Simulated ENSO Sensitivity to Climate

This chapter examines the sensitivity of tropical Pacific interannual variability to changes in the climatological background state. Chapter 5 showed that although a wide variety of climate perturbations can affect the equatorial Pacific, the equatorial structure of the response tends to be similar in each case. Due to strong coupled feedbacks like those active in ENSO, the Pacific climatology generally responds with either an El Niño-like warming (with a weakened zonal SST gradient, relaxed trades, relaxed currents, and a flatter thermocline) or a La Niña-like cooling (with a sharper zonal SST gradient, stronger trades, stronger currents, and a steeper thermocline slope). This chapter therefore focuses primarily on the ENSO sensitivity to a single climate parameter, the strength of the equatorial zonal wind stress. Section 7.1 draws on the ideas of Chapter 6 to explain how the growth of small disturbances is affected by the strength of the climatological equatorial trades, and investigates the extent to which the ENSO sensitivity to other climate parameters resembles the sensitivity to trade wind strength. Section 7.2 then considers how a change in the equatorial trades affects the transient ENSO response to strong wind events, and extends the climate sensitivity map to stronger values of the air-sea coupling. Section 7.3 discusses the effects of climate changes in the stochastic context, focusing on ENSO predictability and the extent to which observed ENSO changes can be detected and attributed to climate changes. Section 7.4 concludes with a climate sensitivity analysis of a hybrid coupled GCM.