AMO vs global warming

I ended my last blog post by asking a few questions related to the future of the North Atlantic Ocean and the AMO. What can we expect looking forward? As I also mentioned in the end of the last post, the AMO is very hard to model and therefore also to predict. Almost all models have a difficulty in simulating the AMO variability. So we really don’t know what to expect into the future…

But we do have knowledge and numerous studies which we can draw theories from. We do know that the AMO is a dominant mode of variability in the North Atlantic SST with a duration of 55-80 years (Wei & Lohmann 2012). But the question we are asking these days, and the same question I asked in my previous post, is to what extent the North Atlantic Ocean is influenced by global warming relative to the internal variability (AMO), and what consequences that may have for the future.

A study based on observational data by Wang and Dong in 2010 finds that both global warming and AMO variability make a contribution to the recent warming in the North Atlantic basin, and that their relative contribution is approximately equal. They also find (after removing a linear trend and the seasonal cycle found in observational records) that atmospheric CO₂ anomalies show a multidecadal variation approximately coinciding with the cold and warm phases of the AMO (see figure below).

They discuss that this may be related to the ocean’s CO₂ uptake through ocean circulation and the strength of the AMOC. As mentioned in my previous post, the phases of the AMO are determined by AMOC variability. There also exists a relationship between solubility of CO₂ and ocean temperature, which says that a warmer ocean leads to a release of CO₂. So a warm (cold) phase of the AMO will lead to a release (uptake) of CO₂ to (from) the atmosphere.  Summarized the warming of the North Atlantic due to AMO variability may influence global SST via the increase of atmospheric CO₂.

So based on findings from this study it seems like there exists a two-way relationship between North Atlantic Ocean temperatures and CO₂ concentrations in the atmosphere. Increased amounts of atmospheric CO₂ contributes to a warming of the North Atlantic basin. The other way around, the warm phase of the AMO with anomalously warm SSTs contributes to an increase in atmospheric CO₂, which further increases global warming. With future predictions of further CO₂ increases this does not look good. But then again the AMO is a mode of natural variability internal to the climate system which likely will change to a negative phase sometime in the future, which will reverse the picture. It is pretty clear, considering the huge climatic impacts of the AMO, that further research is needed in this field.