Abstract
The aim of this thesis is to improve our understanding of El Niño, by using recently
available observational datasets to analyze and verify mechanisms that drive El Niño.
A secondary goal is to improve model simulations of El Niño, which can lead to improved
forecasts.
Chapter 2 gives an analysis of how changes in
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SST are related to thermocline depth
variability. There is a time delay between a local thermocline depth anomaly and the
resulting SST anomaly at the surface. It is shown that the delay varies with longitude.
Two important pathways are distinguished that cause the relation between thermocline
depth and SST. The upwelling pathway (involving kelvin waves, upwelling and mixing)
is found to be dominant in the eastern Pacific, approximately east of 140°W. The wind
coupling pathway (involving convection, mixing and evaporation) is dominant west of
140°W.
Chapter 3 analyzes the mechanisms that are important for the development of SST
anomalies in the western equatorial Pacific, the warm pool region. In a budget study
it is shown how SST anomalies propagate zonally from the western Pacific to the central/
eastern Pacific and back during the development and decay of an El Niño event. An
analysis of the mixed layer heat budget in an ocean model simulation shows that both
zonal wind anomalies (anomalous upwelling and zonal advection) and wind speed anomalies
(anomalous latent heat flux and changes in mixed layer depth) are important.
Eastward propagation of SST anomalies during the growth phase of El Niño is caused
partially by a reduction of the mixed layer depth east of the SST anomaly, and partially
by zonal advection. Westward propagation during the decay phase is caused by warming
in the western Pacific through mean zonal advection across an anomalous temperature
gradient, and radiative cooling east of the SST anomaly.
The theme of chapter 4 is the question "How does El Niño change under the influence
of human induced global warming?". An analysis of 62 coupled ocean-atmosphere
model simulations over the period 1940-2080 shows no significant changes in ENSO
characteristics, despite a simulated global average warming of ∼ 1.2K. A detailed investigation
is performed to find out if flaws in the model used for the simulation are
responsible for the insensitivity of El Niño to global warming. The model behavior is
found to be qualitatively similar to that of a stable system driven by stochastic noise.
The zonal wind response to SST anomalies in the equatorial Pacific is shown to be
insensitive to changes in background SST. The zonal wind response pattern is too narrow
around the equator, which leads to a more stable system, insensitive to changes in
background temperature. Concluding, model deficiencies make the model insensitive
to global warming. It is necessary to analyze and improve coupled general circulation
models with respect to these deficiencies before they can be used answer the questions
whether and how ENSO will change due to global warming.
Chapter 5 discusses the results found in chapters 2,3 and 4 and provides a summary
of the conclusions.
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