| authors | Philip, S.Y. |
| source | Natuur- en Sterrenkunde Proefschriften (2009) |
| full text | [Full text]
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| publisher | Utrecht University |
| document type | Dissertation |
| version | Publisher version |
| disciplines | Natuur- en sterrenkunde |
| abstract |  Increasing our understanding of the equatorial Pacific climate phenomenon El Niño - Southern Oscillation (ENSO) is important for making progress in climate prediction. We wonder whether we can analyse how differences arise in ENSO-like ehavior in global climate models (GCMs). Which mechanisms are important for ENSO and the asymmetry between El Niño and La Niña? Are they modelled correctly in GCMs? We trace the mechanisms governing El Niño observations and in 19 coupled GCMs. To this end, we use a conceptual model that describes the relations between subsystems that play a role in ENSO using local linear regressions. The relations describe a wind response to SST variability, the response of SST to both thermocline variability and wind stress variability and a temporal damping on SST. We categorize six GCMs as having the most realistic balance of the various feedback mechanisms compared to observations. For this subset of models we make projections into a future stable, warmer climate. The signs of the changes in ENSO mechanisms are similar. However, the sign of the small net effect differs from model to model. In general El Niño is larger than La Niña the distribution of SST anomalies in the East Pacific is positively skewed. Can we use nonlinearities in our conceptual model for analysing what is the likely origin of the skewness of ENSO? What is the role of atmospheric noise in this respect? For observations the linear couplings are extended with a new description of atmospheric noise properties and some nonlinearities in the atmospheric terms. The effect of these nonlinearities are studied with an Intermediate Complexity Model (ICM) in which the fitted couplings and noise properties areimplemented, but no further tuning is carried out. The SST skewness is most affected by a nonlinearity in the response of the wind stress to SST anomalies. This is followed by the state-dependence of atmospheric noise. The skewed nature of atmospheric noise has only a minor effect on SST skewness. GCMs tend to simulate lower atmospheric noise amplitudes than observations. The GCMs that show a nonlinear response of wind stress to SST anomalies simulate the most realistic SST skewness. Overall, both a nonlinear atmospheric response to SST and the dependence of noise on the background SST influence the El Niño/La Niña asymmetry. Finally, we investigate the sensitivity of ENSO to uncertainties in the description of physical processes in a GCM by examining a set of GCM experiments with perturbations to key atmospheric and oceanic GCM parameters. |
| keywords | ENSO, El Niño GCMs, atmospheric noise, climate change, atmospheric properties, mechanisms, climate models, atmosphere-ocean couplings |
