Understanding ENSO dynamics through the exploration of past climates

Abstract

Historical reconstructions show that significant changes in ENSO characteristics took place during the Holocene. ?Modern? El Ni?o variability began around 5000-7000 years ago, with only weak decadal-scale events occurring beforehand. There was a gradual strengthening of ENSO thereafter, with evidence of a peak in variability around 1000-2000 years ago. Exploring these changes, using both data and models, provides a means of understanding ENSO dynamics. It can also provide insights into the mechanisms that drive changes in ENSO behaviour. 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 Ni?o events to develop. However, this proposed forcing mechanism is qualitative in nature and has yet to be rigorously tested. To explore this hypothesis, we use the CSIRO Mk3L climate system model to conduct a suite of simulations of the climate of the past 8000 years. We find that the model is able to reproduce the historical trends in ENSO variability, and that it therefore provides a tool that can be used to explore changes in ENSO dynamics. Consistent with the changes in insolation, the mean state of the tropical Pacific remains essentially unchanged over the Holocene, while there are large changes in the seasonal cycle. 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 Ni?o events. We explore a number of ENSO paradigms, including the recharge-discharge oscillator and a rigorous consideration of the energetics. 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 Ni?o events via westerly wind bursts