No Evidence for Ocean Circulation Driving the Atlantic Multi-Decadal Oscillation

The Atlantic Multi-decadal Oscillation (AMO), a basin-wide sea surface temperature (SST) change in the North Atlantic, is a major mode of climate variability. Its many important societal impacts include Atlantic hurricane frequency and droughts in North America and North Africa. While there is no consensus on the causes of the AMO, most previous explanations identify the driver as the ocean circulation, specifically changes in the Atlantic Meridional Overturning Circulation (AMOC). Here we show that main features of the observed AMO are reproduced in models where the ocean heat transport is prescribed and thus cannot drive variability in the atmosphere or SST. Allowing the ocean circulation to interact with the atmosphere does not significantly alter the spatial and temporal characteristics of the AMO in current-generation climate models. We conclude that the simplest explanation for the AMO is that it is the low frequency response to stochastic forcing from the mid-latitude atmospheric circulation, with thermal coupling between the atmosphere and the upper ocean allowing the AMO signal to extend into the tropics. In this view, the AMOC and other ocean circulation changes would be largely a response, not a cause.