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Antarctic Sea Ice Control on Ocean Circulation in Present and Glacial Climates

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Antarctic Sea Ice Control on Ocean Circulation in Present and Glacial Climates Antarctic sea ice control on ocean circulation in present and glacial climates Raffaele Ferraria,1, Malte F. Jansenb, Jess F. Adkinsc, Andrea Burkec, Andrew L. Stewartc, and Andrew F. Thompsonc aDepartment of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139; bAtmospheric and Oceanic Sciences Program, Geophysical Fluid Dynamics Laboratory, Princeton, NJ 08544; and cDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 Edited* by Edward A. Boyle, Massachusetts Institute of Technology, Cambridge, MA, and approved April 16, 2014 (received for review December 31, 2013) In the modern climate, the ocean below 2 km is mainly filled by waters possibly associated with an equatorward shift of the Southern sinking into the abyss around Antarctica and in the North Atlantic. Hemisphere westerlies (11–13), (ii) an increase in abyssal stratifi- Paleoproxies indicate that waters of North Atlantic origin were instead cation acting as a lid to deep carbon (14), (iii)anexpansionofseaice absent below 2 km at the Last Glacial Maximum, resulting in an that reduced the CO2 outgassing over the Southern Ocean (15), and expansion of the volume occupied by Antarctic origin waters. In this (iv) a reduction in the mixing between waters of Antarctic and Arctic study we show that this rearrangement of deep water masses is origin, which is a major leak of abyssal carbon in the modern climate dynamically linked to the expansion of summer sea ice around (16). Current understanding is that some combination of all of these Antarctica. A simple theory further suggests that these deep waters feedbacks, together with a reorganization of the biological and only came to the surface under sea ice, which insulated them from carbonate pumps, is required to explain the observed glacial drop in atmospheric forcing, and were weakly mixed with overlying waters, atmospheric CO2 (17). Here we apply theories of the deep ocean thus being able to store carbon for long times. This unappreciated circulation to illustrate that there is a direct dynamical link between link between the expansion of sea ice and the appearance of a the drop in temperature, the expansion of sea ice around Antarctica, voluminous and insulated water mass may help quantify the the rearrangement of the ocean deep water masses, and the change ’ – ocean s role in regulating atmospheric carbon dioxide on glacial in circulation at the LGM. Hence these are not independent feed- interglacial timescales. Previous studies pointed to many indepen- backs, rather they are expected to co-occur in each glacial cycle and dent changes in ocean physics to account for the observed swings in mayexplainwhytheatmosphericCO2 concentrations dropped by atmospheric carbon dioxide. Here it is shown that many of these the same amount for at least the last four glacial cycles. changes are dynamically linked and therefore must co-occur. Modern Deep Ocean Stratification and Circulation carbon cycle | ice age | ocean circulation | paleoceanography | Munk (18) first argued that the dense ocean waters that sink Southern Ocean through convection into the abyss at high latitudes return back to the surface as they are mixed by turbulence with lighter overlying he Last Glacial Maximum (LGM) spanning the period between waters. In this conceptual model, high-latitude convection sets the Tabout 25,000 and 20,000 y ago was characterized by global at- rate of the overturning circulation, and mixing determines the mospheric temperatures 3–6° colder than today (1), atmospheric deep ocean stratification through a balance between upwelling of – carbon dioxide (CO2) concentrations 80 90 ppm lower than pre- dense waters and downward diffusion of light waters. This view industrial values (2), and extended ice sheets and sea ice (3, 4). ignored the pivotal role of the Southern Ocean (SO) in control- Geochemical tracers suggest that the ocean volume filled by waters of ling both the stratification and the overturning of deep water Antarctic origin was nearly quadrupled at the LGM (5). In this study masses (19–21). In this section we review key elements of the new we apply recent understanding of deep ocean dynamics to explain the paradigm that has emerged over the last twenty years. In the next connection between the observed changes in the atmosphere, the section we discuss its implications for interpreting the changes EARTH, ATMOSPHERIC, cryosphere, and the deep ocean water masses. This connection pro- observed in the ocean at the LGM. AND PLANETARY SCIENCES vides a robust framework to quantify the drop in atmospheric CO2 concentrations, a critical gap in our understanding of glacial climates. Significance Much attention has been paid to the inferred changes in the deep ocean at the LGM, because they are believed to have played a key ’ – The ocean s role in regulating