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A Southern Ocean Mechanism for the Interhemispheric Coupling and Phasing of the Bipolar Seesaw

Thompson, Andrew F. and Hines, Sophia K. and Adkins, Jess F. (2019) A Southern Ocean Mechanism for the Interhemispheric Coupling and Phasing of the Bipolar Seesaw. Journal of Climate, 32 (14). pp. 4347-4365. ISSN 0894-8755. doi:10.1175/JCLI-D-18-0621.1.

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The last glacial period is punctuated by abrupt changes in Northern Hemisphere temperatures that are known as Dansgaard–Oeschger (DO) events. A striking and largely unexplained feature of DO events is an interhemispheric asymmetry characterized by cooling in Antarctica during periods of warming in Greenland and vice versa—the bipolar seesaw. Methane-synchronized ice core records indicate that the Southern Hemisphere lags the Northern Hemisphere by approximately 200 years. Here, we propose a mechanism that produces observed features of both the bipolar seesaw and the phasing of DO events. The spatial pattern of sea ice formation and melt in the Southern Ocean imposes a rigid constraint on where water masses are modified: waters are made denser near the coast where ice forms and waters are made lighter farther north where ice melts. This pattern, coupled to the tilt of density surfaces across the Southern Ocean and the stratification of the ocean basins, produces two modes of overturning corresponding to different bipolar seesaw states. We present evolution equations for a simplified ocean model that describes the transient adjustment of the basin stratification, the Southern Ocean surface density distribution, and the overturning strength as the ocean moves between these states in response to perturbations in North Atlantic Deep Water formation, which we take as a proxy for Greenland temperatures. Transitions between different overturning states occur over a multicentennial time scale, which is qualitatively consistent with the observed Southern Hemisphere lag. The volume of deep density layers varies inversely with the overturning strength, leading to significant changes in residence times. Evidence of these dynamics in more realistic circulation models is discussed.

Item Type:Article
Related URLs:
URLURL TypeDescription
Thompson, Andrew F.0000-0003-0322-4811
Hines, Sophia K.0000-0001-9357-6399
Adkins, Jess F.0000-0002-3174-5190
Additional Information:© 2019 American Meteorological Society. Manuscript received 20 September 2018, in final form 19 April 2019. We thank Raffaele Ferrari and David Marshall for constructive comments to an earlier draft of this study and three anonymous reviewers for their insightful comments. We acknowledge helpful conversations with Paola Cessi, Malte Jansen, Brad Markle, Emily Newsom, Andrew Stewart, and Laure Zanna. We also thank Feng He for providing the TraCE simulation output. AFT received support from the David and Lucille Packard Foundation and from National Science Foundation (NSF) Grant OCE-1235488; SKH received support from NSF Grant P2C2-1503129 and the Lamont-Doherty Earth Observatory Postdoctoral Fellowship.
Funding AgencyGrant Number
David and Lucile Packard FoundationUNSPECIFIED
Lamont-Doherty Earth ObservatoryUNSPECIFIED
Subject Keywords:Ocean; Sea ice; Southern Ocean; Meridional overturning circulation; Ocean dynamics; Climate variability
Issue or Number:14
Record Number:CaltechAUTHORS:20190711-083803362
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:97049
Deposited By: Tony Diaz
Deposited On:11 Jul 2019 16:27
Last Modified:16 Nov 2021 17:25

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