ENSO teleconnections in high resolution AGCM experiments
Ph.D. Thesis
Ute Merkel
Abstract
Successful seasonal forecasting requires a thorough dynamical understanding of the atmospheric response to lower-boundary forcing such as sea surface temperature anomalies.
This thesis focusses on atmospheric teleconnections related to the El Niño/Southern Oscillation (ENSO) phenomenon and the underlying dynamical processes involved in the signal communication to remote regions. They are investigated based on ensemble experiments with an atmospheric general circulation model (ECHAM4).
Part of our experiments have been performed at high horizontal resolution (T106) to examine the role of resolution for a correct simulation of the observed atmospheric anomalies during ENSO events. A clear resolution dependence of the ENSO response is found over the North Atlantic/European sector.
Furthermore, the atmospheric response is analyzed with respect to its antisymmetry (El Niño-, La Niña-type forcing) and is shown to considerably deviate from a simple sign reversal, both in the mean flow and the subseasonal fluctuations.
Due to ENSO, the extratropical transient eddy activity is modified through a meridional shift of the midlatitudinal cyclone tracks. In order to gain more insight into the role of transient eddies in determining the atmospheric response to ENSO, the feedback of the transient eddies onto the mean flow is analyzed in terms of the eddy forcing onto the mean streamfunction. We present evidence that ENSO related changes in the upper-tropospheric transient eddy vorticity fluxes help to reinforce the mean streamfunction response. However, at lower levels, transient eddy temperature fluxes tend to destroy the mean temperature response and thus to reduce baroclinicity.
Considering the ongoing anthropogenic emissions of greenhouse gases and associated changes of climate, it may be asked whether the present-day ENSO impact on the atmosphere is maintained under future climate conditions. From experiments similar to those conducted under present climate conditions, it can be concluded that the atmospheric response to the tropical Pacific forcing remains rather robust.
Finally, we seek to reproduce the observed influence of ENSO on tropical storms (e.g. hurricanes) from a multi-year ECHAM4/T106 integration. Thereby, we also focus on the large-scale circulation providing the background for tropical storm development. It is shown that the vertical wind shear is clearly affected by ENSO.