Hansen, Heiko (2007) Sensitivity of ENSO dynamics to wind stress formulation as simulated by a hybrid coupled GCM. (PhD/ Doctoral thesis), Universität Hamburg, Hamburg, Germany, 93 pp. . Berichte zur Erdsystemforschung, 49 .

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Abstract

The sensitivity of the El Ni ̃no/Southern Oscillation phenomenon (ENSO) with respect to different wind stress formulations is simulated by a hybrid coupled general circulation model (HCM). The HCM consists of a version of the MPI-OM general circulation model (GCM) coupled to a statistical atmosphere model. The design of the HCM allows mean wind stress and the anomalous sea surface temperature (SST) to wind stress coupling to be prescribed. Two main aspects are addressed: First, ENSO variability and equatorial Pacific ocean thermal structure are investigated with respect to variations in the mean background climatology, and second, improved parameterizations of the momentum flux at the ocean to atmosphere boundary in GCMs of the tropical Pacific are analyzed. To address the first aspect, the climatology of the tropical Pacific is varied via the strength of the mean wind stress and the ocean-to-atmosphere coupling. The results of the coupled model simulations show that the ENSO period decreases as the mean wind stress is increased. Furthermore, the zonal SST gradient along the equator as well as the mean thermocline depth increase with increasing mean wind stress, while the thermocline intensity is reduced. The ENSO amplitude is more sensitive to the anomalous ocean to atmosphere coupling, where a stronger coupling leads to higher amplitudes. The results are compared to recent studies with intermediate complexity models (ICMs) and to results from intercomparison studies with coupled atmosphere ocean GCMs (AOGCMs). At first view, the prediction of ENSO period from the ICM contradicts the HCM results. This is explained by the setup of the ICM, where the thermocline intensity is prescribed by a parameter, while in the HCM the thermal intensity is modelled. The AOGCMs do not exhibit relationships similar to the results of the HCM. To address the second aspect, the common bulk formula for wind stress is corrected for a moving ocean surface and improved parameterizations of the drag coefficient Cd. Sensitivity studies are carried out for various parameterizations in coupled and uncoupled mode, to give an overview of possible changes. The results of the uncoupled simulations suggest that the correction for a moving surface and the improved parameterization generally lead to reduced mean wind stress and reduced ENSO variability. The correction partly improves the equatorial east Pacific SST and thermal structure. Accounting for surface motion also reduces the variability in coupled simulations. In some experiments the system moves from a self sustained to a damped ENSO mode. The results partly sustain findings in recent sensitivity studies with the ECHAM5/MPI-OM AOGCM, which account for surface motions. The results emphasize the need for an accurate parameterization of the momentum flux also for very low wind speeds.

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