Low-Frequency Variability of Troposphere-stratosphere Coupling

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9405448 Baldwin On timescales from a week through interannual there is considerable variability of tropospheric flow patterns. This study will focus on understanding how much of the low-frequency variability of the stratosphere can be accounted for by stationary wave patterns in the troposphere. The general approach will be to identify patterns in the mid-troposphere that are linked with stratosphere flow. Particular attention will be paid to the possible influences of El Nino/Southern Oscillation (ENSO) on the stratosphere that may be communicated through modulation of the extratropical tropospheric flow. The PI will use empirical orthogonal function (EOF) analysis and singular value decomposition (SVD) analysis of the tropospheric height field together with correlations, composites and spectral analysis to identify modes which, on interannual and interseasonal timescales, seem to have a significant stratospheric influence. The complementary nonlinear technique of cluster analysis will also be used. A series of experiments using a GCM with simplified physics will be used to study the response of the middle atmosphere to tropospheric circulation anomalies. The experiments will be designed to determine the model's stratospheric response to tropospheric anomaly patterns. Comparing the response of the model's winter stratosphere with the resultq of the data analysis should provide a valuable method of identifying the contribution to winter stratospheric interannual variability from tropospheric anomalies. As part of this study the PI will use gridded ECMWF data analyses in conjunction with rawinsonde data, to determine tropical tropospheric horizontal wave structure to better elucidate the different types of synoptic-scale disturbances that exist in the tropical troposphere and their vertical coupling, if any, to the stratosphere. The focus is specifically on organized, westward- traveling, synoptic-scale waves in the tropical lower atmosphere that may be involved in the excitation of Rossby-gravity waves. Disturbances in both lower and upper troposphere will ne investigated, as will any vertical coupling between the two layers. This research is important because it should enhance our understanding of important climate processes related to stratospheric-tropospheric dynamics.