Dispersion in deep polar firn driven by synoptic-scale surface pressure variability

TitleDispersion in deep polar firn driven by synoptic-scale surface pressure variability
Publication TypeJournal Article
Year of Publication2016
AuthorsBuizert C, Severinghaus JP
JournalCryosphere
Volume10
Pagination2099-2111
Date Published2016/09
Type of ArticleArticle
ISBN Number1994-0416
Accession NumberWOS:000384269600001
Keywordsabrupt climate-change; age-differences; air convection; antarctic ice; atmospheric histories; fractionation; gas concentrations; greenland; ice core; isotopic; porous-media
Abstract

Commonly, three mechanisms of firn air transport are distinguished: molecular diffusion, advection, and near-surface convective mixing. Here we identify and describe a fourth mechanism, namely dispersion driven by synoptic-scale surface pressure variability (or barometric pumping). We use published gas chromatography experiments on firn samples to derive the along-flow dispersivity of firn, and combine this dispersivity with a dynamical air pressure propagation model forced by surface air pressure time series to estimate the magnitude of dispersive mixing in the firn. We show that dispersion dominates mixing within the firn lock-in zone. Trace gas concentrations measured in firn air samples from various polar sites confirm that dispersive mixing occurs. Including dispersive mixing in a firn air transport model suggests that our theoretical estimates have the correct order of magnitude, yet may overestimate the true dispersion. We further show that strong barometric pumping, such as at the Law Dome site, may reduce the gravitational enrichment of delta N-15-N-2 and other tracers below gravitational equilibrium, questioning the traditional definition of the lock-in depth as the depth where delta N-15 enrichment ceases. Last, we propose that Kr-86 excess may act as a proxy for past synoptic activity (or paleo-storminess) at the site.

DOI10.5194/tc-10-2099-20160
Short TitleCryosphere
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