Date of Award

5-2011

Document Type

Honors Thesis

Degree Name

Bachelor of Science

Abstract

Heavy precipitation events in the midlatitudes can be supported by the poleward transport of tropical air masses within the warm sector of extratropical cyclones. Previous studies have established a climatology of the four preferred pathways of tropical moisture export (TME) events into the midlatitudes over the Northern Hemisphere (NH). The present study constructs a similar climatology of TME timing and frequency over the Southern Hemisphere (SH), highlighting three preferential regions for tropicalmidlatitude interaction. These regions correspond to the locations of the: (i) South Pacific convergence zone (Pacific Ocean pathway, PO), (ii) South Atlantic convergence zone (South American pathway, SA), and (iii) South Indian convergence zone (Southeast African pathway, SEA). A Eulerian precipitable water (PW) climatology is constructed to isolate individual TME events within the three preferred pathways in the SH. The climatology identifies PW values along 30°S at 5° increments, extracting values four-times-daily from the 2.5° NCEP-NCAR reanalysis dataset for 1979−2007. Potential TME events are identified when two neighboring grid points have PW values >93rd percentile of their monthly PW distribution for >24 h. Potential events are classified as TMEs following human verification of the event‘s tropospheric structure. The present investigation reveals that TME frequency in the SH varies on intraseasonal and interannual timescales. The PO pathway exhibits the least seasonal variability of the three examined locations, while the SEA pathway is over three times as active during the SH meteorological summer (DJF). An in-depth analysis of the overall synoptic/dynamic mechanisms associated with TME events in the SEA pathway is performed in this study, linking the observed DJF peak in TME activity to the to the active phase of the South Indian convergence zone.

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