Mechanisms of passive tracer interhemispheric transport: An analysis of model-derived and observational interhemispheric transport climatology and interannual variations.

摘要

Examination of the distributions of minor atmospheric constituents may provide valuable insight into atmospheric transport processes. Interhemispheric transport (IHT), the cross-equatorial linkage of the Northern and Southern Hemispheres, is an important aspect of transport that can be explored using the properties of passive tracers. In this dissertation, the principal spatial and temporal features of IHT are examined through modeling and observational approaches.; Using the Goddard Institute for Space Studies-University of California, Berkeley (GISS-UCB) atmospheric general circulation model (AGCM) and its companion tracer transplant model (TTM), IHT climatology is first described. Means of assessing IHT, including a simple two-box model and transport partitioning, are introduced, and the seasonality of IHT is elaborated. Particular emphasis is placed upon the elucidation of the longitudinal and vertical features of IHT. IHT sensitivity to source emission geometry and convective mixing is also explored using both the TTM and a Lagrangian trajectory model (LTM) approach.; This dissertation further addresses the interannual variability (IAV) of IHT within the GISS-UCB AGCM framework. Analysis of several AGCM simulations, including an ensemble of “Identical Forcing” runs, reveals that IHT IAV is small relative to either the mean IHT timescale or its seasonal variations. IHT IAV is linked to both axisymmetric and regional circulations, and both forced and unforced circulation variations contribute significantly to the development of IHT anomalies. Among the mechanisms that modulate IHT on interannual timescales are changes in the intensity/geometry of the Hadley circulation, variations of the strength/displacement of the zonal-mean and regional Intertropical Convergence Zones (ITCZs), the El Niño/Southern Oscillation (ENSO), and the Indian Ocean Monsoon (IOM). The IOM appears to play an especially prominent role in the modulation of IHT.; A comparison of AGCM-simulated and observed IHT is also undertaken. Two bases of comparison are considered: a chemical transport model simulation of a fossil fuel source forced with an approximation to the historic state of the atmosphere and station observations of CFCl₃ (CFC-11). Although some similarities are noted between AGCM-simulated and observed IHT, significant differences, particularly at interannual timescales, suggest that the GISS-UCB AGCM results are either not generalizable or not completely reflective of observed IHT.