Mechanisms of Decadal and Centennial ENSO Variability

The new convection scheme recently introduced to the Community Climate System Model (CCSM) yields substantial improvement in the simulation of the modern El Nino/Southern Oscillation (ENSO), altering ENSO behavior away from a simple delayed oscillator and toward a more stochastic system involving both atmospheric and oceanic feedbacks. The new convection scheme has a significant impact on ENSO dynamics, which exhibit a range of behavior even within a single model run. In particular, the role of pre- existing conditions in the eastern Pacific on amplifying El Nino events is extremely important in the new model. Interesting and unexpected correlations are seen between zonal wind/thermocline depth anomalies and variations in NINO3 index, a direct indication of the importance of stochastic forcing. Our results also indicate that ENSO forcing changes dramatically on decadal and centennial timescales within the model run; this implies that representative ENSO statistics require a much longer instrumental record than previously thought. Future work will focus on quantifying the ability of the model to identify the effects of stochastic forcing within Holocene and other paleorecords.