Simulated Two-Stage Recovery of Atlantic Meridional Overturning Circulation During the Last Deglaciation - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 74

Simulated Two-Stage Recovery of Atlantic Meridional Overturning Circulation
During the Last Deglaciation

Jun Cheng, 1 Zhengyu Liu, 2,3 Feng He , 2 Bette L. Otto-Bliesner, 4 Esther C. Brady, 4 and Mark Wehrenberg 2

A two-stage recovery of Atlantic meridional overturning circulation (AMOC)
during Bølling Allerød (BA) is revealed in the

first transient simulation of last
deglaciation in a fully coupled general circulation model (Transient Simulation of
Climate Evolution over the Last 21,000 Years (TraCE-21000)). After being sup-
pressed during the Heinrich 1 event, North Atlantic Deep Water (NADW) forma-
tion

first reinitiates in the Labrador Sea in stage 1 and then reinitiates in the
Greenland-Iceland-Norwegian (GIN) seas in stage 2. This feature is derived from
the investigation of NADW formation in its two origins with a newly developed
method and is con

rmed by the comprehensive analysis of relative variables. A
new mechanism is proposed to interpret the northward asynchronous reinitiation of
NADW during BA. In addition, our work also points out that the generation of the
AMOC overshoot is associated with the reinitiation of NADW in GIN seas.

1. INTRODUCTION sometimes the pronounced abrupt climate change is also

associated with AMOC recovery after fresh meltwater dis-
charge in the North Atlantic [Ganopolski and Rahmstorf,
2001; Rahmstorf, 2002; Clarke et al., 2002; McManus et al.,
2004; Lippold et al., 2009]. In the first transient simulation of
the last deglaciation with a fully coupled model (Transient
Simulation of Climate Evolution over the Last 21,000 Years
(TraCE-21000)), the abrupt onset of Bølling Allerød (BA)
warming was captured, and it was found that the onset of
BAwarming is dominated by the recovery of the AMOC. A
10°C air temperature increase over Greenland during BA
onset accompanies a 16.5 Sv increase of AMOC intensity
within 300 years [Liu et al., 2009].
The details of the recovery process of AMOC have not
been fully explained in the literature so far. Some studies
simply define one latitude band in the North Atlantic as the
important North Atlantic Deep Water (NADW) formation
region to explain AMOC recovery and do not consider the
details of NADW reinitiation in multiple origins [Krebs and
Timmermann, 2007a, 2007b; Hu et al., 2007; Mignot et al.,
2007]. This simplified method relies upon two points: (1) the
low resolution of the ocean model that employed a longer
integration for AMOC study and (2) the broad convection
area in the model simulation that was not only limited to the

Paleoclimate records and climate model simulations sug-
gest that climate change on centurial-millennial time scales is
largely connected with the variation of Atlantic meridional
overturning circulation (AMOC) [Broecker, 1990; Manabe
and Stouffer, 1995; Vellinga and Wood, 2002; Levermann et
al., 2005]. In addition to the slowdown/shutdown of AMOC,

1 Key Laboratory of Meteorological Disaster of Ministry of
Education and College of Atmospheric Sciences, Nanjing Univer-
sity of Information Science and Technology, Nanjing, China.
2 Center for Climatic Research and Department of Atmospheric
and Oceanic Sciences, University of Wisconsin-Madison, Madison,
Wisconsin, USA.
3 Also at Laboratory for Climate and Ocean-Atmosphere Studies
and Department of Atmospheric and Oceanic Sciences, Peking
University, Beijing, China.
4 Climate and Global Dynamics Division, National Center for
Atmospheric Research, Boulder, Colorado, USA.