atmospheric carbon dioxide on role in reducing atmospheric CO2 concentrations (6 8). The deep glacial–interglacial timescales remains an unresolved issue in ocean contains 90% of the combined oceanic, atmospheric, and paleoclimatology. Many apparently independent changes in terrestrial carbon, and a rearrangement of deep water masses could ocean physics, chemistry, and biology need to be invoked to have a large impact on the atmospheric carbon budget. Fig. 1 shows explain the full signal. Recent understanding of the deep ocean δ13 the distributions of C of benthic foraminifera in the modern circulation and stratification is used to demonstrate that the – and glacial climates along a north south section from the western major changes invoked in ocean physics are dynamically linked. δ13 Atlantic (5). In the modern section one can see the high C In particular, the expansion of permanent sea ice in the Southern δ13 tongue of Arctic origin flowing over a low C tongue of Ant- Hemisphere results in a volume increase of Antarctic-origin abys- arctic origin below 4 km. At the LGM, the Antarctic source water sal waters and a reduction in mixing between abyssal waters appears to fill the whole ocean volume below 2 km squeezing of Arctic and Antarctic origin. a much reduced component of Arctic source waters above 2 km. The expansion of Antarctic-origin abyssal waters, richer in Author contributions: R.F. designed research; R.F., M.F.J., J.A., A.B., A.L.S., and A.F.T. nutrients and metabolic carbon than the deep Arctic-origin waters it performed research; R.F. and M.F.J. analyzed data; and R.F. wrote the paper. replaced, is believed to have reduced atmospheric CO2 by 10–20 ppm The authors declare no conflict of interest. (9). Concomitant changes in the ocean circulation, stratification, *This Direct Submission article had a prearranged editor. biological nutrient uptake, and carbonate compensation have been Freely available online through the PNAS open access option. invoked to account for the full 80–90 ppm drawdown (10). The 1To whom correspondence should be addressed. E-mail: [email protected]. physical feedbacks that have been identified are (i)achangeinthe This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ocean circulation that isolates abyssal waters from the surface, 1073/pnas.1323922111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1323922111 PNAS | June 17, 2014 | vol. 111 | no. 24 | 8753–8758 Downloaded by guest on September 29, 2021 frequency equal to twice the Earth’s rotation rate multiplied by 1.2 the sine of the latitude, and K is an eddy diffusivity that quantifies -1000 .08 8.0 the efficiency of instabilities at slumping isopycnals. This formula holds in a zonally averaged sense or, more precisely, for an av- -2000 8.0 )m(htpeD 08 . erage along the path of the Antarctic Circumpolar Current -3000 (ACC) that flows around the whole globe in the SO. To illustrate the power of the scaling law 1, we plot the pre- 04. −3 -4000 diction for the slope of isopycnal 27.9 kg m , which outcrops in the surface winter mixed layer at 65°S, for a characteristic wind -5000 −2 2 −1 0.4 stress of 0.1 N m and eddy diffusivity K = 1,000 m s (25). The black dashed line in Fig. 2 shows that with these values, the -6000 scaling 1 correctly predicts the slope of isopycnals shallower than 3,000 m; i.e., above major topographic ridges and seamounts, 1.6 which modify somewhat the slope of deeper isopycnals. In the -1000 upper few hundred meters, the isopycnals become flat at latitudes where winter ice melts and creates a shallow layer of fresh water. 0 1.2 -2000 0.8 )m(htpeD This is a transient summer phenomenon. Our scaling applies to 4.0 isopycnals below this layer. -3000 Given the surface distribution of density, the scaling for the 0 slope determines the zonally averaged distribution of density be- -4000 .0- 8 low the shallow wind-driven bowls. The argument is purely geo- -0.4 -5000 metrical, and it is illustrated by the dashed black line in Fig. 2. The isopycnals slope downward from the latitude where they intersect -6000 thesurface(moreappropriatelythebaseofthesurfacemixed -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 layer) to approximately 458 S, the northernmost latitude reached Latitude by the ACC where Eq. 1 holds. North of 458 S, the isopycnals are Fig. 1. Contour maps of δ13C for a section in the Western Atlantic (5) based essentially flat, because the presence of lateral boundaries does not on water samples for the modern climate (Upper) and benthic foraminiferal permit strong zonal flows, which would result in tilted isopycnals. −3 stable isotope data using a variety of Cibicidoides spp. for the LGM (Lower). Density surfaces lighter than 28 kg m come to the surface also at The glacial reconstruction documents the shoaling of North Atlantic Deep high latitudes in the North Atlantic with a very steep slope in re- Water to about 1,500 m, and the expansion and northward penetration of sponse to upright convection driven by strong cooling.
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