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abstracts
Understanding ENSO dynamics through the exploration of past climates
Steven Phipps, Jaclyn Brown
Historical reconstructions show that significant changes in ENSO characteristics took place during the Holocene. “Modern” El Niño variability began around 5-7,000 years ago, with a gradual strengthening of ENSO thereafter and a possible peak in variability around 1-2,000 years ago. Exploring these changes, using both data and models, provides a means of understanding ENSO dynamics.
Modelling studies to date have suggested a mechanism whereby changes in the Earth's orbital geometry explain the strengthening of ENSO over the Holocene. Decreasing summer insolation over the Asian landmass resulted in a weakening of the Asian monsoon system. This led to a weakening of the easterly trade winds in the western Pacific, making it easier for El Niño events to develop. To explore this hypothesised forcing mechanism, we use the CSIRO Mk3L climate system model to conduct a suite of simulations of the climate of the past 8,000 years.
We find that the model is able to reproduce the historical trends in ENSO variability. In the early Holocene, the easterly trade winds are amplified in the western Pacific during the northern autumn, consistent with an enhanced Asian monsoon. The stronger trade winds represent a barrier to the eastward propagation of westerly wind bursts, therefore inhibiting the onset of El Niño events. We find that the fundamental behaviour of ENSO remains unchanged, with the major change over the Holocene being the influence of the background state of the Pacific on the susceptibility of the ocean to the initiation of El Niño events.
Steven Phipps, Climate Change Research Centre, University of New South Wales,, Sydney NSW 2052, Australia, s.phipps@unsw.edu.au
Jaclyn Brown, CSIRO, Marine and Atmospheric Research, Hobart TAS 7001, Australia
Session: CCT3: Modeling
Download Poster:
> PAGES_OSM09_Phipps.pdf
Download Talk:
> YSM09_OralE_Phipps.pdf
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