Evolution of the deep Atlantic water masses since the last glacial maximum based on a transient run of NCAR-CCSM3
During the last deglaciation (from approximately 21 to 11 thousand years ago), the high latitudes of the Atlantic Ocean underwent major changes. Besides the continuous warming, the polar and subpolar ocean surface received a large amount of meltwater from the retracting ice sheets. These changes in temperature and salinity affected deep waters, such as the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW), which are formed in the Southern Ocean and in the northern North Atlantic, respectively. In this study, we present the evolution of the physical properties and distribution of the AABW and the NADW since the last glacial maximum using the results of a transient simulation with NCAR-CCSM3. In this particular model scenario with a schematic freshwater forcing, we find that modern NADW, with its characteristic salinity maximum at depth, was absent in the beginning of the deglaciation, while its intermediate version—Glacial North Atlantic Intermediate Water (GNAIW)—was being formed. GNAIW was a cold and relatively fresh water mass that dominated intermediate depths between 60 and 20°N. At this time, most of the deep and abyssal Atlantic basin was dominated by AABW. Within the onset of the Bølling-Allerød period, at nearly 15 thousand years ago (ka), GNAIW expanded southwards when the simulated Meridional Overturning Circulation overshoots. The transition between GNAIW and NADW ocurred after that, when AABW was fresh enough to allow NADW to sink deeper in the water column. When the NADW appears (~11 ka), AABW retracts and is constrained to lie near the bottom.
KeywordsMOC Paleoceanography Meltwater Modeling climate change Millennial time scale
This work was carried out when JMM was a visiting graduate student trainee in the Department of Atmospheric and Oceanic Sciences (AOS) at McGill University (December 2013–November 2014). JMM wishes to thank the AOS for their hospitality and office facilities during this period. Helpful discussions with Marcos Tonelli, Rodrigo Kerr, Dr. Jaime Palter, Dr. Peter Gent and Marc-Olivier Brault during the course of this research are also gratefully acknowledged. We thank the editor and reviewers for their comments and suggestions, which greatly improved this paper. This work was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) through the Grants 2011/02047-9 and 2013/20977-9, and is a contribution to INCT-Criosfera. We wish to thank P2C2 program/NSF, Abrupt Change Program/DOE, INCITE program/DOE, NCAR and Bette Otto-Bliesner for making the TraCE-21K simulation available.
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