HOLIVAR2006 Abstracts
Using coupled model simulations of Mid-Holocene ENSO to constrain uncertainty in predictions of ENSO response to future climate change.
Jo Brown1, Mat Collins2 and Sandy Tudhope3
1Centre for Global Atmospheric Modelling, Department of Meteorology, University of Reading, Reading, UK
2Hadley Centre for Climate Prediction and Research, Met Office, Exeter, UK
3School of Geosciences, University of Edinburgh, Edinburgh, UK
Contact: Jo Brown (josephine.brown@reading.ac.uk)
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The sensitivity of the El Niņo Southern Oscillation (ENSO) to changes in mean state is investigated in simulations of Mid-Holocene, pre-industrial, and future climate with the Hadley Centre coupled ocean-atmosphere model, HadCM3. Coupled model predictions of ENSO response to future climate change are highly uncertain, with the latest IPCC AR4 simulations giving a wide range of predictions for changes in ENSO and the mean state of the tropical Pacific. Given this uncertainty, the ability of models to simulate changes in ENSO characteristics under palaeoclimate conditions may be used as one criterion to assess the credibility of future predictions. In the Mid-Holocene, orbitally forced changes in insolation resulted in changes in seasonality which may have altered ENSO behaviour. Proxy records of ENSO from the early and Mid-Holocene generally imply that ENSO was weaker than present or absent at this time. The HadCM3 simulations are compared with fossil coral isotopic records from the western Pacific Warm Pool which show a substantial reduction in interannual variability at ENSO periods in the Mid-Holocene (Tudhope et al. 2001).
The model simulates a modest reduction in ENSO amplitude in the Mid-Holocene, with a damping of sea-surface temperature (SST) anomalies from late boreal summer onwards in response to enhanced easterly trade winds in the central and western Pacific (Brown et al. 2006). A stronger Asian summer monsoon also contributes to a strengthened Walker circulation. The Mid-Holocene zonal SST gradient in the tropical Pacific is slightly reduced in the annual mean due to a relative warming of up to 0.5°C in the equatorial eastern Pacific in the boreal summer. As the response of ENSO to climate forcing may be influenced by the representation of physical processes in the model, we carry out additional simulations of Mid-Holocene, present day, and future climate with versions of the model in which selected physical parameters are perturbed from their standard values (Murphy et al. 2004). These perturbed parameter model versions simulate a range of ENSO amplitudes for each climate scenario, but do not produce changes in ENSO characteristics in response to Mid-Holocene orbital forcing.
Brown, J., Collins, M., Tudhope, A., 2006. Coupled model simulations of Mid-Holocene ENSO and comparisons with coral oxygen isotope records, Advances in Geosciences 6, 29-33.
Murphy, J., Sexton, D., Barnett, D., Jones, G., Webb, M., Collins, M., Stainforth, D., 2004. Quantification of modelling uncertainties in a large ensemble of climate change simulations, Nature 430, 768-772.
Tudhope, A.W., Chilcott, C.P., McCulloch, M.T., Cook, E.R., Chappell, J., Ellam, R.M., Lea, D.W., Lough, J.M., Shimmield, G.B., 2001. Variability in the El Niņo Southern Oscillation through a glacial-interglacial cycle, Science 291, 1511-1517.
Jo Brown is a postdoctoral researcher in the Centre for Global Atmospheric Modelling at the University of Reading, UK. She completed a PhD in 2003 in the Department of Earth Sciences at the University of Melbourne, Australia. The title of her PhD thesis was "The response of stable water isotopes in precipitation and the surface ocean to tropical climate variability". Her research interests include ENSO and tropical climate variability, palaeoclimate modelling and proxy data-model intercomparison, and the use of stable water isotope tracers in GCMs. Her current research is funded through the NERC Rapid Programme.