Normal-mode analysis of the role of tropics in global atmospheric predictabilityKosovelj, Katarina (Avtor) Žagar, Nedjeljka (Mentor) Madden-Julian Oscillationnormal modesICTP AGCM (SPEEDY)general circulationpredictabilityTropical circulation features associated with convective heating affect the extratropical variability trough various teleconnections. Some properties of this tropical-extratropical coupling are addressed in this thesis. The novelty of the applied methodology is the modal decomposition technique that provides scale-dependent diagnostics in terms of the Rossby and inertio-gravity (IG) waves. These waves are eigensolutions of the linearized primitive equations, termed the normal modes and their horizontal structure is described by the Hough harmonics. Known for a long time, the normal-mode functions (NMFs) have recently been applied also for the validation of weather and climate model variability. The central question addressed in the thesis is the quantification of the global Rossby and IG wave response to tropical heating perturbations resembling different phases of Madden-Julian Oscillation (MJO). This question is addressed by numerical simulations using the ICTP AGCM (SPEEDY) model. Although simplified with respect to the full scale state-of-the-art climate and weather prediction models, SPEEDY proves capable to reproduce large-scale patterns of both transient (short-term) and average (climatological) circulation. The model simulates the partition of the spatial and temporal variability between the zonal and meridional modes, zonal mean state and waves, the Rossby and IG part of circulation reasonably well in comparison to the ERA-20C reanalyses. It is shown that initial uncertainties is SPEEDY grow in a similar fashion as in the numerical weather prediction systems. The model is able to fairly reproduce forecast uncertainties associated with the Rossby and IG modes with temporal and spatial characteristics found in an operational ensemble prediction systems. This justifies using SPEEDY for studying the transient properties of the global response to tropical heating perturbations. Four ensembles of simulations with the MJO-like heating perturbations in 100 winters with different background conditions are performed. Two ensembles are forced with dipole and another two with monopole heating and cooling perturbations, mimicking various phases of the MJO life cycle. The perturbations are applied on temperature tendencies and include feedbacks from the model convection and large-scale condensation schemes. The results show that the realistic formulation of heating perturbations involving moist feedbacks does not produce significantly different results from dry model with imposed temperature perturbations used in previous studies. The response is mostly defined by a positive heating anomaly, and the interaction of the two poles of dipole acts to localize the tropical response and to affect the shape and orientation of the Rossby wave train emanating from the subtropics. The short-term response is projecting predominantly on the IG modes, especially on the Kelvin wave. In the medium time range, the Rossby modes dominate the response. Although the n=1 Rossby mode is the main contributor to the Rossby wave variance at all time scales, it is shown that the modes with the meridional index n>1 contribute over 50 % of the medium-range Rossby wave response. Finally, numerical experiments without large-scale orography components in the northern hemisphere (NH) are performed to study the orography impact on the extratropical propagation of the Rossby wave developed in response to MJO heating perturbation. A relatively small statistically significant impact is found.20192019-10-17 07:45:18Doktorsko delo/naloga111890VisID: 103431COBISS_ID: 385193sl