the climate reconstructions. These are the same climate models as those used in the Intergovernmental Panel on Climate Change’s future climate change simulations, but run over the past millennium. By comparing model simulations with different external ‘forcings’, such as the effect of changes in solar output, volcanoes and GHG emissions, we will better understand the sensitivity of the ENSO system to such forcings and consequently, will be better able to predict its likely future response.
1) Detecting changes in ENSO variability that are robust against the range of unforced climate behaviour is challenging. Empirical constraints derived from CMIP5 GCM control simulations lower the apparent significance of changes within the instrumental record, but the near-centennial time-scale changes present between 14/15th C and 20th C coral records remain unlikely to have originated through unforced variability alone.
2) Isotope-enabled model work suggests that interpretation of western Pacific coral δ18O records in terms of changes in ENSO variance is generally robust, but with some tendency to underestimate the magnitude of extreme events.
3) Inferring changes in ENSO variability from tele-connected regions requires either very strong calibration relationships, or very long records.
4) Fossil and living corals in Galapagos preserve in the oxygen isotopic composition of their skeletons robust records of past seasonality and ENSO variability. Fossil corals are providing 30-70 year-long windows into climate variability in the region over the past 4,000 years.