Interhemispheric gradient of atmospheric radiocarbon reveals natural variability of Southern Ocean winds 1AOS Program, Princeton University, Princeton, NJ, USA
26 Oct 2011
2National Institute of Water and Atmospheric Research, Wellington, New Zealand
3Department of Earth and Planetary Sciences, McGill University, Montreal, QC, Canada
4Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
5Carbon Dating Laboratory, University of Waikato, Hamilton, New Zealand
6Stazione Zoologica Anton Dohrn, Naples, Italy
7Rutgers, The State University of New Jersey, New Brunswick, NJ, USA
8Institute for Environmental Physics, University of Heidelberg, Germany
9German Aerospace Center, Institute for Technical Thermodynamics, Department of System Analysis and Technology Assessment, Stuttgart, Germany
10Queens University, Belfast, UK
Received: 14 January 2011 – Published in Clim. Past Discuss.: 25 January 2011 Abstract. Tree ring Δ14C data (Reimer et al., 2004; McCormac et al.,
2004) indicate that atmospheric Δ14C varied on multi-decadal to
centennial timescales, in both hemispheres, over the
period between AD 950 and 1830.
The Northern and Southern Hemispheric Δ14C
records display similar variability, but from the data alone
is it not clear whether these variations are driven by the production
of 14C in the stratosphere (Stuiver and Quay, 1980)
or by perturbations to exchanges
between carbon reservoirs (Siegenthaler et al., 1980).
As the sea-air flux of 14CO2 has a
clear maximum in the open ocean regions of the Southern Ocean, relatively modest
perturbations to the winds over this region drive significant perturbations
to the interhemispheric gradient.
In this study, model simulations are used to show that
Southern Ocean winds are likely a main driver of the
observed variability in the interhemispheric gradient over AD 950–1830, and
further, that this variability may be larger than the Southern Ocean wind
trends that have been reported for recent decades (notably 1980–2004). This
interpretation also implies that there may have been
a significant weakening of the winds over the
Southern Ocean within a few decades of AD 1375, associated with the
transition between the Medieval Climate Anomaly and the Little Ice Age. The
driving forces that could have produced such a shift in the winds
at the Medieval Climate Anomaly to Little Ice Age transition remain unknown.
Our process-focused suite of perturbation
experiments with models raises the possibility that the current generation
of coupled climate and earth system models may underestimate the natural
background multi-decadal- to centennial-timescale variations in the winds
over the Southern Ocean.
Revised: 03 August 2011 – Accepted: 04 September 2011 – Published: 26 October 2011
Citation: Rodgers, K. B., Mikaloff-Fletcher, S. E., Bianchi, D., Beaulieu, C., Galbraith, E. D., Gnanadesikan, A., Hogg, A. G., Iudicone, D., Lintner, B. R., Naegler, T., Reimer, P. J., Sarmiento, J. L., and Slater, R. D.: Interhemispheric gradient of atmospheric radiocarbon reveals natural variability of Southern Ocean winds, Clim. Past, 7, 1123-1138, doi:10.5194/cp-7-1123-2011, 2011.