Simulated decadal oscillations of the Atlantic meridional overturning circulation in a cold climate state |
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Authors: | Masakazu Yoshimori Christoph C Raible Thomas F Stocker Manuel Renold |
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Institution: | (1) Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland;(2) Center for Climate System Research, University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa Chiba, 277-8568, Japan;(3) Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland |
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Abstract: | The significance of the Atlantic meridional overturning circulation (MOC) for regional and hemispheric climate change requires
a complete understanding using fully coupled climate models. Here we present a persistent, decadal oscillation in a coupled
atmosphere–ocean general circulation model. While the present study is limited by the lack of comparisons with paleo-proxy
records, the purpose is to reveal a new theoretically interesting solution found in the fully-coupled climate model. The model
exhibits two multi-century-long stable states with one dominated by decadal MOC oscillations. The oscillations involve an
interaction between anomalous advective transport of salt and surface density in the North Atlantic subpolar gyre. Their time
scale is fundamentally determined by the advection. In addition, there is a link between the MOC oscillations and North Atlantic
Oscillation (NAO)-like sea level pressure anomalies. The analysis suggests an interaction between the NAO and an anomalous
subpolar gyre circulation in which sea ice near and south of the Labrador Sea plays an important role in generating a large
local thermal anomaly and a meridional temperature gradient. The latter induces a positive feedback via synoptic eddy activity
in the atmosphere. In addition, the oscillation only appears when the Nordic Sea is completely covered by sea ice in winter,
and deep convection is active only near the Irminger Sea. Such conditions are provided by a substantially colder North Atlantic
climate than today. |
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