Diagnostic studies of extratropical intraseasonal variability in the northern hemisphere
Robinson, Dennis P.
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A comprehensive analysis of midlatitude intraseasonal variability in extended integrations of General Circulation Models (GCMs) developed at the National Aeronautics and Space Administration Goddard Space Flight Center is conducted in the first part of this thesis. The model statistics, three-dimensional structure, and dynamical characteristics of storm tracks and large-scale weather regimes are diagnosed and directly compared to parallel observational analyses. Results indicate that several aspects of simulated intraseasonal phenomenon are linked to errors in the zonally asymmetric circulation. During the boreal winter, anomalously large meridional temperature gradients and enhanced zonal winds in the simulations are linked to suppressions in baroclinic wave activity in the models. As a result lower-frequency modes in the GCMs are more strongly driven by baroclinic dynamics. Nonetheless, the GCMs successfully reproduce the North Pacific midwinter suppression phenomenon. In an effort to provide insight into the North Pacific midwinter suppression, the second part of this work stratifies the boreal cool season into three stages, where the early and late (middle) stages approximately correspond to the seasonal maxima (minimum) in synoptic eddy activity that occurs over the North Pacific. Analyses using this unique approach reveal that during the midwinter suppression period, cyclonic perturbations entering the North Pacific storm track core from Asia are already deficient in magnitude compared to early and late winter stages. In both observations and model simulations, the North Pacific midwinter suppression feature is discovered to have a clear organized extension upstream into Siberia. Thus, the final portion of this thesis examines the causes for the midwinter suppression of upper tropospheric Rossby wave packets propagating across Asia prior to entering the North Pacific storm track. The study unveils an increased sensitivity of synoptic-scale wave packets with a large-scale, upper-tropospheric flow pattern over Siberia during midwinter. This interaction with the large-scale pattern over Siberia results in a general dampening of synoptic eddy amplitudes over Asia, which is proposed to be the contributing factor to the North Pacific midwinter suppression phenomenon.