Multi-centennial variability controlled by Southern Ocean convection in the Kiel Climate Model
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A quasi-oscillatory multi-centennial mode of open ocean deep convection in the Atlantic sector of the Southern Ocean in the Kiel Climate Model is described. The quasi-periodic occurrence of the deep convection causes variations in regional and global surface air temperature, Southern Hemisphere sea ice coverage, Southern Ocean and North Atlantic sea surface height, the Antarctic Circumpolar Current and the Atlantic Meridional Overturning Circulation (AMOC). The deep convection is stimulated by a strong built-up of heat at mid-depth. When the heat reservoir is virtually depleted a coincidental strong freshening event at the sea surface shuts down the convection. The heat originates from relatively warm deep water formed in the North Atlantic. The several decades lasting recharge process of the heat reservoir depends on the AMOC and the Weddell Gyre and sets a minimum delay for the deep convection to recur. While the strength of the AMOC increases, the Weddell Gyre weakens during the non-convective regime. Convection onset and shutdown also depend on the stochastic occurrence of favorable sea surface conditions, which contributes to the multi-centennial period of the phenomenon. The shutdown triggers a century-long deviation in AMOC strength caused by significant reductions in bottom water formation and surface salinity in the Southern Ocean’s Atlantic sector. Additional numerical experimentation reveals that sea ice has an important effect on the frequency of occurrence and intensity of the deep convection. Further, we find intriguing similarities to the Weddell Polynya observed during the 1970s.
KeywordsMulti-centennial climate variability Deep convection Southern Ocean Weddell Polynya Coupled climate model
This work was supported by the BMBF—supported Nordatlantik and DFG—supported SFB754 (www.sfb754.de), and the European Union’s Seventh Framework Programme (FP7/2007–2013) under grant agreement no 212643 (THOR: ‘‘Thermohaline Overturning–at Risk’’). The model integrations were performed at the DKRZ Hamburg and the Computer Centre at Kiel University.
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