Two regimes of Atlantic multidecadal oscillation: Cross-basin dependent or Atlantic-intrinsic

The Atlantic Multidedal Oscillation (AMO) is a prominent mode of sea surface temperature variability in the Atlantic and incurs significant global influence. Most coupled models failed to reproduce the observed 50-80-year AMO, but were overwhelmed by a 10-30-year AMO. Here we show that the 50-80-year AMO and 10-30-year AMO represent two different AMO regimes. The key differences are: (1) the 50-80-year AMO involves transport of warm and saline Atlantic water into the Greenland-Iceland-Norwegian (GIN) Seas prior to reaching its maximum positive phase, while such a transport is weak for the 10-30-year AMO; (2) the zonality of atmospheric variability associated with the 50-80 year AMO favors the transport of warm and saline water into the GIN Seas; (3) the disappearance of Pacific variability weakens the zonality of atmospheric variability and the transport of warm and saline water into the GIN Seas, leading to the weakening of the 50-80-year AMO. In contrast, the 10-30-year AMO does not show dependence on the variability in Pacific and in the GIN Seas and may be an Atlantic-intrinsic mode. Our results suggest that differentiating these AMO regimes and a better understanding of the cross-basin connections are essential to reconcile the current debate on the nature of AMO and hence to its reliable prediction, which is still lacking in most of coupled models. (C) 2019 Science China Press. Published by Elsevier B.V. and Science China Press.