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theses

The South Pacific Convergence Zone (SPCZ) is simulated as too zonal a feature in current generation climate models, including those in Phase 5 of the Coupled Model Intercomparison Project (CMIP5). On synoptic timescales, the SPCZ structure is controlled both by the amount of low-level inflow from the relatively dry southeastern Pacific as well as the tropical-extratropical interaction between transient synoptic systems and the mean background state. However, the fidelity with which CMIP5 models are capable of simulating these interactions has not been previously examined. Building on the analysis of observed SPCZ-region synoptic scale variability by Lintner and Neelin (2008), composite analysis of two reanalyses and 17 CMIP5 models reveals both individual models and their ensemble mean capture patterns of wind, specific humidity, and precipitation anomalies consistent with reanalysis and observational results. To further explore the difference between weak- and strong-inflow conditions, both are instantaneously imposed in an ensemble of experiments using the Quasi-equilibrium Tropical Circulation Model 2 (QTCM2). While imposed circulation anomalies lead to observed moisture and precipitation anomalies, imposed moisture anomalies manifest a weak circulation anomaly. Thus, it appears more likely that the initiation of SPCZ shifts is dynamic in nature. Analysis of synoptic variability in the simulated subtropical SPCZ reveals that the basic mechanism of tropical-extratropical interaction is generally well simulated, with storms approaching the SPCZ along comparable trajectories to observations. However, there is a broad spread in mean precipitation and its variability across the CMIP5 ensemble. The region of mean negative zonal stretching deformation or “storm graveyard” in the upper troposphere is displaced in CMIP5 models to the northeast of its position in reanalysis data, albeit with pronounced (≈25°) inter-model longitudinal spread; SPCZ precipitation is similarly displaced and models with stronger storm graveyards show higher precipitation variability. Overall, these results further confirm that SPCZ errors are primarily related to a biased background state. Additionally, SPCZs simulated by CMIP5 models are not simply too zonal; rather, in models the subtropical SPCZ manifests a diagonal tilt similar to observations while SST biases force an overly zonal tropical SPCZ, resulting in a more discontinuous SPCZ than observed.

ETD_6191

Ph.D.

Includes bibliographical references

by Matthew J. Niznik

doi:10.7282/T3611249

ETD doctoral

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