Major Atlantic ocean current system might be approaching critical threshold -- ScienceDaily

The major Atlantic ocean current, to which also the Gulf stream belongs, may have been losing stability in the course of the last century. This is shown in a new study published in Nature Climate Change. The Atlantic Meridional Overturning Circulation, or AMOC, transports warm water masses from the tropics northward at the ocean surface and cold water southward at the ocean bottom, which is most relevant for the relatively mild temperatures in Europe. Further, it influences weather systems worldwide. A potential collapse of this ocean current system could therefore have severe consequences.

"The Atlantic Meridional Overturning really is one of our planet's key circulation systems," says the author of the study, Niklas Boers from the Potsdam Institute for Climate Impact Research, Freie Universität Berlin and Exeter University. "We already know from some computer simulations and from data from Earth's past, so-called paleoclimate proxy records, that the AMOC can exhibit -- in addition to the currently attained strong mode -- an alternative, substantially weaker mode of operation. This bi-stability implies that abrupt transitions between the two circulation modes are in principle possible."

It has been shown previously that the AMOC is currently at its weakest in more than a 1000 years. However, so far it has remained an open question whether the observed weakening corresponds to a change in the mean circulation state, or whether it is associated with an actual loss of dynamical stability. "The difference is crucial," says Niklas Boers, "because the loss of dynamical stability would imply that the AMOC has approached its critical threshold, beyond which a substantial and in practice likely irreversible transition to the weak mode could occur."

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