El Niño-Southern Oscillation: Asymmetry, nonlinear atmospheric response and the role of mean climate

Abstract

The differences between the warm and cold phases of the El Niño-Southern Oscillation have profound implications on their potential socio-economical impacts and predictability; and understanding them provides insights into the fundamental dynamics of the coupled climate system. In this dissertation, these differences are systematically quantified in terms of their amplitude, duration and sequencing preferences. The role of atmospheric nonlinearities in causing these differences is investigated using a hierarchy of models. It is found that the equatorial surface zonal wind response being stronger during El Niño than during La Niña can lead to the observed ENSO asymmetry in a consistent manner.

The nonlinearity in the zonal wind response strength has not been previously explained. With the use of a linear shallow water model, it is demonstrated that the nonlinear surface zonal wind response is related to the nonlinearity in the precipitation response. By decomposing the precipitation anomalies into components attributable to adjustments of the Walker circulation (zonal redistribution) and the local Hadley cell (meridional redistribution) respectively, it is shown that during La Niña, the meridional adjustment acts to reduce the climatological precipitation available for the zonal adjustment to take place, therefore weakens the La Niña surface zonal wind response and enhances the zonal wind response nonlinearity. As the equatorial climatological precipitation is found to be linearly correlated with the surface zonal wind response during El Niño and La Niña, it follows that the mean state climatological may have a strong control on ENSO characteristics. This hypothesis is tested by flux adjusting a state-of-the-art coupled climate model to different ocean surface climatological states. Some, but not all of the ENSO statistics and feedbacks are shown to depend systematically on the the mean climate state. Several hypotheses on the causal relationships between ENSO and the mean climate are also tested. The extent to which ENSO depends on the mean climate state is quantified and discussed.