[A-CG44] Extratropical oceans and atmosphere

[ACG44-01] Western boundary currents and their extension regions as conduits for the ejection of bomb-radiocarbon from the thermocline

★Invited Papers

*Keith Bradley Rodgers^1,2, Ping Zhai³, Richard D Slater³, Brendan R Carter^4,5, Daniele Iudicone⁶, Olivier Aumont⁷, Jesse Farmer^8,9, Ivy Frenger¹⁰, Yves Plancherel¹¹, Laure Replandy¹², Andrea Fassbdner¹³, Katsuya Toyama¹⁴ (1.IBS Center for Climate Physics, Busan, Republic of Korea, 2.Pusan National University, Busan, Republic of Korea, 3.Atmospheric and Oceanic Sciences Program, Princeton University, NJ, 4.Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA, 5.NOAA Pacific Marine Environmental Laboratory, Seattle, WA, 6.Department of Integrated Marine Ecology, Stazione Zoologica, Anton Dohrn, Naples, Italy, 7.Sorbonne Universités, UPMC, Univ. Paris 06-CNRS-IRD-MHNH, LOCEAN/IPSL, Paris, France, 8.Princeton University, Department of Geosciences, Princeton, NJ, 9.Climate Geochemistry, Max-Planck INstitute for Chemistry, Mainz, Germany, 10.GEOMAR Helmholz Centre for Ocean Research, Kiel, Germany, 11.Grantham Institute, Imperial College, London, UK, 12.Princeton Environmental Institute, Princeton, NJ, 13.Monterey Bay Aquarium Research Institute, Moss Landing, CA, 14.Oceanography and Geochemsitry Research Department, Meteorological Research Insistute, Japan Meteorological Agency, Tsukuba, Japan)

Keywords:Western Boundary Currents, Air-Sea Interaction, Radiocarbon, Thermocline

The western boundary currents and their extension regions have long been expected to play an important role in the ocean’s carbon cycle. However, it is still uncertain how western boundary currents impact the ocean uptake pathways of anthropogenic carbon (C_ant), and if they sustain a new transfer of C_antfrom thermocline to subpolar water masses. Here we consider water mass transformation diagnostics applied to a forward ocean circulation model in conjunction with an analysis of bomb-DI¹⁴C (bomb-radiocarbon) as a proxy for a “pulsed” atmospheric source function of C_antover the period 1955-1963. Our main result is that the extension regions of the western boundary currents play an important role in sustaining rapid (32±3 years) renewal timescales for the full ocean volume above the density horizon of the base of the thermocline. This rapid renewal timescale sustains a vigorous exchange of the waters with maximum bomb-DI¹⁴C content. In 1995 in the model (mid-WOCE) 66% of the global cumulative air-sea flux of bomb-¹⁴CO₂enters the ocean directly into thermocline densities, but with only 42% of the inventory in the thermocline. Approximately 70% of the net transfer of bomb-DI¹⁴C from subtropical to subpolar waters in 1995 is shown to occur south of 30°S, sustained largely through the densification of near-surface waters through the release of heat to the atmosphere within the western boundary currents and their extension regions. The results may help to understand pathways of C_antwithin the ocean under a future transition to zero or negative emissions.