Do the modern AMOC and PMOC form a natural climate seesaw?

Thursday, December 13, 2018 - 17:45
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Mechanisms governing the setup of modern Atlantic Meridional Overturning Circulation (AMOC) are still not well understood. Commonly it is thought that the much saltier Atlantic Ocean favors the formation of deep convection in the subpolar North Atlantic while the fresher Pacific inhibits the occurrence of deep convection. Here we use the Community Earth System Model version 1 (CESM1) to explore whether the AMOC and PMOC form part of a natural climate seesaw. We examine whether a collapsed AMOC would change the salinity contrast between the North Atlantic and North Pacific, and whether deep convection in the Pacific (PMOC) could develop if we added freshwater into the subpolar North Atlantic. In the model simulations, the additional freshwater is compensated for either by (1) global oceans (other than the subpolar North Atlantic) or (2) by the subpolar North Pacific. Results show that the AMOC collapses in all experiments, but that the PMOC is only established in experiments where the added freshwater is compensated for by water from the subpolar North Pacific. Further analysis of the global compensation experiments indicates that with a collapsed AMOC, Bering Strait throughflow is severely weakened (or even reverses direction), keeping more freshwater or bringing in additional freshwater to the subpolar North Pacific, and preventing deep convection from occurring. In the subpolar North Pacific compensation experiment, deep convection (PMOC) forms in response to the additional salt input, mimicking the formation of AMOC in the North Atlantic. With a collapsed AMOC, the Antarctic circumpolar current (ACC) strengthens by ~ 30% in all experiments relative to the control run, suggesting that the upper branch of the return flow for AMOC may impede the ACC. At the same time, Indonesian Throughflow weakens by 50 to 80% in experiments with an active PMOC, and weakens by ~20% in experiments without an active PMOC, suggesting that part of the Indonesia Throughflow is driven by AMOC. With an active PMOC, the west Pacific warm pool significantly weakens, (possibly associated with weaker easterly winds), leading to a dramatic reduction in Indonesian Throughflow. We conclude that the AMOC and PMOC do not form a natural seesaw under modern climate conditions with an open Bering Strait unless significant salt is added into the subpolar North Pacific.

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