High-resolution climate models have projected a “decline of the Atlantic Meridional Overturning Circulation (AMOC) under the influence of anthropogenic warming” for decades (Lobelle et al., 2020). New research that assesses changes in the deeper layers of the ocean (instead of “ignoring” these layers like past models have) shows instead that the AMOC hasn’t declined for over 30 years.

The North Atlantic has been rapidly cooling in recent decades (Bryden et al., 2020Fröb et al., 2019). A cooling of “more than 2°C” in just 8 years (2008-2016) and a cooling rate of -0.78°C per decade between 2004 and 2017 has been reported for nearly the entire ocean region just south of Iceland. The cooling persists year-round and extends from the “surface down to 800 m depth” 

Image Source: Bryden et al., 2020
Image Source: Fröb et al., 2019

Those who favor an anthropogenic explanation for these rather inconvenient cooling trends have leaned on the climate models that say the  ocean’s dominant heat transport mechanism – the AMOC – has been declining in response to anthropogenic greenhouse gas forcing.

But now a new study (Worthington et al., 2021) throws cold water on this claim too.

Image Source: Worthington et al., 2021

Reconstructing over 30 years of AMOC variability (1981-2016), the authors use a “higher-fidelity empirical model of AMOC variability” that, unlike past assessments, does not “ignore changes in the deep circulation”.

The authors do indeed find there was a brief dip in the AMOC from 2004-2012. But even this temporary decline was dominated by internal variability rather than being associated with anthropogenic forcing.

In fact, Worthington and colleagues have determined that there has been “no overall AMOC decline” since monitoring began in 1981. This contradicts the results of high-resolution climate models.

Image Source: Worthington et al., 2021

Consequently, the cooling in the North Atlantic can no longer be dismissed as a response to an anthropogenically-weakened AMOC.

Something else is driving the cooling.

via NoTricksZone


February 25, 2021 at 08:53AM