Analysis of Passive Tracer Transport as Modeled by an Atmospheric General Circulation Model

Abstract

Tracers without feedback on the atmosphere are used to probe tropospheric transport. Such passive tracers are considered for two important anthropogenic sources, Europe and eastern North America. The linearity of passive tracer continuity allows transport to be formulated in terms of a Green function, G. A coarse-grained Green function is defined that is suitable for numerical investigation with a GCM. An ensemble of independent realizations of the atmosphere is used to obtain the model’s ensemble mean, or “climate” Green function. With increasing time, the individual realizations of G converge to their climate mean and this convergence is quantified in terms of the decay of ensemble fluctuations. Throughout, G is analyzed with the goal of gaining new insight into the tracer climate that results from constant sources. The climate Green function is used to identify transport timescales, pathways, and mechanisms. The Green function is zonally mixed after about 3 months. The time to mix G to within 10% of its asymptotic value exceeds 1 yr at high-latitude lower levels, while the interhemispheric two-box exchange time is ∼7 months. Tracers from Europe and eastern North America follow different pathways with distinct seasonality. Eddies play a key role in transport. Transport in the Southern Hemisphere is dominated by transient eddies resulting from tracer injected ∼4 months earlier. These transient eddies extend throughout much of the troposphere, and align to a large degree with contours of zonally averaged mixing ratio. Large seasonal changes of the mean-motion part of the tracer flux are primarily compensated by the standing-eddy transport. Ensemble fluctuations of G decay with an approximate t−3 power law. Eddy conversion provides a source of fluctuations, while dissipation damps ensemble fluctuations with a timescale of ∼10 days. In the GCM context, the relative importance of parameterized versus resolved vertical transport is examined.