Articles | Volume 10, issue 5
The Cryosphere, 10, 2099–2111, 2016

Special issue: International Partnerships in Ice Core Sciences (IPICS) Second...

The Cryosphere, 10, 2099–2111, 2016

Research article 15 Sep 2016

Research article | 15 Sep 2016

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

Christo Buizert1 and Jeffrey P. Severinghaus2 Christo Buizert and Jeffrey P. Severinghaus
  • 1College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, USA
  • 2Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA

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 δ15N–N2 and other tracers below gravitational equilibrium, questioning the traditional definition of the lock-in depth as the depth where δ15N enrichment ceases. Last, we propose that 86Kr excess may act as a proxy for past synoptic activity (or paleo-storminess) at the site.

Short summary
The upper 50–100 m of the world's ice sheets consists of the firn layer, a porous layer of snow that is slowly compacted by overlying snow. Understanding air movement inside the firn is critical for ice core climate reconstructions. Buizert and Severinghaus identify and describe a new mechanism of firn air movement. High- and low-pressure systems force air movement in the firn that drives strong mixing, called dispersion. Dispersion is the main mechanism for air mixing in the deep firn.