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.

Shortened Duration of Global Warming Slowdowns with Elevated Greenhouse Gas Emissions

Continuous emissions of anthropogenic greenhouse gases(GHGs)and aerosols in the last 160 years have resulted in an increasing trend of global mean surface temperatures(GMSTs).Due to interactions with natural variability,rates of the combined anthropogenically and naturally induced warming trends are characterized by significant slowdowns and speedups on decadal timescales.Here,by analyzing observed and model-simulated data,we investigate how the duration of these episodes will change with different strengths of GHG and aerosol forcing.We found that the duration of warming slowdowns can be more than 30 yr with a slower rate of anthropogenic emissions but would shorten to about 5 yr with a higher one.This duration reduction depends on both the magnitude of the climate response to anthropogenic forcing and the strength of the internal variability.Moreover,the warming slowdowns can still occur even towards the end of this century under high emissions scenarios but with significantly shortened duration.