Role of Cold Fronts in South American Monsoon Onset

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Emily Wright (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: During the austral spring, the interactions between the southward-moving intertropical convergence zone with cold fronts that progress equatorward contribute to the complexity of the timing and regional differences in South American Monsoon onset. This thesis studied the impact of cold fronts on the onset of the South American monsoon. Previous studies have shown that cold fronts undergo a seasonal transition in structure, intensity and propagation. In fact, whereas wintertime cold fronts bring relatively little rainfall but strong temperature anomalies to South America, summertime cold fronts affect the rainfall patterns well into the tropical portions of the continent but cause very small changes in temperature. Cold fronts are the dynamical trigger of monsoon onset in at least some regions of South America.   In this study the 1979-2007 the National Center for Environmental Prediction and the National Center for Atmospheric Research reanalysis data (NCEP/NCAR) and Global Precipitation Climatology Project (GPCP) data are used to produce a composite analysis of the dynamic and thermodynamic structures, intensity and propagation of cold fronts that occur before ("pre-onset), during ("onset), and after ("post-onset") monsoon onset in the SACZ region. Those composites are used to study the effect of cold fronts on the abrupt onset of the monsoon season in the region of the South Atlantic convergence zone (SACZ).   The "pre-onset", "onset", and "post-onset" composites showed differences in speed and propagation of the cold fronts. In the "pre-onset" composites, cold fronts passed through South America quickly and did not reach the SACZ region. The "onset" composites showed cold fronts moving through the SACZ region and becoming stationary, bringing precipitation to the region. The precipitation in the region due to cold fronts is an indicator of the abrupt and dynamic onset in the region. Therefore at the time of "onset", there is a radical change in the structure and propagation of cold fronts propagating through South America that triggers monsoon onset in the SACZ region.   Years with normal, early and late onsets were compared to understand how thermodynamic changes, such as volumetric soil moisture, and sensible and latent heat fluxes, contribute to monsoon onset in the region. Quick changes in the thermodynamic variables right before onset in the SACZ region indicate that there is a fast onset triggered by cold fronts. In the WR, since there is a slow change in the thermodynamic variables before onset, WR onset does not seem to be associated with the passage of cold fronts.  The effect of the Madden-Julian Oscillation (MJO) on SAMS onset was also analyzed. The WR and the SACZ did not show a clustering of onset dates within the enhanced phases of the MJO for South America, therefore onset in those regions is found to not be affected by the MJO.  

Additional Information

Publication

Thesis

Date: 2010

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